MALE AND FEMALE WOOD DUCK, TO SHOW SECONDARY SEXUAL CHARACTERS.
LFronL photographs of slufled specimens in the Collection of the Maryland Academy of Sciriices 1
THE
LAW O F HEREDITY. A STUDY O F TEE
CAUSE. O F VARIATION, AND THE
O R I G I N OF
LIVINGORGANISMS, BY
W.
I(. BROOKS,
ASSOCIATEIN BIOLOBY, Joms HOPKINS UXIVEBSITY.
BALTIMORE AND NEWY O R E : JOHN XURPHY & CO., PUBLISHERS.
COPYaIOaT,
1883,
BY
JOHN IklURPHY & CO.
TO
THEMEMORYOF
CHARLES DARWIN, FROM WHOSE STORE OF PUBLISHED FACTS I HAVE DRAIVN NOST OF THE MATERIAL FOR THIS
.
V 0LU M E
PREFACE. The subject which is tre:itcd in this book has occupied my thouglits for ten years or more, but I have refriiined from pubIishing my views, as I hope that I may some time be able to submit them to the test of experinlent. Many experiments have suggested theniselves t o me, but as most of them involve the cultivation ar,d hybridization, for many generations, of such animals arid plants as mill thrive aiitl multiply in confinement, they can only be carried out by some one who has the niems for experimental researches, arid who lias also a permanent home in thc country, where organisms of many kinds may be kept under observation for years, and where many specimens of hybrids between various wild and domesticated spec& can be reared to maturity. My own studies hare been in a different province of natural science, and it has therefore seemed best t o publish this volume in order to call renewed attention t o this most fascinating subject. I hare little hope that my views will be permanently accepted in the form in which they are here presented, but I do hope that they may Berve to bind together and
viii
Preface.
to vitalize the mass of facts which we already possess, and that they may thus incite and direct new experiments. If this book should serve to turn the attention of others into this channel, and should thus ultimately help us to a clearer insight into the nature of the forces which have acted, a d still act, to guide the evolution of life, this result will far outweigh the acceptance or rejection of the epculations which we here advanced.
CONTENTS. CHAPTER I. WHAT IS HEREDITY
?
The development of an animal, with the complex and beautiful structural adjustments, the instincts, habits, and individual traits of its parents is one of the most wonderful phenomena of the material universe-Heredity is not due to the external conditions which act upon the ovum, but to something within the ovum itself-The phenomena of reversion-Asexual and sexual heredity-Possibility of a n explanation of heredity-Characteristics which are now hereditary were at one time new variations-Heredity and variation are opposite aspects of the same problem-We may hope that a more perfect acquaintance with the laws of heredity will remove many objections to the theory of natural selection.. .. ... .. . . .
. .. ..... . . ...... ... . .....
6
CHAPTER 11. HISTORY OF TEE THEORY OF HEREDITY.
Requisites of a theory of heredity-Historical sketch of speculation on heredity-Evolution hypothesis of Bonnet and Haller-Ovists and spermists-Modern embryological research has sbown that it is impossible to accept the evolution hypothesis in its original form-Buffon’s speculations upon heredity fail to account for variation-Hypothesis of epigenesis-This hypothesis is logically incompleteThe analogy between phylogeny and ontogeny gives no real explanation of the properties of the ovum-Haeckel’e plastidule hypothesis-This hypothesis is not logically complete unless it involves the idea of evolution-Jager’s hypothesis-Ultimate analysis shows that this is at bottom an evolution hypothesis-No hypothesis of epigenesis is satigfactory-No escape from some form of the evolution 16 hypothesis-This conclusion is accepted by Huxley
......
x
CONTENTS.
CHAPTER 111. HISTORY O F THE THEORY OF HEREDITY-(&?LtZhUed).
Some form of tlre evolution hypothesis a logical necessityDarwin’s pangenesis hypothesis-This is an evolution hypothesis, since all the cliaracteristics of the adult are supposed to be latent in the germ-Miscellaneous objectious to it-These objections do not show that it contlicts with fact-Difiiculty in imagining detailed working is no reason for rejecting it-Galton’s experimental disproofThcre are m m y reasons for believing that the sexual elements have differetit functions-The evidence from partlietiogeuesis-Polar-cell hypothesis-The evidence from hybrids, from variation, and from structures confined to one sex-The pangenesis hypotliesis recognizes no such difference 10 tlie functions of’ the reproductive elementsWe must therefore distrust its absolute correctness-Sum47 mary of last two chapters
.............................
CHAPTER IV. A NEW TEEORY OF HEREDITY.
The objection to the liypotliesis of pangenesis would be almost entirely removed if it could be simplified-statenicnt of a new theory-Heredity is due to the properties of the egg-E:rcli new character lras been impressed upon tlie egg by the transmission of gemmnles-Tendency to form genimules is due to the direct action of external conditions-The ovum is the conservative element-The male cell is the progressive element-This theory has features of resemblance most of the liypotheses wliich have been noticed-It fills most of BIivart’s conditions also-It is not necessary to assume that the ovnm is as complicated as the adult-There are many race characters w u c h are not congenital-There are many congenital characters which are not hereditary-Direct action of external conditionsOur theory stands midway between Darwin’s theory of natural selection and Lamarckianism.. 80
.................
CHAPTER V. ON TEE OPINION THAT EACH SEX MAY TRANSMIT ANY CHARACTERISTIC WHATEVER.
The argument from hybrids-This argument is inconclusive -The argument from the homology between the ovum and the male cell-Homology does not involve functional similarity-The argument from the dual personality of each individual; from reversion; and from polymorpliism -These phenomena admit of a simpler explanationQ9 Summary of chapter..
.................................
CONTENT8.
xi
CHAPTER VI. THE EVIDENCE FROM HYBRIDS.
Importance of the subject-It
fUrRiSlJeS a means Of analyzing or isolating the influence of each sexual element-Hybrids very variable-Hybrids from domesticated races more v;~ri:tblethan those from wild races-The descendants of b y brids more variable than the hybrids themselves-The offspring of a male hybrid and the female of a pure ppecies are inucli more variable than those of a feniale hybrid and the i n + of a pure species-These facts inexplicable on any view. except the one liere presented-Reciprocal crosses-They differ in fertility and in structure-The difference is exactly what our theory requires-Difficulty in expliniug transmission of characters witiioiit fusiouIteversion caused by crossing-Two kinds of reversion.. . .... 118 Summary
.......... .... .. . ........ ....
.........
CHAPTER VII. TEE EVIDENCE FROX VARIATION.
Causes of variation-Changed conditions of Iife induce variability-No particular kind of change is necessary-Varinbility is almost exclusively confined to organisms produced from rertilized ova-Bud variation very rare-History of the Italian orange-The frequency of variation in organisms produred from sexual union, as compnred with its infrequency in those produced asexu:dly, receives a direct explanation by our theory of heredity-Bud variation more frequent in cultivated than in wild plants-our theory mould lead us to expect this-Changed conditions do not act directly, bnt they cause subsequent generations to vary-Tendency to vary is hereditary-These facts perfectly explicable by our theory-Specific cliaracters more variable than generic-Species of large genera more variable than those of small genera-A part developed in an unusual way liiglily variable-Lnw of equable variation -Secondary sexual characters variable-Natural selection cannot act to produce permanent modification unless many individuals vary together-Our theory is the only explanation of the simultaneous variation of many individuals-This theory also simplifies the evolution of complex structures-Saltatory evolution-This is explained by our theory of heredity-Correlated variation of homologous parts-Parts confined to males more vari. able than parts confined to females-Males more variable 140 than females-Summary of last two chapters..
... .. .. ..
CHAPTER VIII. THE EVIDENCE FROM SECONDARY SEXUAL CHARACTERS..
... 166
Xii
CONTEiVT8.
CHAPTER IX. TEE EVIDENCE FROM SECONDARY SEXUAL CHARACTERE coxT1NUED.-TEE CAUSE OF THE EXCESSIVE MODIFICATION O F MALE CIIARACTERS.
The explanntion of Daines Barrington and Wallace-Reasons for considering it inadequate-Darwin’s cxplanationHistory of domesticated races shows t l m this does not go to the root of the matter-The view that the male js more exposed than the female to the action of selection-A more fundamental explanation is needed-This is furnished by our theory of heredity-Special difficultiesSummary 207
............................................. CHAPTER X.
THE EVIDENCE FROM TEE INTELLECTUAL DIFFERENCES BETWEEN MEN AND WOMEH..
............................
2&
CHAPTER XI. TEE THEORY OF HEREDITY CONSIDERED AS SUPPLEMF,HTARY TO THE THEORY O F NATURAL SELECTION.
Darwin believes that variations are purely fortuitous-Natural selection cannot give rise to permanent race modifications unless many individuals vary in nearly the same n’ay, at about the .same time-The chances against this are very great if variations are fortuitous-Argument from North British Review-Dakwin acknowledges tlie great weight of this objection-It is removed by the theory of heredity -The co-ordinated modification of complicated organsThe time demanded by Darwin practically infiniteMurphy’s argument from the complexity of the eyeHerbert Spencer’s illustration-Our theory removes this difficulty-Mr. Conn’s objection-Saltatory evolutionEvidence that it occurs-Spike horn buck-Ancon and Mauchamp sheep-Black-shouldered peacock-The theory of heredity accounts for saltatory evolution-Parnllel variation-Evidence of its occurrence-Evolution of the medusse-General and special Homologies.. 275
.............
CHAPTER XII.
......................
RECAFITULATION AND CONCLUSION..
3ia
HEREDITY, CHAPTER I. WHAT IS HEREDITY?
The development of an animal, with the complex and beautiful structural adjustments, the instincts, habits, and individiial traits of its parents is one of the most wonderful plienomcna of the inaterial universe-Heredity is not due to the external conditinnq wliieli act upon tbe ovum, hut to something within the ovum itself-The phenomena of reversion-Asexual and sexual heredity-Possibility of a n explanation of heredityCharacteristics aliicli are now hcreditaiy were at one time new variations-Heredity and variation are opposite aspects of the same problem-We may hope that a more perfect acquaintance with the laws of lieretlity will remove many objections to the theory of natural selection.
TOthe ordinary nnscicntific reader the word heredity may perhaps suggest nothing more than s few curjoiis cases where an odd peculiarity of the parent has been transmitted to the children, or i t may recall the hereditary transmission of a tendency to certain diseases, or the mental or moral idiosyncrasies of the parents. To the breeder of domestic animals or plants it has a somewhat wider significance, and recalls the transmission by choice or fancy breeds of the features which give them their value. To him heredity is the law which enables him t o modify his animals and t o build up and perpetuate new varieties.
6
Heredity.
T o the naturalist, on the contrary, the word is filled with deep meaning, and instead of recalliIig to liis mind a few odd eases, tlie tricks and accidents of heredity, i t briugs before him the most i n a n ellous of all the plienomena of the material universe: the production from a simple egg of a living animal, with tlie intricate structure and complex bodily and mental functions of its proper species. Thoughtful men i n all ages have regarded the strncture and faculties of tlie higher animals as :I proper field for life-long study. Yet the acute intellects, thc powers of patient observation and profound reflection which generations of naturalists hare brought to this fascinating subject, have not yet given l i b iL complete knowledge of the life of a singlc animal. I n crery agc and country wlicre science Iins flourished men liave devoted tlieir lives to this subject, and h v e felt that tlieir lisrdly-enrned resnlts could scarcely be called :I beginning. So vast is tlic field, so many are the plienoniena, that tlic povincc of natural science is practically infinite, for each animal and each plant presents special problcms which open out in all directions before tlie student in an endless vista. Wonderful and various as tlic attributes of each :minial are, Iiowevcr, they ;ire not myste:.ious; for, at tlio same time t1mt we discoiw in an organism tlie power to do wonderfiil things, TVC also find in i t :Lmaterialorganization, a Inccliaiiism, adopted to do these very thiiigs. It is true tliet we cannot perfectly understand this meclianism, tlmt i n nmiy cases we fail completely i n our attempts to trim its working, i t n d that in most cases our insight is very crnde indeed. Still me are able to sliow that the niacliinery exists; and anatomy, or the study of structure, goes hand inhand with the study of the bodily
WXat is Heredity?
7
anil mental :tctivitics of animals. We do not underEtarld the ni:teliinery, but we find that i t is there, and we c:m intcrrulbt it5 work by obstructing or iiijnring it. Oar rvondcr is iiot :I feeling of mystery, a sense that tlic l)lienornc~i:itranscend kiio~ledgcj i t is due to a perceptioii of tlie :tmouiit of knowledge required. We rcgard an :tdult anirnal with feelings siniilsr to those with mliicli an intelligent savage might regnrd a telephone or a steamboat. A dog, with a11 the powers and faculties which enable him to fill his place as m;tii’s companion, is a wonder almost beyond o w powers of expression; but we fiiid in his body tlie machincry of muscles and reins, digestive, rcsi)iratory, and circulatory organs, eyes, ears, etc., mliicli adatpts liim t o his place; and study lias taught us ciiough about the action of this machinery to ‘‘1-Bsure us greatcr knowledge woiild show 113, in tlic structure of t h e dog, an esplmation of all that fits the dog for Iiis Iifc; an explanation :13 satisfactory as that which a s:ivage might rcacli, in the case of tlic steambod, by studying its anatomy. Lct our savage find, however, while studying an iron stcambo:it that small masses of iron, withont structure, so far as tlic mc:m at his command sllow liim to examine sad decide, are from time to time broken off arid thrown o v e r b o d , and that each of tIicse contains i i i itself the power to build up all the macliiiiery and applimces of a perfect stemiboat. The wonderful thing now is not the adaptation of woiiderf id m:icliiiiery to produce wonderful rcsults, but the prodiiction of wonderful results without any discoverable meclianism; and this is, in on~line,tlie problem which is brought before tlic mind of the nltturalist by t h e word heredity. Every o m knows that each dog exists at some time
S
flerediity.
as an egg, and the microscope shows in this egg no traces of the organs of the dog’s body or of anything a t all like them. So far as oiir means of examination go the egg is 110 more likc a dog tliaii the mass of iron is likc a stcamboat. I t may be said, though, that the dog’s egg is not left to itself, but is fertilized and is carried inside the body of the mother until the new animal is matured; that i t is there iiourished and built up from substlances supplied through the body of a full-grown dog; t h t it may be acted upon a t this time by agencies mliicli liare a direct tendcncy to build up out of it an organism like the parent; that the egg does not wtually contain a 110tential dog, but simply supplies the proper material to be acted upon by the surrounding conditions, and that the structure of tlic new animal is due to tlicsc conditions; that the embryo becomes a dog because it is bathed by a dog’s blood, nourished through a dog’s body, and is complctcly surrounded by influences which arc peculiar t o dog nature. Those persons who arc not nucumlists derive their knowledge of the animal world chiefly from our common domestic animals, and to such persons tliis explanation may seem probable; but naturalists, with wider experiencc, know that animals which carry tlicir young inside their bodies arc exceptions, and that tlie organization of thc future animal must cnist potentially in the egg, since the conditions to which it is exposed cannot possibly have any tcndcncy to prodrice from it :L being ml~ichdoes not already exist, in some form, within it. A bcc is almost as wonderful as a dog; its anatomical structure is cxqiiisitely dclicnte and complex, and every one is acquaintcd with the wolidcrful work mhich it accomplishes. A t the time it is laid tlie egg which is to become a worker-bee contains no ~iisibletrace of it6
What is Eieredity?
9
body, or of anything like it. It has becn carefully stndicd with all t h e resources of modern science, but examination shows nothing within it which is more like a bce than a mass of iron is like a n iron ship. This egg is not evcii fertilized, but i t dcvelops into a perfect worker, with all its wonderful structure and instincts, by virtue of something which it contained when i t left the ovary of its mother. It is true that it is not left quite to itself, but is carcfiilly attended and caredfor by other bees; but ercrytliing which they d o for i t might be done just as well by dc1ic:tte machinery, and t!ie attention has no tcndency whatever to miinufacture a bee. Proper heat a n d access t o air are :is necessary as attcntion, and attention hns no more power to prodnce it bee than air or heat. No onc who is familiar with marine anim4s can believe for a n instant t h t tlic conditions to wliich an egg is exposed liavc anything whatever to do with the character of tlic animal t o which i t gives rise. We map artificially remove eggs from the ovaries of severs1 dillercnt m i m d s , fertilize them :Lrtificially, a i d tlicn p h c c thcni together i n a tnmbler of fie%m:iter, and expose thcm to exiictly siniihr e s t e r i d conditions, yct c : d i one will follow its own d ~ ~ t ~ ~ i i conrse, i i ~ i e dand wc nxty rear in tlio Fame tumbler of wtrter from eggs which :&re h d l y diatinguishalle animnls which liare less in common than a dog :md a bird. Jf tlierc is no mystcry in the p~rform:tnceby the complicated organs of : ~ nadult aniiml of all its complicated IiinctionS, mliat slid1 me sny mlicn we find the ~ O W Wto perform thcsc functions cxisting i n a 1:ttent state in the egg, witliont tlio corresponding organs? Thi3 is tlic problem of heredity. I n thc mind of the natur:dist tlic word calls np t h e greatest of all tlic wonders of tlic niatcrial universe: the cxistcnce, iu a simple,
10
Heredity.
unorganized egg, of a power to produce a definite a d d t animal, with all its characteristics, even down to the slightest accidental peculiarity of its pauents; a power to reproduce in it all their l i d i t s and instincts, and even the slightest trick of speech or action. This is by no means the whole of the problem of heredity. One of the most interesting plicnorncnil coilnccted with our subject is what is known as rcversion, or the appearance in the child of peculiarities which were iiot present in either parent, but are diic to inlieritsnce from a grandparent or a more remote mcestor. An interesting illustration of this law is the occasional appearance in horses of stripes on tlie body and legs. Such stripes are not usually present in the horse, although Darwin has given reasom for believing that our liorscs :&redcseended from a striped zebra-like ancestor. The power to revert to this :tnccstrd form is handed down from generation to generation in the egg, acd i t may show itself a t any timo by the production of a striped colt. &version is, in a certain sense, exccptional, but it is not a t all rare, and me must add this power t o thc wonderful properties of the egg. Darwin gives the following case, which mill serve to illustrate the nature of reversion: A pointcr bitch produced some puppies; four were marked with blue :uid white, which is so unnsual :I color in pointers that she was thought to have played false with onc of the greyhounds, and tlie .whole litter '(VRS condemned, but the gamekeeper mas permitted to s aw one as a curiosity. Two years afterwards a friend of the owner saw the young dog, :md declared that lie mas tlic image of his old pointer bitch, S:tpph, tlie only blue :mtl white pointer of pure descent which he had eyer seen. This led to close inquiry, and it was proved that he was the great-
WiLat is Heredity? -
11 ~-
great-grandson of S:~pplio; SO that, according to the comnion expression, he had only one-sixteenth of her blood in liis reins. Another aspect of our subject must be kept constantly in mind. Among tlic higher aniin;rls heredity usually maiiifests itself oiily by what is known as sexual reproduction,-that is, the dcvelopment of iiew individnala from fertilized eggs; but in the lower forms of life another k k d of reproduction, the development of new inclividuds by bndding or by analogous processes, is even more common. Among the liydroids lierecljty may manifest itsclf by the formation of ncw :mimals, widi all tlic cliaracteristics of the parent, on almost any part of tlie body of the latter, and in certain plants the smallest fragmcnt of tissue may become a new and perfect p!mt, e:tpable of producing otlicrs in the same may or by seeds. The most surc and rapid way to get iicw sea-anemones is to tear an old one to pieces. As a rule this power i, confilled to the lower forms of life, but certain animals wliidi are by no means low or simple in strnctnrc multiply ;~scxu:tlly,and the offspring thus produced inlierit, like those developed from eggs, all the cliaracteristics of Clie parent. This then is the problem of hercdity, certainly one of the grandest secrets of nature. When we reflect upon its obscurity and complexity we may fairly ask what hope tlicre is of discovering its solution; of reaching its trno meaning, its hidden laws and causes. If i t is true lliat, i n each egg, all the functions and faculties of a dcfiliite mature allinla1 lic hidden, withont any corresponding organs, must we n o t regard heredity as a mystery too grc:it for solution; as something which must be acccptcd as it is without scientific explanation? Thirty years ago the adaptation of each organ of axi
12
IIeredity.
adult anim;il t o its proper purpose seenied to be a m:stery of the snnie hind. and ni:tny profouiid tliitiI\ers $:it isfied themselves and t:hiIglit otl1er3 that this adaptat ion iviis n o t broiiglit ahout by the laws of matter and 1)y secondary canses; t1i:it it must be accepted in itself, without explanation, and that the methods o f yhysical science are liere of no use. Darwin’s work has taught 11s that this is not trne; t h a t in the law of natnml Election we luivc a t least a p a r t i d cxplanation of the origin of the adaptiltioiis of nature; that while natural sclcction may n o t be the exclusivc means by ~vliichthey have been produced, it is, so far :is i t goes, :I true scientific explanation, for it cien pats i t iu our power t o produce, in domestic animals, similar adaptstions to special purposes, by the selectio~iof t h e fit test r a i h t ions. Darwin, in his first and in all his later bools on the snbjcet, pointcd out that liis discovcry did not complete the solntion of t h e problt3m; t h t ’ I natui*~il sclcctiori is ii great h i t not the exclusive means of motlification.” Tlic greatest valne of liis work lies in the yroof ivliicll he lias furnislicd, tlint tlic origin of the structure of animals is not be~oiidour rcacli, but that obscrwtion and reflection, tlie mcaiis ivliich 1ioi.e unlocked for 11s so many of the secrets of inorganic natnre, arc eqiuilly uscful in this fieltl; that tlic adaptations of nature nisy be studied and understood liltc a problcm in astronomy or physics. The aim of this work is to sliow that the eamc thing is trno of the problem of heredity. We may n o t be able. as yet. to pciietrdc its secrets to their inmost dcptlis, I ) a t I 11ol)et o d10w that observnt’ion and reflection do enable US to discoier sonit3 of the l a m upon which heredity depends, and do farnish us with at
What is Heredity?
13
least a partial solution of the problem; that we have every reason to hope that in time its hidden causes will all be made clear, and that its only mystery is that which it shares with all the phenomena of the universe. In this introductory statement we have presented one side of the problem of heredity: the transmission from parent to child of the established congenital hereditary characteristics of the race. We must not forget, though, that there is another aspect which is fully eqnal to this in importitnce. We know that each characteristic has been gradually acquired through a long series of modifications; that all thc wonclcrful adaptations which fit animals to their surroundings, and meet their particular needs, have been evolved step by step by the natnral selection of the fittest congenital variations. Each racecliaracteristic has at one timc been a new variation, and the process of modification is still going on and perfecting the harmony between the structure of each organism and its needs. No theory of heredity has any value unless it explains the may in which new features, which may become hereditary, continnally make their appearance as congenital variations, a t the same time that it accounts for the may in which established peculiarities are handed down from generation t o generation. The problem is two-sided; what is nom hereditary vas tit one time variation, and each new variation may soon be hereditary. Heredity and variation are opposite aspects of the same thing, and an explanation must be examined and tested on the one side, as well as on the utlier, before i t can be accepted. There is still another conoideration which remains to be noticed. Darrrin has never failed to perceive, and he has frequently pointed out,that the law of natural selection is not
14
Heredity.
complete explanation of the origin of species, and that it is exposed t o certain very serious difficulties.
zb
Still he concludes that the theory is supported by such a mass of evidence that we may fairly believe that our own knowledge, not natural selection, is a t fault, and that furtlier research will remove tlie difficulties by tlie discovery of otlier laws. Naturalists all over the world have acknowledged the justice of this claim, and some, less candid arid broadminded than Darwin, seem to have even lost sjglit of the difficulties. Kom natural selectiou can act only by the preserratiori of such variations as chalice to appear, and until we know the laws which govern the appearance of variations it miist be iinpossible to decide how far the course of OF ganic erolution has been determined by these uuliiiown laws, and how far by natural selection. We may tlicrefore entertain a rensonable hope, tliat when the true tlwory of heredity is disco\-ered, it mill, by revealing to 11s the laws aiid causes of variation, place the law of natural ,.election upon a firmer basis, and E ~ O W that its apparent diEEcnlties are simply due to the aarrow Iiniits of our knowlcdgc. K i t h this introduction I mill pass t o the discussion of our subject, tlie riatiire of heredity. The attempt t o generalize from the whole field of natural science is bcset with many difficulties, s ~ c the e field is so vast that an attempt to give in advance a statement of all tlie facts upon wliich rcasonii~gis based would simply confuse the mind of the render, arid burden liim with a mass of detail. I t seems best tlien to start with the generalizntions which are believed to bind the facts together, so that the reader may then approach the specific proofs with more
WLat is Heredity ?
16
interest. The latter method is open to objection, since the reader may bc called upon to listen to views which are opposed by accepted authorities, and to wait until the proofs are presentcd in due course. I must therefore request the reader to suspend judgment, and to lay aside established opinions, until he has examined the subject upon all sides. The examination of the history of the subject will furnish an introduction to its scientific discussion, and I have therefore adopted the following plan: 1 shall give, first, an outline of the chief hypotheses which have been proposed, from time to time, as an explanation of heredity, with reasons for rejecting them. I shall then present briefly, i n outline, a statement of what I believe to be the true explanation. I shall then try to show that this theory furnishes a basis or foundation for the theory of natural selection, and removes the most serious difficulties which have been urged against thc latter theory. I shall tlien show that there is no a priori reason for rejecting the theory of heredity; and that i t furnishes an explanation of many well-known facts which cannot without it be seen in their true relations. I shall then attempt to show that i t is supported by direct proof, and finally I shall give a statement of the theory in a more extended form.
CHAPTER 11. HISTORY OF THE THEORY OF HEREDITY.
Requisites of a theory of heredity.-Historical sketch of spcculation 011 heredity-Evolution hypothesis of Bonnet and 1131ler-Ovists and sperrnists-Modern embryological research 118s sliown that it is inipossible to accept the evolution liypothesis in its original form-Buff on’s speculations upon heredity fzil to account for variatien-Ilypotlicsis of epigenesis-This liypotliesis is logically incompletc-The analogy between pliylogeny and ontogeny gives uo real explanation of the properties of the ovum-IIaeckel’s plastidule hypothesis -This liypotliesis is not logically complete unless it involves tlie idea of evolution-Jager’s hSpotliesis--ultimate analysis sliows that this is at bottom an evolution hypothesis-No hypothesis of epigenesis is satisfactory-No escape from some form of the evolution hypothesis-This conclusion is accepted by Huxley.
9 1. Requisites of a theory of 7~eredity. The following list is a brief summary of what seem t o me the most important characteristics of the reproductive process in living things: 1. New organisms may be produced by tlie wrious forms of asexual generation and from ova. 2. Ova may develop, in certain cases, without fcrtilization. 3. As a rulc the ovum does not develop iiito a new organism until it has been fcrtilized by union with a malc cell. 4. The ovum and male cell will not unite unless t h y are derived from organisms with the same or nearly the same systematic affinities.
History of the Theory of Heredity.
17
5 . T h e new org;inisni, mliether produced sexually or asexnully, is essentially like its ancestors, although it m:Ly be quite different from its immediate ancestor, a s i n cases of alternation. 6. Orgiinisms prodnced from fertilized ova differ in t h e following points from those produced asexually: a. As a rule the development of the egg embryo is indirect, and a more or less complicated metamorphosis or alternation of generations must be passed tliroiigli before tlic adult form is reached, and t h e circuitous p a t h thus trarersed bears a resemb1:mce to the line of evo111tion of tlie species. An organism formed ascxmlly traverses o n l so ~ much of this path as remains to be traversed by the orgrnnism which gives birth to it. 6. Reversion, or the appearance of characteristics not exhibited by the parents, but inherited from remote ancestors, is not a t all unusnal in egg embryos, b a t it is niore rare in those prodnced asexually. c. New variations, or features which are not inherjtcd, appear con tinnally in organisms prodnccd from fez= tilized ova, and they may be transmitted either sesnnlly or asexually to future generations, thus bcconiing establislied as hereditary race-cliarncteristics. Hereditary variations are extremely rare in organisms produced asexually. 7 . Tlie ovnm and the male cell are homologous with each other, and are morphologically equivalent to the other cells of the organism. We must therefore believe tlint their distinctive properties ha1.e been grdiially acqnired, and that their specialization has been bronglit about by the action of the same laws as those in accordance with wliicli tlie other specializatjons of t h o organjsm have been produced. 8. Changed conditions do not act directly, b u t they cause subsequent generations to vary.
18
Heredity.
3. In the higher animals, where the sexes h a ~ clong been separated the male is more variable thaii the female. 10. The result of crossing is not the same wlien crosses are made reciprocally. 11. The sex of the parent-species affects the degree of variability of hybrids; arid when a hgbrid is used as the father, and either one of the pure parent-species, or a third species, as the mother, the offspring arc more wriable than when the same hybrid is used as the molhcr and either pure parent-species or the same tliird species as the fdher. There may pcrliaps be other requisites wliicli should be included in this list, but I think there can be no doubt that a theory of heredity must recognize and be in harmony mith all mliich are here given. $ 2. A sketch of the history of speculation on ihs timry of heredity. The laborions rcsearclies of the students of tlie science of embryology hare yielded a ricli harvest of valnable facts, and we now know that the process of cell division by rvliich an unspecialized unicellular egg becomes converted into a many-celled, highly-specialized organism bears the closest resemblance to the process of growth or of ordinary cell-mnltiplication. We know that all the various forms of reproduction, cell-multiplication, fission, gemmation, conjngation, sexual reproduction, and parthenogenesis, are inter-related in such a way that me must believe that they are different manifestations of the same power, and that they have been evolved one from the other. We know that direct development, metamorphosis, and alternation of generations are not separated from each other by any hard and fast line, and we know too that
Bistory of the Theory of Heredity.
19
~-
the changes through which the embryo passes on its road from the egg to maturity show a wonderful parallelism to the series of changes tlirough which tlie organism has passed during the liistory of its evolution from lower forms. These results are well worth the labor they have cost, and they illustrate, more clearly than any otlier facts in biology, the common naturc of 311 living things. They do not, however, contribute directly to a clearer insight into the lams of heredity. IIere we are still compelled to go beyond the visible plienomena, and to attempt by tlie scientific w e of the imagination to discover the as yet unseen relations d i i c h bind them together. As me.entcr upon this snbject it will be well to bear in mind tlie wide diffcrencc between the end we lime in vicnr-the discovery of the secondary laws of heredityand llie attcrnpt t o undcrstand its ultimate cause. The power to reproduce itself, to imprcss upon dead inorgmic niattcr its own distinctive properties, is oiie of the fund:imcntal cliaracteristics of living matter; and mliile im may hope that increase of knowlcdge may some day enable us to trace the origin of this power, such an attempt forms no part of our present undertaking. We shall accept without explnnation the fact that living matter does thus reproduce itself, and we shall coniinc oursclvcs to the attempt to discover why tlie egg of a star-fish for instance, reproduces a stnr-fish, and the egg of a bee a bee; to discover the origin of the differenccs betmecn the various forms of reproduction, rather than the cause of what they have in common. Tho plicnomena of heredity in the higher animals, as well as the niechpiism of ova and male cells through which these phenomena are manifested, hwe certainly
20
Neredity.
been produced by slow modification, throngh the inflnenc2 of conditioris which arc to a great extent open t o study. ‘I’hc attempt to trace tlieir origin and significiincc is not a pure speculation, but a legitiniate exercise for the scicntific intellect. As we should expect from the fascinating nature of the subject, there has been no lack of speculation in tlic past, and various hypotheses have been proposed from time to time t o account for the phenomemi of heredity. These hypo theses diEer greatly aniong themselves, but they may be rouglily classed as epigeiicsis ligpotlieses, and evolution hypothesis: the word evolution being hcre used, of course, in its old sense, as contrasted with epigeucsis. The liypotliesis of evolution, pnre and simple, as advocated by Bonuet and Ballcr, is that thcrc is contained in the egg or seed or in tlie male element a perfect but minutc orgmi3in, and tliat tlic subsequent development of the cgg is simply tlic “evolntion” or unfolding of this germ. Up to tlic end of the last century the prevailing opinion was that each egg contains, in alatcnt or dorinant state, a completcly formed o ~ p n i s n i . The fertilization of the egg was supposed to awaken this dormant germ, t o call its latent potential life into activity; and the process of devclopmcnt was regarded as the iinfolding and growth of the already fully formed and perfect embryo. The embryo m:is held to be not prodnced by, but simply anfolded from the egg, and tlie act of reproduction was therefore regarded as eduction riot pi*odiiction. According t o Hnxley (Eneyc. Brit., Art. Evolution) “Boiinet affirms that before fecnndstion the hen’s rgg contains an excessidy minutc but complete chick, and tliat fecundation and incubtition simply cause this germ t o absorb nutritions matters, which are deposited in the Snterstices of the elementary structure of which the min-
IZstory of the Theory of Heredity.
21
i.iturc chick or germ is made np. The consequence of tiiis intiissusceptive growth is the “ devclopnient” or “evoliitioii” of this germ into tlie visible bird. Tli~is nil orgnnizcd individual “is a composite body consisting of tlie original or elcmentary parts, and of tlie rnattcrs wliicli have been associated with tlietn by the aid of nutrition,” so t h a t if tliese matters could be extracted from the individual, it monld, so to spealc, become c b x n t r a ted in a point, and wonld thus be restored t o its prirnitivc condition of a germ “fjnst as b y extracting from a bone the c;ilcarcoud substance mliicli is tlie source of its hardness it is reduced to its primitive state of gristle and mcmbrane.” cLEuolzitio~zand develnpmeitt are, for Bonnet, synonymous terms; and since, by evolution hc means simply the expansion of t h a t which was invisible into visibilitv, lie was natnr,:lly led t o the conclusion, at wliicli Lcibnitz liad arrived by a different line of reasoning, that no such thing as generation exists in nature. Tlic growth of a n organism bcing simply a process of enlargement, as a particle of dry gelatine may swell iip by the intussnsccption of miter, its death is a shrinkage, such as the nielted jelly niiglit undergo on desiccation.” M,~nchmore anciently the erolution hypothesis found acccptance i n a somewhat different form. and the rniniature organism W:LS believed to exist in t h e male element, a i d t o receive from t h e cgy t h e nonrishmenb needed for its growth and perfect dcvelopmcnt. Leeuivenhoek’s discovery of the motile spcrnmtozoa of animals was reg:irdcd as a new basis for this view, anJ the “ sperrIi-aiiiniiilcule ” w:is held to be the perfect and living animal ready for unfolding or evolution, tlie term “sperniatozoon,” still retained in scientific nomenclature, being a remnant of this old hypothesis. Leeuwenhoek’s
22
€leered ity.
discovery inaugurated, in tlie first lialf of the last ccntn-
ry, the warm dispute between the Aninidculists and the Ovists, one side holding that the germ is contained in the egg, and the other that it exists as tlie seminal animalcule. It is obvious that, in either form, the evolution theory, as above stated, is logically incomplete, siiice it only accountsfor a single generation. Its advocates mere therefore compelled to enlarge it, and to assume that, as each organism thus exists, in a perfect form, in the prcccding generation, each germ must contain, on a still smaller scale, the perfect germs of a11 subsequent generations. Thus Bonnet held, in his hypothesis of eniboitement, "that a11 living things proceed from pre-existing germs, and that these contain, one inclosed within tlie other, the germs of all future Iiving things; that nothing really new is prodnced in the living world, but that the germs which develop have existed since the beginning of things." (Huxlcy, Evolution.) The advocates of thc evolntion hypothesis appealed to such facts n5 the presence Qf u minute plant inside the acorn, or to the butterfly inside the pupa-skin, in srrpport of their views; but the hypotliesis, i n its crude state, was quickly overthrown by the first discoveries of modern microscopic embryology. Harvey's studies on the development of the chick, followed by the researches of Wolff, Pander, Von Baer, and thehost of embryologists of the l a t fifty years, show concInPiveIy that the embryo is not nnfolded out of, but gradnally built np from the egg. We now know that the eggs of all animals, mhen t h y are not complicated by the presciicc of food, or of peculiar coverings for protection or attachment, are essentially alike in optical structure, and that they are not only like
History of the Theory of Heredity.
23
each other, but like tlie constituent cells of ail parts of the body of the organism. Fiir from being preformed in tlie egg, we know that the body is built up gradudly, step by step, by repeated cell-division, and that tlie earlier stages of development do not result in tlie formation of the parts of the perfect body at all, but that they simply give rise to germ-layers, or tracts of cells out of which organs’ are gradually formed, and that cells which were a t first quite widely separated in the embryo may come at last to enter into tlie formation of a single organ. For instance, wlien tlie nervous system of a vertebrate first makes its appearance in the embryo, there are no traces of the brain, of the spinal cord, of the nerves or of sense organs. It at first consists of a long group of cells running along themiddle line of the body, and presenting no difference from the otlier cells of its surface. I n most cases this elongated group of cells bccornes converted into a furrow, and afterwards into a closed tube, tlie nerve-tube, by tlie folding together of its edges. Tlie primitive nerve tube is at first simply a long tube of embryonic cells running along tlie middle line of tlie back, and it is a veIj different thing from tlie find nervous system of an adult mammal, nor is it in any sense a mammaliua nervous system in miniature, for tlie changes by wliicli i t becomes converted into the latter are great arid numerous, as well as gradual. Certain parts, such as the eye, are fornied only in part from this tract of ce!ls, for tlie vertebrate eye is the result of the combination of an outgrowth from tlie embryonic nervo:is system and an ingrowth from the surface of the head. The whole history of tlie nervous system and sense organs is thus seen to directly oppose tlie view that these organs are present in miniature in the germ.
24
Heredity.
Still more opposed t o the hypothesis of evolution is tlie remarliable fact tliat tlie clianges which take place i n tlie derelopingegg are not sucli :ts would lcad directly to the formation of the adult animal. I n most c a m a circuitous or indirect path is followed, a i d this indirect path leads at first towards tlie adnlt form of lower members of tlie group. This, the most suggestive fact of modern embryology, may perliaps be made clearer by an illnstr at’ion. Let us try to c o m p r c the growth of an egg into an adult animal with tlic growth of some manufactured prodiict in the 1iaiids of its m‘‘I 1-Ier. Tlie evolutionist I iew of the clewlopment of an organism may be illustrated by tlie tnannfactnre of a yarn base-ball. A boy, wishing to makc n y a m ball, procures, if possible, a small rnbber ball, and winds his yarn on to this until tlie desired size is reached, tlie only clianges during tlic growth of the ball being the change of size and of material. The observed facts of embryology show that tlie developmcnt of an. enibryo does not take p1:ice in any sucli vay as this. It may, liomrcr, be illnetrated by the growth of a ste:im-sliip in tlic hands of the builder, wlio first lays down an indefinite skclcton, arid outlines i n a vague my the niorc prominent featiires, before any of t h e details are finished. 111order to make tlie illustration perfect, howerer, n-c must imngine the builder to commetice work upon liis steam-ship by 1;ij ing oiit tlie skeleton of a big triareme; me must imaginehim tocnrry this some stiiges towards completion, and to p u t into it certain contrirauccs, snch as i~owers’-benches, ~vliicliare of 110 nse i n a steam-ship. We must imngiiie that lie then abanclons liis plan, tcars down his benclies, a11d nEes tlrc material to mukc a deck; that he cliaiiges 11ic shape and
IIistory of the 7’JLeory of Heredity.
25
proportions of his Iiull a little to fit i t for sailing instead of rowing, tliat he piits in masts and spars, and makes everything rcndy for a sliip’s rigging; that he then changes the shape of liis hull once more; tears out part of liis cabiil, puts i n bnlklieads, coal bunkers, and an engine and boiler ; shortens 111s masts, alters his rigging, and fiunlly converts liis unfiiiished ship into a finished stcamer. This is not by any means a forced illnstration, bnt a very fair outline of tlic dc~elopmcntof an animal. In nearly every case TVC Gild that the development of tlic embryo as :I,n~hole,or else the d e d o p m e n t of certain organs, takes place in this ronndabout, indirect way, and repeats, nsnally in :in imperfect manner, the structnrc of a related but lower animal. As an example, we may refer to tlic history of the bIood-vesscIs of a m:inimnl. The breathing organs of tlic lower vertebrates arc gills on the sides of the neck, and tlic venous blood is driven from the heart through a scrics of branchid srtcries to the gills, where it is aerated and conveyed into a scries of brancliial veins which carry it, not back to the heart, but to the various organs of the body. In a mammal thercare no traces of gills at any stage of development; the adult animal brentlies by lungs, and tlic blood which has been aerated in the lungs goes back to the heart; before it is distributed throughout the body. Now the early stages in the d e v c l o p c n t of the bloodvcsscls of a mammal would, if carried out to completion, Icad to thc formation of the system found in fishes. The manimalian embryo has no gills, but it does have bmncliial arteries and veins, and its blood at first follows tllc sainc conrsc tlmt i t follows in a fish. I t is plain h a t the fishlike circulation is not an outline or sketch Df that of a niammal; that i t is not R ncccssary stage in
26
Heredity.
the formation of the latter, for tbe branchial vesscls are soon, in part pulled down and destroyed, and in part profoundly modified, in order to conform to the mammalian type. Cases of this kind are almost univcr~nl,and the law of resemblance between the early stages of higher mimnls and the adult condition of lower animals is a fundamental law of embryology. I t is obvious that the hypothesis of evolution of a ~ m * fectly formed germ contained in thc egg, is utterly irreconcilable with this lam, and me may therefore state with confidence that this hg'pothesis is refuted by the observed facts of embryology. We must not forget, hon-erer, that there were other less superficial forms of the evolution hypothcsjs, and that these cannot be d i s p r o ~ dso easily. Buffon, for jnstance, held that the embryo is built up by the union of organic particles mhicli are given off from every part of the body of the piirent, and which, assembling in the scxnal ~ e r r c t i o i i assume ~, in tlie body of the offspring positions like those wlricl~they occupied in the parent. This is e~sentiallyan evolntion hg'potliesis, bnt it is logically complete, since it acconntsfor the prodiiction of siiccessive generations witliout the ncccssity for assuming that they were all contained in cmbryo i n thc body of a reniote ancestor. Microscopic exnmination cannot orerthroiv this hypothesk, for a f;tilnre to diseowr these organic p"rtic1es -with an? payiicular mngnifying power does not, of coiirse, dispro~etheir existcncc any more than a failrire to see them without a microscope. Although Bnffon's h ~ p o t h e s i sdoes n o t nccount for the fact that derelopment is indirect in most cases, t h a t the egg does not build up the adult aninial in the simplcst way, but takes a roundabout circuit, this fact is 112t
History of the Theory of Heredity.
27
directly opposed to liis liypothesis, for .we can easily conceive that after an indirect method of deve1opme:it Ii:is been establ;~l~ed i t might be perpetuated by E~iffm’s orgnuic niolectil~,provided tliese are given off by the p r c n t organism at all stages of its life, and not simply after it lias reaclied its final form. Tlicre is, however, another class of phenomena of even greater importance-the 1)lienomcna of variation. Buffon’s liypotlicsis accoimts for the rebemblaiice between tlic cliild and tlie parents, but we now know that tlic cliild is not exactly like its parents or eyen inidway between tliem, tliat nnimds aiid plants are born with tl tendency to vary, that this variation may affect any part of tlic body, and tliat by tlie selection of tliese congenital varistions the most profound changes of hereditary strusture may be produced. Tlie fact of congenital variation is as profound, as universal, and as cliaracteristic of living things as tlie fact of heredity, and tlie constant appearance of new variations is as fatal to B~iffoii~s liypotliesis of evolution as it is to tlint of Bonnet. With tlie growth of tlie modern science of morphology tliese liypotlieses have been abandoned arid tlie liypothesis of epigenesis almost universally accepted in their place. This hypothesis, first bronght into notice by the researches of Harvey and Wolff on tlie development of tlie chick, lias gradually assumed a more definite shape with the progress of embryology, and has been especially mod~ficdby the growth of the cell theory. In its modern form i t may, for convenience of discussion, be divided into two parts-a statement of the observed facts, and an explanation of the origin of the phenomena.
28
Her edit y.
So far as it is a statement of facts, it cannot be called an hypothesis, for it simply affirms that t h e rgg is optically an ordinary unspecialized cell; that i t gives rise, during tlie process of segmentation, in q miinircr vliicli is identical with ordinary groivtli by cell division, to a number of cells mliicli gradually become specialized for ccrtain functions, and are set apart as tlie foundations of tlic various organs of the body; that the repetition of this process gives riae, a t last, to the perfect body of the mature animal; that the reproductive elements which are to give rise to the nest generation, originate, like d.11the cells of the body, by cell division during the process of development, and that they are simply cells specialized for’ tlie reproductive function as other cellsare specialized for other functions. Every one who has the slightest acquaintance with modern biology will accept this statement, not as an hypothesis, but as an obserwd fact, and will agree that between this and tlie old evolution hypothcsis there can be but one choice. The old liypotliesis of evolution, lionww, c1:iimccl to be sometliing more than a statcmcnt of fact, for the presence of the germ within the cgg accounted for tlie wonderful properties of the egg itself. We are at once compelled to ask, then, horn, on the hypothesis of epigcnesis, has the egg acquired these properties ?‘ If it is simldy an nnspecialized ccll; if, as cfegenbaucr states, “ the egg is notliiiis more nor less thim a cell; tlie egg-cell does not diffcr from other cells in any essential points” (Conip. Anat., Bcll’q Trans., 13. 18), how can tlie cgg of a horse develop into a horsc, wliilc another cell, mliich “does n o t differ from i t in any cssclltial points,” develops into a bee or an alligator or an oyster ? Nothing in nature is more marvellous than the devel-
History of the Theory of Neredity.
29
opmcnt of each egg into its proper organism, and if i t is true tliat tlie egg which is to give rise to a man differs in no essential point from that wliicli is togive rise to :in insect, mc m iy conclude tliat the mystery is too great to be fnthomcd by oiir intelligence, and we may fairly ask what possi blc explanation can, on this hypothesis, bc given of tlic wondcrful properties of tlic egg. The answer wliicli has been given, and wliicli seetna t o linve been tlionglit satisfactory by many students, is tliis: We know, from a mass of evidence mliicli is constantly and rapidly increasing, and to wliicli each new observation adds cumnlntire weight, tIiat the various forms of life liave been slowly evolved, during long ages, from older and simpler forms; tliat as me trace back tlie history of any two animals or plants we find eridence tliat in tlic past tliey liad for a common ancestor a species mliicli liad iiot yet acquired tlic distinctive fenturcs of either of them; that a little fiirtlier back we trace tliis s p c i e s to :m :tiicestor with still midcr relationships. Every day tlie evidence grows strongcr to sliow that morc complete knowledgc will id timately provc that tlie same tliing is true of still larger groups ; tliat families, classes and ordcrs of organisms I i a ~ ebeen formed in the samc wiry by gradual modification and divergence ; tliat complctc liiiowledge of tlic ancestry of any organism would Icad us back tliroiigli simpler and simpler forms to a remote unspecialized nnicelliilar ancestral form. It is unnccessary to review in this place the evidence for this conclnsion, for t h e fact that i t is fully acccptcd by those best qiialified to judge of its truth, is perfectly familiar to all students. Now it is said, and tlie explanation is pretty generally accepted, t h a t since any particular organism, a horse for
30 Instance, lias been slowly erolred from an nncestral rliizopod, and siiicc tlie ovum of a liorsc is Iiomologous with a rliizopod, or is morpliologically eqiiirulent to it, me have iu tlic gradual phylogeiietic erolutioii of tlie horsc species from an unicellular ancestor, a satisfactory cxplanation of tlie ontogcnetic derelopnient of tlie individtial liorse from an unicellular ovum. As soon as atteiition is fairly fixed upon tlie subject, tlie meakriess of this cxplaiintion bccoines so evident that I take tlie liberty of m:iliing the follorviiig qnotation from a well-known antlioritp, in order to sliow that the explanation has been soberly advanced. I n making tlic extract from Haeclicl’s writings I am not actuated by a desire to attack liis views, for tlie same idea can be found, expressed pretty definitely, in the works of many other writers, and tliis particular selection is simply a matter of convenience. 1i:ieckel says : ‘‘ Uiitil reccutly the greatest students of embryology, Wolff, Baer, Reniack, Schleidcn and tlie whole school of embryology founded by tlieni, have regarded tlie science as cxclusively the study of individual development. Far otlierwise to-day, ~5 lien tlic mysteries of the wonderfnl history of tho development of indiriclual organisms no longer face 11s as an iiicompreliensil~le riddlc, hut Iiave clcarly revealed their deep sigiiificancc : for the clianges of form which tlie germ p s s e s tlirongh under oar eyes in a short time are, by tlie lam of inheritance, a condensed and shortened repetition of tlie corrcspondiiig changes of form mliicli the ancestors of tlic organism in question have lmsed through in tlie coiirse of many million years. To-day, wlien me lay a hen’s egg in an incubator, and in twenty-one days see t h e cliick break out of it, we no longer gaze in dumb wonder on the marvellous changes which lead from the simple egg
IZistory of the T h o r p of IIeredity.
31
to the two-layered gastrula, : from tliis to tlie worm-like and skulless gcrm, and from this to later stages wliicli rcpcat, csseiitially, tlie organization of fish, ampliibiun, reptile, until a t last we liave a pcrfcct bird. On the contrary, me unravel from this history the corresponding series of ancestral forms, which linve led u p through tlic a m a h , tlic gastraa, the worms, tlie acrania, t h e fiJics, the ampliibia and tlic reptiles to t!ic bird. ‘‘ Tlic scrjcs of c1i:~ngcs i n tlie llrii’s cgg gives u s a n ontlinc s!cetcli of tlie series of ancestors. This ancestra2 oi*pJi!jlogeiictics i ~ ~ ~ i ~ f i c?f n rthe ~ cplieiioineua c of ontogeny or iiitliridzud decelopnieizt i s up t o the present time the only explaiiatiou, of the latter.” (“ Gcsanirnelte Pupulsire V o ~ t r i i g e , I[., ~ ~ 1’. 103.) ‘‘ Any one who ‘accepts tlie law tlint individual dewlopmcnt is a rccapitnlation of the evolution of tile species toill find it simp1y siatwal tlint tlie microcosm of tlic ontogenetic cell-tree should be tlie dimiuntivc, and i n part distorted, reflection of the tnacrocosni of the pliylogcnetic gcnca!ogical tree of tlie species.’! (“ Gesammelte Populiirc Vortrige,” II., p. CS.) No one can set too liigli a d u e upon the scientific law liere expressed-that individual dcvclopment is a recapitulation of the history of the evolution of the species. It must be regurded a3 one of tlic greatest generalizations of modern science, bot I do not think i t is possible to agree with IIaeckeI that with its discovery tlie mystery of individnal dcvclopment lias clearly revealed its deep significance, and iio longer faces us as a riddle. I t may be trnc tliat i t is “ simply natiiriil” t h a t t h e egg of a horse should recapitidate tlie ancestral history of horses, and tlic cgg of a bird the ancestral history of birds, but the statement tliat this is tlie case is in no sense an explanation of heredity. For that matter it is
Her edity . “ simply natural ” that a bird’s egg should give rise to a bird, and a horse-ovum to a horse, but n o one mould accept the statement as an eqdanation. We have in the natural selection of variations a true ezpZnnatiol8 of the manner in which an unicellular rhizopod lias bceii slowly and gradually modified by an almost iiifini te number of slight clianges, extending through countless millions of generations, into a bird. The clinngc is one of the most monderful of tlie phenomena of nature, but i t is in no sense a mystery, for the skill of the breeder may even now, by the employment of tlie same means, produce similar results, only on a much smaller scale ; by tlie nietliodical selection of congenital variations an organisni may be, in a few gctierntions, sliglitlg modified in atiy desired direction, aiid we can fairly and truly a E r m that we understand tlic evolution of birds from their aniceIlular aiicestors ; biit the resemblance between the evolution of birds from tliesc remote ancestors by natural selection, and tlic derelopnierit of an individid bird from an uniccllulnr ovum, is simply an analogy. I t is true that i t is an aimlogy of the grentest significance, but we must not lose sight of the fact that the nicaiis by which the end is acconil~lisheci-the natural selection, through a long series of gcncrations, of congenital variations-is absent i n the second case. If the epigenesis hypothesis is true, if the egg is simply, like the rliizopod, an unslmializecl cell ; if the egg of a bird docs not differ from tlie egg of a star-fish in any essential points, we rnnst aclriioivletlgc that the mystery of individual derclolinicnt is iiot only as yet unsolved, biit absolutely insoluble. The student at the sea-shore may collect at the siirface, with his dip-nct, t h e e similar traiisparent splierical eggs. Each of these is, optically, simply a nucleated
Nistory of the Theory of Heredity.
33
cell, and each mlicn placed undcr tlie microscope will 60011 bc seen to pass through almost exnctly the same cliangcs, giving rise by division to a spherical h j e r of cells. Yet i1 these three eggs are placed together in a tnmblcr of water and exposed t o identical coiiditions, one may a t last become a star-fish, anotlier a crustaccau, and anotlicr a vertebrate. Similar tliings tinder similar conditions cannot give rise to widcly diffcrent results, and there seems no escape from tlie conclusion that tliesc threc eggs are not similar, or eTcn cssentially alike, but tliat one of tlicm is a potcntial star-fish, anotlicr a potential crnstaccan, and a third tl potential vertebrate. Tlitit tlicrc is in cach of them tl something which scparatcs it very widcly from the otlier two, and determines its future histoiy. ‘i’lie liypotlicsis of epigcncsis proves, then, on careful annlysis to be as unsatisfactory as the speculations of Bonnet and Bnffon, and mc must acknowledge tliat we are as yet unablc to picture to onrselves the hiddcu significance of tlie plieiioniena of individual dcvelopmcnt, witliout returning to sonic modification of the old evolution hypothesis. The attempt to escape this necessity, 2nd. to hold fast to the hypothesis of epigenesis, lias given rise witliin recent years to much* ingenions speculation, and an examination of some of tlie published papers will help, rather tlian retard, our argnment. Among tlicse, o m of the most jiigcnions and snggcstivc is IIacckel’s paper, “ Ueber die Wellcnzciignng der Lebenstlieilclien oder die Pwigcnesis dcr P1:tstidulc.” Tlie following extract ( I ‘ Gcsammclte PopulBre Vortrigc,” II., pi?. 66-72) mill, I hope, give a sufficiently clear statement of his views: “ I n order to penetrate still farther into t h e mechan-
34
Heredity.
ics of the bingenetic process, we must descend into the deep obscurity of pl~~stid-life, and search for its trne efficient cause in tlic motion of organic molecules (Plustidule-BeweSrutiS). “ I n fine, this question remzjns ta be anstrered, Are we in a position, by tbe aid of coniparison mitli annlogous plienoniena, of motion, to form a satisfactory provisional hypotliesis regarding the trne nature of the plastidiile motions wliicli are hidden from onr direct observation? Oar hypothesis of pcrigenesis is an attempt to answer this question in tlie affirmative. “As we reviem, from tlic IiigIiest and most cornprehensi-rc point of view, the sum of tbc phnomcna of organic der;elopmeut, the most general resnlt of our survey is the conclusion that t h e biogenetic process is a periodic motion, which we can best pictore to on;.selves as P wave motion. Adliering a t first to facts which are beyond dispute, and which admit of direct observation, we may commence witli our own ancestry: either confining ourselves to the so-called historic period, in wliicli we can pass from man to man by direct proof; or else following, by tlio methods of anthropogeny, our ancestry still further back, through the Tertebrates to amphioxus, and tlirougli the group of invertebrates to the gastrm, and nt last to the amceba and the moner. I n either case the course of development (entwickelungsbemegung) of onr series of ancestors can be most simply represented by a wave-line, in whicli tlie individanl life of each orgauisnz answers to a single wave. ‘‘ If now me enlarge our field of view t o embrace not simply our own direct ancestry but the whole of our blood-relations, we can make clear by a genealogicul tree their relatioiisliip to each ohher. As the history of the
IILstory of the Theory of Heredity.
35
cT-olution of enell person is represented by a wave-line, the entire tree will have the form of a branched mavemotion, Q ramified undulation. “ A nntural system of classification is notIiing but a gcncalogieal tree of allied species of organisms, a n d each branch a n d twig of tlie tree corresponds to a greater or smaller group of descendants from a common ancestral form. ‘f’his commuidty of descent unites all the forms of a class, an order, and so on. Since each class is divided into various orders, each order into several fitniilies, each family again into various genera, each genus into a number of species a n d l-arjeties, there is a similar brancliing i n tLe wave-motion wliicli is carried from the eoniiiion aceestral form to tlie entire group of its descendants; and each iindulating branch iniplanCs i n the same way its individual motion on its various descendants. “Now the fundamental lam of embryology jeaciies us that this liistory of t h e phylogenetic evolution of organisms is mirrored in miniature in tlie ontogenetic development of each individual. Here the single waves answer to tlic life of the constituent plastids (cytodes a n d cells). T h e cytiila, or tlie first segmentation cell mliicli originates from tlie fertilized egg, and out of wliicli the muny-celled organism is developed, bears the same relation to tlie various cell-generations which originate from i t by division, and which give rise later by specialization of function to the vnrions tissues, that the stcmform of a class or order bears to tlie various families, genera and species whieli diverge from it, and wliieli have been diffcreiitly evolved through adaptation to diversified conditions of existence. ‘“I!lie ontogenetic ‘cell-tree’ of the former bas exactly the same form as the phylogenetic ‘species-tree’ of t h c
...
36
EIeredity.
lattcr. The developing impulse which in tlie one case is trsnsfcrrcd from the ancestral species to t h e entirc group of species, and in tlic other case from the anccstral cell to tlic cntirc group of cells, assumes in both cases tlic same forin of a branching wave-motion. Any one wlio accepts the fundamental law of d e v c l o p c n t Trill find i t oiily natural that the microcosm of the ontogenetic ‘ cell-tree’ should be a diminution, and to some degree distorted reflection of the pliylogenetic ‘speciestrce. ’ “As we can only explain and render intelligible a complicated atid obscurc plienorncnon by dividing i t into its separate elements, and by tlic exact analysis of theso p u t s , so i t is necess:iry to licnetratc to t h e nltimate elemcntary f : t t s of our nicclianical tlicory of developlllcn t. “ Now tlic biogcncfc process as a whole is the liiglily compouncl resnltant of the developmental history of cll species of- organisms. These consist :(gain of tLe life histories of the individuals, just as tlic latter are again made up of the histories of thc constituent ldustids. “ T h e development of each plastid, howcvcr, is in its t u r n only the prodnct of tlie active rnovemcyts of its constitocnt plastidules. Now we 1iaT-e seen that the devclopmcntal impulse of the branches and classes, tlic orclcrs and families, the genera and epccics, the individuals and plastids, always and everywhere has for its fnnd i m m t a l characteristic the branched navc-motion. Accordingly tlic molecular plnstidulc-motion, which lies a t tlic bottom of rll tlie plienomena of life, can liave no other form. Wc must conclndc tliat tliis ultimatc cause of all tlic phrnomcna of lifc, tliat the invisible activity of t h c organic molccnlcs is a branclicd wavc-motion. This true and ultimate cazisa ejicieus of the biogenetic
History of the Theory of Heredity.
37
process I propose t o designate by a single mord-Perigeiiesis, tlic periodic wave-generation of the organic molccnles or plastidules. " This mcclianical hypothesis is a true cxplmation of tlic proces3 of orgmiic development. . " The designation of this branched mare-motion of the plastidule by the word perigenesis or wave generation serws to emphasize the distinctive characteristic which separates this 6 m n c h e d motion from all similar periodic phenomena. Tliis peculiarity depends upon tlic reproductive power of the plastidule, and this again is brought about by its peculiar atomic composition. This power of reproduction mliicli alone renders possible tlic multiplication of the plastids is, however, the eqnivalcnt of the memory (Gediichtness) of the plastidule. " This brings us tQ Ewalii Hering's ably established view that unconscious memory is the most importnnt clinracteristic of organized matter, or more properly of thc organizing plastidnles. Memory is the chicf factor in tlic process of dcvclopmcn t of organisms. Through tlic memory of tlie plastidnles the plasson has the power t o carry over from generntion to generation by inheritance, in continuous periodic motion, its characteristic peculiarities, and to add to these the new experiences which the plastidnles have acquired through adaptation in the course of their evolntion. "I have shown that each organic form is the necessary product of two mechanical factors-an inner factor, heredity, and an outer factor, variability, or a powcr of ada13t nt i on. " By tlie hypothesis of periqenesis me are able to more sharply define these two fnndamental lmvs of the modification of organisms, for heredity is the memory of the plastidules: viiriaBility tlreir power of perception (Die
. .
3s Erbliclikcit ist (Ins (>ciliiclitniss dcr Plnstitliilc, die TT3riabilitiit is die F’:tssltngslir;tft tler l’1;isticltilc). ! h e onc brings about thc const:tiicy xiid tlic other tlie diversity of orgnnic foriiis. In t h very sinil.de :md pcrsisknt forms of life tlic plastidnles li:tve, EO to y i e d i , 1c:iriicd nothing :11id forgotLen nothing. I n Iiiglily perfected and variable orgi~iiismsthe plastidulcs 1i:~veLoth 1e:iriied and forgot ten m ach ” This sonicnliiit long qnotation coiitnins n tliorongh and csliuiistivc statement of the Iiwigenesis Iiy~otlirsis, and it. is tliei-cforo interesting to irotioe that its only rral claim to rccngiiitioii : I S :I true c,rplmdioib of the ])lieno men:^ of lieredit? is I)iised upon or a t IC:ISL dcmaiitls the acccptiince of some forin of tlic evolution Ii~-potltesis. 1Iowever p e n t niny be tlic im1mrt:iiice of the anallogy between the g r a d i d erolntion r.)f tho species liy tlie specialization of t . 1 1 ~consiitr1c:it iiidivi~I~i:~ls, arid tlic develol)ment of the individu:tl l i tllc ~ spcci:tliz:ition of cclle, and plnstidiilee, we II:LVC itlreiitlp pointed oii t iliatr i t is in no sense nu es1)l:in:ition of tlic latter, Eiiice the r e d cause of tlic evolution of the species, tlie selection of congmitul ~ i w i : i t i o ~ i sis, nlmnt. !l’lie on?? p r t of 1T;icckcl’s liypotlicsis of perigenesis which 1i:u :my clilirn to be considered nn explanation of t,he rcprodnctivc power of :~iiinials,is tlic stnt.emcnt t l ~ t heredity is memory, n i i d wri:ibility the :tcquisitim of new experiences. Slated by itself, without expl:iti:~tion, this nia! seem to tltose who are nnf:umili:ir wj111 tlIe siihject F C ~ J : nincli like nonsense, for the pr(ilount1 truth upcm which it wats is not :it all obviorrs :It first sight. IIerbci-t Sprnccr Im, i n his m:istcrly tlisrnssim of the natnrc 2nd diatinctivc c l i : ~ r : \ c t ~ r i ~ of t i c life, given 118, as tlic sum and substaiicc of his analysis, the statcment
.
History of th.e Theory of IJeredity.
39
t l i ; ~ t“life is the continuous adjustment between intern;d reli~tionsand. esteriid rc1:Ltions.” ‘lliis, like 1I:ieckel’s st:ttenient, that 1ieredit.y is mcmoiy, is not very clcar willio\it e q ~ h i i : & m ,but its nie:uiing may perliays be broiiglit out by ail illnstixtioii. If 1 kick :istone I prodiiee in it ccrtnin oliaiiges, siicli as motion, hciit, ctc.; tliese cliariges being directly prodiiccci by tlic kick are siinplv manifestations of ilie eiicrgy traneferrcd from my foot to the s t m e . If, instead of :I stone, I kick a clog, I produce :Lsimilar set of cliunges, niid sonietliing more. Ylic cxpcrieiice of tlic dog*aiicl of his nnccstoibs lias tanglit hiin t1i:it siicli violent ntt:icks :ire : t l w ; i y nssociiitcd mitli :t rl isposition t o commit still fartlicr violencc, SO, when the dog feels tlic blow lie iniincdiately performs actions -wliicli have its tlicir object, esc;q)c from or nroi~liuiceof the danger mliicli lie 11as not p t cspcricncccl, b u t wliicl~lie knows to bc imminent. Tlicsc actions :we not t h e cffoct of tlic liiclc, for tlic cncrgy expencled m:ig bc linnclrcds of times gmiter. Their c1iar:ictcr is ilc:crmincd, riot IJJ any cli:iiigc in tlie dog, but by tlic character, tlic tlisposition, w\\ich he lins iiilicbritei\; uiid x\ieiher he retaliates by :in :tttnck on his o \ v n lxirt, piits Iiis tail b e t w e n his legs :ind riiiis, or cronclics at niy feet, his actions :ire tlic effect, riot of tlic kick, but of p s t experience as to tlie best mcans of cscn.ping further iiijury. !Tl~ercis n relation, cxtcninl to the dog, bctwc.cn t h e kick arid 5 disposition t o iiijiire tlic dog, and tlrcrc is within the clog a rc1:ttiou between the scnsntion of injury :mtl the actions vliicli e s l m % w x lins shown t o be tlie proper ones for cscnping fnrtlier iii,jnry. T h ; vliich cliatitigiiishcs tlic dog from tlie stone is t h e power t o adjust these internal relations t o the esternal relations, to conform his coiiduct to the lams of
40
Heredity.
the world around him. The dog, as a living thing, differs from all inorganic bodies, in his power to nia1;c tliis adjustment: so long as lie retains this power he lives; liis life is a (‘continuous adjustment between internal rela tioiis and extcrnnl relations.” I t is plain that this power depeiids npoii experience, but experience dcpends upon “ memory.” So we may state, witli truth, that in a certain sense, life is memory; and as tlic power to reproduce its like is c1i:iractcristic of all living tliings, we see that there is in €Iaeckel’s Etaternelit a profound
t 1’11t 11. We know memory, hornever, only in conncetion n i th organization, and i f i t is true that heredity, the power of an organism to reproduce its like, is simply the memory, by the ovum, of the expcrience of its ancestors, we must believe that therc csists in the o w m an organization of some kind to correspond to each of tlicsc past experiences. We are therefore driven by thc liypothesis of perigenesis back from tlic hypothesis of epigenesis to some form of tlic old evolution hypothesis, for TC caunot conceive that cornplicatcd experiences should exist without complicated structure. We are thus compelled to conclude that, while it 1111doubtcdly expresses a great truth, IIaeckel’s hypotlicsis of perigenesis is not a satisfactory aiid final exl)lanation of the phenomena of reproduction. A satisfactory theory of heredity must explain what i t is, in the strncture and organization of tlie ovum, which determines that each ovam should produce its proper organism. To statc that this organization can be expressed in ternis of memory, is. simply to statc thc familiar truth that matter and force are different aspects of the same thing; that all problems of matter may be put iiito the
Ifistory o f the Theory of Heredity.
41
terms of force. The statement does not help 11s at all to picture to ourselves the essential hidden structure of the egg, the organization upon wliich its wonderful prol'erties depend. JIiger has recently brougli t forward an hypothesis whicli seems at first sight to be a satisfactory epigenesis. hypdthesis, but exaniinatiod sliows that this too, like IIaeckel's perigenesis hypothesis, must be turned into an evolution hrpothesis before it can be accepted. Tlic follomiug extract from his paper (" Zur Parigenesis," von Prof. Dr. G. Jager. x o s r t i o s iv. 376. 1879) gives, I believe, a fair statement of his Triews. ' ''Each organ and tissae of a n animal or plant contains, in the molecules of its albumen at least, a specific ~flnvo~-n~ibodo~.-substniice (Daft-nnd-Wiirzestoff) which me can easily recognizc by our chemical sense, for each orgnu of an animal 1 ~itss distinctive fl:~vor. Wlienever a full-grown animal experiences hunger, decomposition of albumen takes place in all its o r p n s and tissues, so that their various ~avor-and-odor-sii~stalices, that is their soul-substance (Seelenstoffe), become free, and penetrate to all parts of the body. " Now, if there exists in any part of the body protoplasm with the power to attract this substance, this protop1:ism acquires in this way its sires formatiam. " I have already referred with empliasis to the embryological fact that the formation of the reproductive elements takes place a t a rery early stage i n the embryonic life of an animal, and I have designated this as the reservation of germinal protoplasm. As soon as the embryonal cells of the developing animal have become specialized into ontogeni tic and phylogenetic cells, khe following will occur. Whenever any decomposition of albumen occurs in the developing organism, from hunL
42
hk-edity.
ger or any otlicr cause, the ontogenetic cell-material whicli builds up t h e org:inism will set free sod-stuff. ‘‘ By t h e law of gascons diffusion this will not oiiiy escape from the body as an excretion, but j t wjll also p e n e trate to the germinal or pliglogenctic protolhsm. This process I sliall now term soul-reception (Seelenfiingcrci) in the following sense. The diemica1 substance wliich forms the greater part of the ova and male cells ha Iittely been callcd nuclein, since i t shows the closest YCsemblance to thc cell nucleus. T h e jolk-enbstance is now rcgnrded, not :is vite!lin, b u t egg-nuclein, rind the substancc of the male cell riot spermatin but sperm-nuclein. Wc also know that nuclein consists of albumin, and phosplioric lecithin. “ T h e qiiestion then is the origin of the nncIcin in the egg, and the male cell, and this may bc ansncrcd as f 0110 ws : ‘‘ The reproductive orgins do not receim albnmen from the body of the mother, since ;iccording t o the law of Trsnbe, the moiecnles of a substance which fornis B membrane cannot, on ncco~intof flicir size, pass through the porcs of t h a t membrane. Thc germ-ccll is an albuminous membrane. and hence i t .will not aIIow the passage of albumin molecules. “It simply contains the albumin-nuclens, which PCmains after the decomposition of the sod-substance, niid this is n peptonelike substance ~rhicli,having lost its soul-eubstance, has a smaller moIccule. It is therefore nnspecializcd, or deprived of its sod (mtepcsificirt, entseelt), and t h e process of assimilation in tllc germ m:iy be termed Eoul-restoration (~-iederbeseclung). Tlic necessary soul-substance is supplied by the decomposition of albumen in the ontogenetic cell-rnateri:~l. ‘‘Thus, for examplc (p. 380), it is kiioxn that tho re-
Bistory of the Theory of IIeredity. -
43
~~
lwocluctire organs of :t catcrpilhr are already formed bcfdrc i t lcadcs tlic cgg. During its lifc 111 t h e egg, and :i$ ;I ci~tciyillar,ci~t~rl)ill:~r-~in~leiil is formed iii its gcrmi11;11 cell-material. During tlic pupa stage pupa-iinclein is storcd 111) i n its iel)roCltictivc clcniciits, arid finally, wlicii it bcconies :I butterfly, butterfly-nucleia is stored up. ‘i’lic ripe cgg and tlic rilbe niule cell thcreforc contain nnclcin of three kind?, catcr~illar-nuclcjn,p u p i i ~ ~ c l c i iand i , butt crfiy-niiclein.” It will be sccn t l i i t t Jiiger’s hypothesis is, in n certain sense niidmay between evolution aiiti epigeuesia. lie liolds that at first both thc oviini :tiid tlic mile-ccll arc iinspccialiaed (cntscclt) ; tliat they exist i n th e w r y piing embryo :is cinbryonic ova or spermatozoa, :uid that, as the embryo gFows up, the reprodiictivc crIIs g~ttluallybecomc spccidizcd by tlie :mimilstion of soulstnff, whicli is thrown off by the decomposition of :tlbnmcii in varions parts of tlic body of tlic growing oYg:inism, and pcnctrating to tlic cnibryonic O V R a i d sl)crmatozos is asPiInilatcd by tliern, so that when the :mirn:tI becorncs se.uu;~llym:itiire, tlie cells of its reprodwAivc org:ins cont:iin :ill the “soul-stuff ” ncccssary to 1nwdiicc n nc\v organism like tlic parent. Tile statcmcnt wliicli I linve given is a frce translation of Jlgcr’s outiinc of his tlieory, and I think it may be rcg:irdcd ;IS a fair exposition of his tiews. d fatal objection to his hypothesis is found in the fact that wlicrc a parcut givcs birth to young beforc it hns rcaclicd f u l l maturity and before it lius acqnircd :dl the cliaractcristics of tlic spccics, tlie yonng n c ~ e r t l i c less inherit thesc chctructcristics. Tlic jouiig which w e h r i i c by a Cpcedomia lurm inlicrit :111 tile cliuractcristics of the full-grown ndtrlt insect, a d a brill may transmit t o fcmalo childrcii the good milking qualities of his
44
Heredity.
mother. I t is plain that the child of a be;irdlcss boy could not inherit tlie “ soul-stuff ” of a beard i n the way Jager imagines, arid this f w t alone js erioiigli to s l i o ~ tliat he h ~ i snot discovcrecl tlie true secret of Iicrcdily. We know, too, tlint ~ C V C I ~ S ~ O the I I , :ip~)e:~r:tnce in the child of tlie features inherited from a remote nnccstor but not shared by its parents, is not a t all unnsnal, and mast be regtardecl as otie of tlie lcading cliaracteristics of Ilercdity. I t is plain that if tlicenibr~-onicoviini is, ns Jager states, in specialized or ” de-souled,” rcvcrsion is inesplicable. Accordingly, when lie conies to discmss reversion lie makes ii fiindamcntal cliange i n liis ligpotliesis, nnd holds that wlien tlie oviim divides, a t a very early stage of its development, into two parts, zn ontogenetic portion, which givcs rise to the iicw orgaiijsni, and a pliylogenetic portion, mliicli ultimately forms tlic germinative cells of its rcprodactire organ, tlie second part is not unspccinlizcd or ‘‘ clc-soulecl”at all, but really retains all the clinractcristics of tlic ovum irliich gives risc to it, and is tlicrcfore capable, like tlie ovuni, of giving rise t o a new organism. As thus remodelled, I bclicw, and hope t o show in the scquel, that Jager’s hypotlicsis is a close approximation to tlie truth, but i t is only fair to point out tlint in its nltcred form it is not original with JHger. Tlie author piiblislied almost exactly the same view in 15’76 (“ On a Provisional IXypothesis of Pangcnesis,” Pmc. Amer. Assit., 1S76, and Aiiiericau A’ntzii*alist, March, 1877), and i t liacl been stated as long ago as IS49 by Prof. Omcn, i n his p:ipcr on Parthcnogcuesis, nlthough this author, in liis ‘‘ Anatomy of Vvrtcbrates,” afterwards states tlint lie now lsclicves it to be funclamentnlly erroneous. It. is plain, too, that in its sccontl form Jager’s hypothesis is one of evolution, pure and simple,
Bistory of the Theory of Heredity.
-~
46
for the egg is, at no stage of its growth, unspccialized, : ~ l di t does not require the assimilation of “ soul-stuff ” iii ordcr to devclop into a n organism. We must conclnde, then, that however satisfactory anll :tccordant with obscrved fact t h e hypothesis of epigcncsis seems to be a t first sight, more careful analysis sliows that it is i n no sense a true explanation of the p11en om en o 11 of d eve1opni cn t. Tlic analogy betwcen t h e evolntion of the species from an nnicellular mce&oy, and t h e development of the individual from ~ 1 ~~nicclInIar 1 egg, is simply an analogy, for tlic cause of Llie first plienomeiion, the selection of cong r n i t d mriation., is wanting in tlic second case, and tlicrc is nothing to take its place if i t is truc t h a t a n egg is rcally, like a rliizopod, an unspecializcd ccll. H;ieckcI’s statement tliat heredity is memory, howc ~ c truc r it may be, cannot Le accepted as an explanation, for me liavc no knowledge of the existence of memory apart from organiza,tion, and we cannot conceive t1i;it a n ovuni can rctain tlic mcniory of the past history of its species, iinless it possesses a corresponding organiznt ion. Jiigcr’s view that ‘the embryonic ovum is unspecializcil, and that its specialization is gr:idnally assirnilntcd during tlic development of t h e organism mhicli contains it, fails l o acconnt for the plienorncna of rerersion, and to acconnt for rcrcrsion lie is comIwlled to assume t h a t tlic c g is organized from the time of its origin i n tlie dcwloping egg of thc preceding generation. In each case we are drircn to tlic same conclusion, tli:it the cpigenesis Iiypothesis is inadequatc; and me are forccd t o accept some form of the evolution hypothesis. This neccssity Iias not escaped the notice of some of our most acute thinkers. Huxley, for example, says
46
Heredity.
(Encyc. Brit., Art. Evolution), " Harvey's definition of a germ as matter potentially alive, and having within itself the tendency to assume a definite living form,' appears t o meet all the reqnirements of modern science. For notwithstanding it might bc' justly qnedioned whether a germ is not merely potentially but rather actually alive, though its vital manifestations are reduced to a minimum, the term pot'cntial may fairly be used in a sense broad enough to escape the objection. And the qualification of potential has the advantuge of reminding ns that tlie great cliaracteristic of the germ is not so much wlint i t is, but what it may under suitable conditions become. '' Prom this point of view the process, which in its superficial aspects is epigenesis, appedrs in essence to be evolution, and deyelopment is merely the expansion of a potential organism or organic preformation according to fixed laws."
...
CHAPTER 111. HISTORY OF THE THEORY OP HEREDITY-(
Continued).
Some form of tlie evolution hypotliesis a logical necessityDarwin’s pangenesis hypotliesis-This is an evolution hypothesis, since all the cliaracteristics of the adult are supposed to be latent in the germ-Miscellaneous objections to it-These objections do not show that it conflicts with fact-Difficulty i n imagining detailed working is no reason for rejecting it -Gilton’s cxperimental disproof-There are many reasons for believing that tlie sexnal elements have different functions -The evidence from parthenugenesis-Polar-cell hypothesis -The evidence from hybrids, from variation, and from structures confined to one sex-The pangencsis hypotliesis recognizes no such difference io the functions of the reproductive elements-We must therefore distrust its absolute correctness --Summary of last two chapters.
8ome Form of the Bvolution Hypothesis a Logical Necessity. Most of the hypotheses which have been proposed, of late years, to account for the phenomena of heredity, are like the two we have quoted, epigenesis hypothesis, for they are attempts to show that the ovum is in reality, as well as in form, an unspccialized cell. Analysis shows, however, that they all rest ultimately upon the assumption that this is not true, but that the ovum really contains, in some form or other, actually or potentially, the €uture organism, with all its hereditary characteristics. We know that eggs which are to all appearances essentially alike, may, when artificially removed from the ova-
Neredit y.
48 ~
ries and artificially fertilized, and when kept niidcr csactly the sanie conditions, develop into widely differciit organisms, and as like things csnr ot, under. lilic conditions, give rise to different results, me are forced to conclude that these eggs are not essentially alike, but that each contains within itself in some form tlie organism to which it is to give rise-that iiidividual development is, in some sense, the unfolding of a germ which already exists in the egg. There is no escape from this conclusion, a t least there is none which can be accepted by the scientific student, and we see that logical tliinkers like Prof. H L X Xare ~ Cdriven ~ to conclude that the process which in its superticial aspects is epigenesis, appears in essence to be evolution.
Darwin’s Hypothesis of Pangenesis. In contrast to the views already quoted me hare the well-known pangenesis hypothesis of Darwin, an hypothesis which is thoroughly one of evolution, since Darwin believes that the whole organization of the syecics’ is present not only in the egg but in the male cell also; that each of these not only contafns the complete organization of the parcnt, but an indetinite series of similar organizations, inherited from a long line of ancestors. It is true that Darwin does not believe that each of these ancestors is represented in the ovum and in the male cell by a minute but perfect animal, like those imagined by Bonnet, but he imagines what is essentially the same thing, that each of the cells of each parent, and every cell of each ancestor for a long aiid practically an unlimited series of generations, is represented in each ovum and each male cell by a germ capable of producing that particular cell with all its distinctive characteristics. Darwin’s original statement ( Va?.iuiio?a,chaps. xxvii.
History of the Theory of Heredity.
49
and xxviii.) is rcadily accessible, but it mill not be out of place to quote it before entering upon its critical discnssion. IIc says: “In tlie previous chapters large classcs of facts, such as those benriiig oil bud-variation, tlie various forms of inlieritance, the caiiscs and laws of variation, have been discusscd, and it is obvious that tliese subjects, as well as tlie several modes of reproduction, stand in some sort of relation to each other. I hare been led, or rither forced, to form a view, which to a certiiin extent connects tlieso facts by a tangible method. Every on0 Tvould wish to explain to himself, even in an imperfect manner, horn it is possible for a character possessed by some remote ancestor suddenly to reappcar in tlie offspring; how the effects of increased nsc or disuse of a limb c n ~ be i transmitted to the child; how the male scxual clement can act not solely on the ovule, but occasionally on tlie niotlier form; horn a limb can be reprodaced on the exact line of amputation, vith ncither too inucli nor too little added; how tlie rarions forms of rcprodnction are connectcd, and so forth. I am amare tlint my view is nicrcly a pro~-isionalhypothesis or spcculntion, but nntil a better one be adranccd i t may be scrviccable by bringing togetlier a multitude of facts which are at prcsent left disconnected by any efficient came. As TVhewell, the historian of tlie inductive scicnces, rcmarks, hypotheses may often be of service to science, wlien they involre a certain portion of incomplctcucss or ercn of error. “ Under this point of vicm I ventnre to advance the liypotlicsis of pangenesis, m1iich.implies that the whole organization, in tlie sense of cvery separate atom o r unit, rcproduces itsclf. Hence ovules and pollen grains-the fcrtilized seed or egg, as well as buds-include and con-
80
Heredity.
sist of a multitude of germs thrown off from each separate atom of the organism." From the extract we see that the hypothesis is a n attempt to shorn that all the phenomena of generation and development, including those of variation as well as those of heredity, depend upon the fact that each strnct u r d unit of the body is the direct offspring of a similar unit i n the body of a parent or of ,a more remote ancestor. The cells of the body of one of the liighcr organisms are not only niorphologically but ;ictnally indcpendent inclivid i d s , reprodncing themselves directly. in the next generation: and the germ of such an organism is in reality an aggregate of these cell-germs. Stated more at length, the li~'p0t11esisis as follows : " 1 assume that cells, before their conversion into 'form matcrial,' throw off minute granules or atoms, which circnlate freely throughout the system, and when siipplied with proper nutriment, multiply by self-division, snbsequently becoming developed into cells like those from which they were derived. These granules, for the sake of distinctness, may be called gemtnules. They are supposed to be transmitted from the parent to the offspring, and are generally developed in the generation which immediately sncceeds, but are often transmitted in a dormant state during many gcnerntions and are then developed. Their development is supposed to depend on their union with other partidly developed cells or gcmmules, which precede them in the regular order of growth. Why I use the term union will be seen when we discnss the direct action of pollen 011 the tissues of the mother plant. " Gemmules are supposed to be thrown off by every cell or unit not only during the adult state but during all stages of development. Lastly I assume that gemmules
. . .
€-€istory of the Theory of Beredity.
61
in their dormant state have a mutual affinity for each otlier, leading to their aggregation either into buds or into the sexual elements. Hence, speaking strictly, it is not the reproductive elements nor the buds whicli generiite new organisms, but the cells themselves throughont the body. These assumptions canstitate the provisional hypothesis of pangenesis.” Darwin’s genimules are, of course, entirely imaginary, that is, a belief in their existence does not rest upon direct observation. We cannot deny that the liypotliesis furnislies an explanation of most of the phenomena wliicli Iic attempts to interpret by it, although it seems possible that there may be a simpler explanlttion. If the existcnce of tlie gemmules were proven we could nnderstand not only the rvonderfnl facts of ordinary inheritance by sexual reproduction, but the various forms of asexual reproduction as well. We should have a simple explanation of the manner in wliich the characteristics of a remote ancestor may suddcnly reappear after they have been dormant for many generations. We should understand how the embryological history of a species may become simplified by tlie omission of larval forms or appendages. I n a mortl, nearly all the phenomena of heredity admit of explanation by the liypothesis, and those who have criticiscd it have not usually attempted to show that it conflicts with fact, but have simply objected to it as apurely imaginary explanation. It is urged that the transmission of all tlie cliaracteristics which we know to be inlicrited from near and remote ancestors demands that the number of gemmules should be almost unlimited and practically infinite; that not only are the gemmules imaginary, but that the aggregation of such numbers in masses as small as the reproductive elements requires
62
Beredity.
that they sliall bc of inconceivable minuteness, and that naturc fiiriiislies no analogy for :tttribotiiig t o such s n i A prticlcs the vital properties which we know otily it1 bodies which itre compiwatively gigautic. It is also urged that the gemmnles ninst be endowed with eiitircly imaginary and wonderfully spccinlized elcctive affini Lies, in virtue of which e x I i dcveIops onIy at the proiler tiuic and place. In order to account for the ni:mtier i l l which tlie characteristics of each parent are mingled in 1110 child we must regud each individual as tlie product of a struggle for existence among the gcmmules, meult\og in the selection a d development of the fittest. The for. mation of several indiyiduals :isexiially by budding from a parent stock demands tliut the gcniniules thcnieelves must be c a p b l c of mnltiplication, and that they niust have the power to transmit tlieir ln-operties to tlieir offspring. To explain alternation of generations we must suppose that the enibrgo receives scvcnil complete scts of gemmules, which are not dnpkutcs, and it is almost impossible to follow ont, in thought, tlie complicated relstions mliicli must exist between the gcmniulcs of tlic egg-embrjo of such an organism as a Sipliouophorc. These aid similar objections may be fairly iu’gcd, and mliile tlicir great weight is oblions, we must iiot attwli undue importance to them, for they do not show tlmt the hypothesis conflicts with any known law or ubs c r ~ c ifact, l and the great drafts made upon tlic im:igiiintion should not, alone, r re vent its pro~isionalaccclltanco so long as me liave no simple esplaniition of tlic phenomena, for difficulty in imagining tlic dctidls of :in hypothesis is a purely snbjectirc matter, vliich vuics will1 the age and with the individual.
History of the Theory
of Heredity.
53
Gcr lt on’s Expe~ intents. Bcsidcs these theoretical objections, we hnve the experimental disproof furnislicd by Galton. In order to test the hypothesis this cxpirimcntcr selected the silvcrgray rabbit-a variety mliicli has, in itself, little tendency to ~ar.y,although i t readily crosscs with otlier varieties, and breeding freely with them gives birth to hybrid offspring. Into the bodies of eighteen of tliese silver-gray rabbits Iic trailsfL1scd tlic blood of otlicr varieties, in some cases replacing onc lialf of the blood. F r o m the ciglitecn rabbits tlina opcratd upon eighty-six soung wcrc produccd, ancl i n no case did tlic offspring cxliibit any of tlic charactcriatica of tlic variety from mliicli tlic blood was talicn, but all of tlie eighty-six mcrc pure silver gr‘iy. Froin tliesc cxpcrimeiit s Galton concludes tlint “ t h e doctrine of pangencsis, pure a n d simple, is incorrect;" a n c l I tliink mc ninst agree with him tliat this conclusion is justified by tlic results mliicli h e rcaclied, altliongli I hope to sliow tliat i t is possible t o restatc tlic hypothesis in a form which is so modified as to escapc this objection.
The Sexual Elenieiats Perfom Different Fuiictions i7a I h e d i t y. There is another objection which seems to me to be of almost eqnal weight, b u t which has never, so far as I a m amare, been pointed ont. T h e early writers upon heredity attributed certain fanctions to the male cell and others to the ovum; but me now know that their means of observation mere so inadequate, and their knowledge so limitcd, t h a t tlicir conclusions wore of littlc value, and that both orjsts and spermists wore wide of the mark. T h e fact that they crroneously attrib-
Her edity.
54 ~~
~~
~
-
~-
uted certain fnnctions to tlic ornm and certain otliers
to thc male cell does not, of course, prore that there is no difference in tlie fnnctions of these elements; but in modern times we actually find that thinkers hare gone t o this opposite cxtremity of the subject, arid liare either tacitly implied or directly accepted tlie vicw that the two sexn:tl elements play similar parts in heredity. Nei thcr IIacckel’s hgpotliesis nor Jiiger’s recognizes any difference in tlieir fn?ctions, w.hile JBger secrns to believe, and Darwin explicitly statcs, that tlieir sliarcs in hereditary transmission are alike. Many facts indicate t1i:it this view is, to eny tlie lcast, very improbable, and I mill give, bricflp, a statenicnt of some of the arguments against it, and will then devote a little space to a discussion of tlic reasons wliich 1i:ire been given by Darwin and others for accepting it. The structural difference between the ovum and the male cell is one of the most widespread and fundamcntnl characteristics of organic beings, and it is found in all except tlic very lowest animals and plants. I t is, to eny the least, very improbable that a strnctnrnl difference so fiindamental and SO nearly universal should 1i:lre 110 functional significance, and the fact that in many marine animals, when the ripe unfertilized ova arc thrown ont into the ocean, like the ninle fluid, to be swept away by the tide, the sexual elements differ in tlie same a a g that they do in aniinals whose eggs are fertilizcd inside the body of the female, forbids 11s to believe t1i:it the difference depends simply upon the fact that tlic nialc ccll must make its way to the ovum. Many of the secondary cliaracteristics of thc ornm, such as its p e n t size in birds and reptiles, and tlic 1msence in it of food-material in so many animals, are no
Bistory of the Theory of Heredity.
55
doubt traceable to the fact that, in most animals, the
cgg is stationary, while the male cell can be conveyed irom place to place; but me must bclieve that there is some more fundamental and primitive difference. Even if the phenomena of Parthenogenesis did not slioiv us that the part played by the ovum is more essential to the perpetuation of the race than the part played by tlic malc cell, we should still be justified in the belief that the difference in form corresponds to some profound difference in function, and the possibility of Parthenogenesis shows beyond question that this is the case. Pnrtlieiaogenesis. Siebold has proposed the term parthenogenesis for the yowx which is possessed by certain female animals, cspcci;~llythe arthropods, to produce descendants without scsnal union with a male. Tlie existence of this power was first pointcd out by Aristotlc (DE Geiisratioiie Animnlittnz, Lib. III., Cap. 10, 21, 22, 23). As this remarkable dmeruer had no menus for exact research at his command, he was, of course, unable to-make w e of rigid tests, or to furnisli the scvcrcly exact proofs wliicli have been given 11s by more modern naturalists ; but he gives many reasons for suspccting that the unfertilized eggs of the honey-bee may give rise to perfect animals without sexual union ; and although we now know that some of the reasons he urges do not really prove tlie case, yet modern science has given tlie most convincing proofs of the correctness of liis general conclusion. I sliall dcrote considerable space to this subject in ordcr to sliom the unscientific reader that the existence of fertile virgin female animals is proved by the obser-
56
Herediiy.
vations of a great number of compctcnt nntnralists; that the subjcct has been thoronglily and carefully studied, with every precsntion against error, and i l x t oiir belief in its existence docs not rest upon tlie unverified statenients of a f e v observers. I n this summary I sliall give many referenccs t o authorities, bnt as my p r p o s e is not to give a conlpletc bibliography, but simply to ~shomhow tlioronglily the subject hns been studied, many imrnes are omitted. Most of the following facts arc takcn from Gerstaecker's history of tlie subject in Volume V. of lh 0 1 1 11's El(isseu 0)d)m):gleu rEes TlLierreidis, altliough I Iiare rcfcrrcd to many of tlie origiiiul pipers and have :dded many facts which are not nientioiied by Gerst:iecGer. 'the snbject is perfectly fsrniliar to most naturalists, and the amount of space devoted to i t may seem oniicccssarily grcat to such 11wsons, bnt it is importnnt to j m p e s s u p o ~n n s e i c ~ ~ t ~ f i c readers :I scnsc of the exact and definite character of tlre cvidcncc for the cxistcnce of partlicirogencsis, and a short history of tlic subject seems the most effective means for accomplishing this prpose. Among the crnstnccn and iiisccts, lmrthenogenesis is by no mcans unusnal. It occurs in some groups mlicre impregnation by m ; h s is so nearly oniversol that nnturalists hare been slow to credit any exceptions. I n 0 t h gronps it is the general rule, and fcitilization by a male is the exccption. In some genera and species the powr is shown only by a few individuals, ~vliile in others it is shnrcd by d 1 the females. In some cases ihe unfertilized eggs give rise to feniales only, in other cases to males, and in still other cases to both Ecxes. In 1'775, Sciibffer, of Begensbnrg, discorcrcd its occurrence in fresh-water crustacea, although Dr. Albrecht
Nistory of the Theory of Heredity.
57
-
]lad made the same discovery in insects in 1’701. Scliiiffw found (‘
68
Heredity.
may be produced by isolated females whose mothers and grandniotliers liave been isolated ull their lives. Claus has shown that the eggs begiu to deveIop in the fenide Evadne, a form closely related to Daplinia, before tlie animal is born; a i d impregnation would liere seem to be impossible. In Daphnja, and related forms the parthenogenetic eggs usndly give rise to females only, but cxperiments have shown that the approach of winter or the failure of the snpply of food causes males as well as females to be produced. Schgffer, the discoverer of parthenogenesis in Dal)Iinia, also discovered that Alms, a crustnceati which belongs to another order, lays eggs which give rise without impregiiation to fertile females, and that this may go 011 for an indefinite number of gencriitioiis. In A p s , and in most of its allies, the malcs are cxtremcly Rue, although the females may Le very t~bundant,and one observer, Joly, foiind only one male spccinicn of Artemia sulina among 3000 femslcs. Psrtlienogenesis is known to occiir in many insects. It is rare and exceptional i n Eome of tlicm, while in otliers it is as freqnmt and normal as it is in Daplinia. Among the butterflies and motlis, sesunl union is tlic rule, and parthenogenesis a r:wc exception, biit in 1701 Dr. Albrecht made thc remarkable discomry that a fcmale BombTx, which had escaped from its pupa under B glass shade, and which could not have been risitcd by a niale, laid fertile eggs. As scxnal nnioii is known to be almosi unircrsnl in the Bombycidz, this obscrrntion was a t first discredited, but the phenomenon has i n more modern times been observed with crery possible precaution in Bombgs mori by a number of most competent obserrers, among whom are Schmidt, BnrthEl-
History of the meory af Heredity.
59
em?, Jourdan, Siebolcl arid others. They all agree tliat wliile lmthenogenesis is rare i n this species, i t does sometimes occur, and i t is known tliat tlie partiienogeiicric cggs give rise t o fertile males aiid fertile females, wliicli may unite sexnaliy and thus 1woduce fertile eggs. Dr. Kipp lies reared another form, Sv/eriut?izis populi, from eggs fertilized by a male whicli liatclied from a partlienogenetic egg, and laid by a female whicli had been reared i n tlie same may. I n Uroiin’s KIasseia ‘ ~ m dOi~diwagen,Gerstncckcr gives tlic followiug list of moths in wliicii par-,l:enogenesis has been observed, with the name of the observer. “lie list might be greatly enlarged by tlie addition of cases wliicli have been recorded since its compilation, but it is suffizient for our purpose, mhicli is simply to show t h a t llie fact has been verified repeatedly by many observers.
........................... .
Sphinx ligustri, once.. .Treviranus. Smerintlius populi, four times.. .Nordmann, Brown, Newnham, Kipp. Smcrintlius ocellatus, once. .......................... .Johnston. Euprepia cnja, five times.. .Brown, Lehocq, Robinson, Schlapp. Barth&my. .Stowell. Euprepia villica, once.. ............................. Saturnia Polypliemus, twice. ................Curtis, De Filippi. Gsqtropacha pini, three times. .....Scopoli, Suckrow, Lacordaire. Basler. Gastropaclia quercifolia, once. .......................... .Burmeister. Giistropacha potatoria, once. ...................... .Pleininger. Gastropacha quercus, once. ........................ Liparis dispar, once ................................. .Carlier. Egger Moth” (Liparis dispar ?), once.. ................Tardy. .Popoff. Liparis ochropotla, once.. ............................ Orpyia pudibunda, once ............................ Wernberg. .Rossi. Psyche apiformis, once.. .*............................. Bombyx mori, many times.. ..Schmidt, Siebold, Jourdan, Barthilemy, and others.
.
..
.......
Heredity. Although these c a m make a long list, mliieli might be greatly increased, t l i q :re still exceptional, for in a11 tliese species almost all the eggs fail to develop unless they are fertilized by a male; but in somc otlicr groups of insects partlienogenesis occurs more fieqocntlg, aiid seems to be perfectly normal. The most remar!iable instances are those which occur in the social iiisects, such as the bees. It is well known that a community of honey-bees consists of individuals of three kinds-the workers or rudimentary females, wliich are the most numerous; the perfect females or queens, of wliich only one is usually present in R hive; and the drones or malcs. I n the workers, or as they arc sometinics falsely called tlic neuter bees, the female reprodnctive org:tns are very imperfectly developed: the vagina is so sinall tliat miion with a male is hardly possible, and the reccptnculumseniinis is very rudimentary, yet it is ~vcllknomn to a11 bee-cultivators that they do sornctirnes lay eggs wliich are capable of development, not only in the honey-bee biit in otlicr species also. Among tlic Iioncj--bccs such fcrtilc workers are dwajs found in a hire wliicli lins lost its queen, and tliey h a w been called ‘‘ drone mothers,” from the fact tlist tlieir eggs produce only drones or malcs. Tlie queen-bee is the only member of the hire ivliich unites sexually with tlie nmles, and Iicr rrpodrrctiw organs are w r y large and well developed, as contrasted with those of the worker. Her receptaculum scminis is large enough to retain a sufficient supply of tlie male fluid to serve for fcrtilizing great nnnibcrs of eggs, and i t is usually f o n d to contain a coixidelnllc quantity. Sexual union takes place during flight, and qnec~iswith imperfect wings are never impregnated, and Siebold,
History of the Theory of Heredity.
61
Leuckart, Berlepscli, and others have shown, by microscopic examination, that in such cases the receptaeulumseiiiinis is empty, and the queeii is a virgin. I n such cases, as well ;LS in hives, where the receptaculum-seminis of the queen lias been exhausted by old age, or has been removed, it is well known to bee-cultivators that only drones are produced, while eggs destined to give rise to females, to workers or perfect queens, are produced only by qneens which have been impregnated and have some of the male fluid in the receptacle. This fmt, considercd in connection with the fact that the eggs laid by workers prodnce only droiies, indicates that tlic drone eggs laid by an impregnated qneen are not fertilized; and Siebold has found actire spermatozoa on newly h i d worker-eggs, but lias failed to find thcm on drone-eggs. We are, therefore, conipelled to believe that the queen is able to lay both fertilized and parthenogenetic egg:”. It is stated that when a queen of the common German variety is crossed with a drone of the Italian bee she produces hybrid workers, while her male offslning :we all pure German bees. In certain Lepidoptera, as in the bees, parthenogenesis seems to be normal, and it has been observed in Solenobia and Psyche by a great number of ancient and modern naturalists, including Schrank, Rbaumer, Pallas, De Geer, Scriba, Speyer, Reutti, Siebold, Lenclrart, Hofmann, and others. Their observations show-lst, that the wingless female is abundant and widely distributed a t all seasons, while the winged males are seldom met with, and are found only in certain restricted localities; 2r1, that there is only one form of female; those mhieli iunite with the male, as well as those who do not, have perfect reproductive organs wliieli resemble those of other butterflies. Parthenogenesis is the rule, and the females
62
Heredity.
lay eggs as soon as t h y hare passed tlirongh the pupa stage. These l7arthenogenetic eggs girc rise oiily to females, and these may give rise to female dcscendaiits in tlie sarnc way for an indefinite number of geiierations; 3d, in a t least one species (XoZenobin ti*zquet~dZn),Clie eggs wliicli are laid by impregnated females gire rise t o both sexes. Dufur, Kessler, I-Iartig, Walsh, and many other naturalists have sliown that certain female gall-maeps are parthenogenetic; witliiii recent years Baesctt and Adler hare made most interesting observations upon these wasps. In 1873 Basectt showed (Ca?zadic~zEiita71iologist, 1873-75, p. 91) that great numbers of male and female wasp escape ~ I JI u n e from c c r f d n g:ills wl~jeh arc found in very great abiindmce on the lcaves of an oak. Late in t h e summer tlic fcmalcs lay their eggs i n t h e leaves of the same oak, and give rise to galls, aliicli, however, are of quite a different c1i:imcter from those in which tlie insects mere born. Early in the Eollo\ying spring a brood of females hatch from tlieze winter gallsJ and at once lay parthenogenetic eggs, irliich gke rise to t h e summer galls, and 1i:itcli i n J ~ i n cinto males and females. Bassett and Adler have extended these obserrations to a great. number of species, and the following account is taken from a paper by the latter v r i t e r (“Ucber d e n Generationsweclisel der Eiclien-Gallmespcn,” Ton Dr. II. Adlcr, Zed. f. TTias. Zool., xxxv. 151), who has carried on a long series of tlie most painstaking experiments, using every precaution against error. H e reared a great niirnhcr of small oak-trees under glass c a m , ant1 tlien, introducing the wasps, traced tlwir whole life history, and h e found that in many specics there is a winter gall, which is produced in the fall by a
History of the Theory of Heredity.
63
fertilized fenxile, aud whicli gives rise early in the spring to a brood of fenialcs without males. Tliese at once lay their eggs and form summer galls, from which both scses are born. In all cases the parthenogenetic forms are so different f v ) m the sexual forms tliat tliey liad previously been described as distinct species, aiid i n most cases tliey had been placed in distinct genera. The following example selected from Adler's paper will give an idea of tlie cliaracter of his experiments: Neuroterus leuticularis is a wasp wliicli is born within n small round gall wliicli appears in July on the lower surfaces of oak leaves. Tlie galls continue to grow until the end of September, wlieii tlie leaves drop off and fall t o tlie ground. In'the spring the insects escape, and all of tlicm arc females, with their ovaries f u l l of eggs, and the male of this species was unknown previously to 9 d ler's cxpcrimcn ts. He gatliered the fallen leaves, and rearing tlie wasps in isolated captivity found tliat, soon aftcr tlic female is born, slie pierces tlie leaf buds of the oak, and lays her eggs. Adler marked by pieces of thread all tlie buds wliicli the insect was actually seen to pierce, and iu a few days he found on tlie leaves which expmclecl from tliese buds a great number of minute young galls, wliicli soon became large enough to show tliat tlicy were very different from the winter gall in which tlic paleiit was born. This new gall proved to be one with wliich entomologists liad long been familiar, as the birtliplace of wliat had always been regarded as a wasp of quite a different gciins-~~atlio~ast ebnccnrum. r It is a soft green gall, I)niictntcil with rcd spots, and it grows entirely through the leaf, so t1i:tt part is on tlic upper and part on the lower sarfacc. The oak trecs with tliese galls were kept
64
Heredity.
carefully protected from the access of other insects until about the middle of June, when male and female specimens of Spathogaster baccarum were produced. The sexes united at once, and the females were then isolated and placed in captivity, each with its little oak tree. They soon laid their eggs i n the leaf bnds, atid thus gave rise t o the winter galls, which, i n the following spring, produced a brood of tlie parthenogenetic female Neuroterus lenticulnris. He lins made similar camful obserrations on many other specks, and he gives the following table to exhibit his rcsults: Parthenogenetive form born from a winter gall, and producing a xummer gall.
Sexual form born from a summer gall. and producing a winter gall.
Neuroterus lenticulnris. Neiiroterus ltrvi~~sculus. Neuroterus neumismatis. Neiirotcriis fumipennis. Apliilotrix rndicis. Apliilotrix Sic1)oldi. Apliilotrix corticis. Aphilotrix globuli. Apliilotrix collaris. Apliilotrix fecundatrix. Apliilot rix c:illidnma. Apliilotrix Mdpighii. Apliilotrix :iatumii:ilis. Drj-oplianta scutellaris. Dryophmta longiventris. Dryoplien ta divisa. Biorliiza nptern. Biorliizn renum. Neuroterus ostreus.
Spathogaster b:iccarurn. Spathogaster albipes. Spathogaster vesicatrix. Spathogaster tricolor. Andricus noduli. Andricus testaceipes. Audricus gemmatus. Audricus inflator. Audricus crirrntor. Andricus pilosiis. Audricus cirratiis. Andricus nudus. Andricus ramuli. Spathogaster l’asclienbergi Spatliagastcr sirnilis. Spathogaster verrucosus. Terus tcrrninalis. Trigonaspis crustnlis. Spathogaster aprilinus ?
I n the following four species no males were discomred, but the parthenogenetic females gave birth to females like themselves:
HLstory of the' Theory of Heredity. Apliilotlirix seminationis. Aphilothrix marginalis.
65
Apliilothrix q~iadrilinintus. Aplliiotlrric albopunctata.
Thesc are all of them insects ~vliicliform gdls on oak leavcs, but Acllcr finds tlint the same powcr to lay parthenogcnctic eggs exists in some other wasps. Ptcromalus ptqmrzun lays its eggs in the bodics of butterfly larva, and thus givcs birth to both males and females. The sexes are so diffcrcnt that there is no difficulty in separating tlieni as soon as t h y are born. Adler fonnd tliat femalcs wl~iclimere thus isolated, and wliicli were shown by microscopic examination to be virgins, nevertheless laid eggs as soon as a caterpillar was furnished them. Among 806 females which hatched from tlicse eggs there were only 9 males, so that there is, in this species, a strong tendcricy for parthenogenetic eggs to produce females. I n the rose-gall-wasps Adler finds that tlie males me very rare, about one to fifty fcmales, and he believes that they are superfluous, since the females j n two species, Rhodites TOSQ and RlLodites eglanteriw, are perfectly partlicnogenetic, giving rise to partlienogenetic fern:& off3priiig. The instances of partlienogene~jsin larval or jrnrnature insects are extremely interesting, but as they will be referrcd to at some length in another place I will not dwell upon them at present, as the cases wliich have been givcn are enough for 0111' purpose, which is simply to show the satisfactory and exhaustive c1iar:icter of the proof that nnfertilizcd eggs do in many animals develop and give rise to organisms wliich w e in all respects like those born from fertiIized eggs. I n Nematus ventricosus the r i d e s are not uncommon,
66
Ner ed ity.
but Adlcr has vcri6cd Sicbold’s statement that in this species parthenogenesis of tlic ordinary females is not a t all infrequent. Althongh pnrtheuogenesis is more frequent aniong the insects and crustacca than it is in other animals, it is not confined to tliesc groups. Colin lins given good yeasons (Zeit. f. 1Ti.s.s. ZOOZ., xii., 1563, p. 107) for believing that among tlie Rotifera the summer eggs, which give rise to both males and females, are parthenogenetic; wliile the irinter eggs, which liatcli into fctnales exclusively, are tlie only ones which are fertilizcd. Tlicre is 110 reason for doubting the correctness of this conclusion, but it has not been placed beyond tlie possibility of all doubt, as is tlie ease with so many insects. Many observers have thought that they have found evidences of pnrtIienogenesis in groups of animals w.here such an occurrence would be e r y cxccptional, bnl i n most of these cases tlierc is much chance fur error. Tlins it has been stated that tlie eggs of echinoderms sometimes develop iritliout imprcgnntion, but when we recollect that both male and female ecliinoderms in most cases discharge thcir reprodnctivc elements into the water, we can see that it must be almost impossible to state that the sen-water in which the eggs arc placed cont;iins no spermatozoa of the same species. Dr. J. 11. Wilson has recently undertaken some experiments on this point a t my suggcstion. Hc fertilized a lot of eggs from one of o w common sea-urcliins, Strongplocentrotus, with male fluid from :inotIier of a distinct genus, Arbncie. A lot of Arbacia eggs wcre fertilized with :L male Strongjlocentrotus,a lot from c ; d i form w i t h fluid from a male of tlie same species, :mil eggs from each species were placed in water without fertilization.
History of the Theory of Heredity.
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In all six cases the eggs gave rise to normal embryos; b a t that this was really due to the presence of spermatozoa in the water, was shown by the fact that no snch snrprising result followed in a second set of experiments where especial effort was made to get pure sea-water. Many of the recorded cases are open to the Fame objection; and in other cases, as in the virgin sow referred to by Bisclioff, there seems to be some doubt whether the ova were really undergoing development; bnt Oelacher’s observations on the eggs of a virgin hen (“ Die Veriincl ern n gen d es 11ii be f r uch t et en Kei mes d es II ii11n ere i es im Eileiter niid bci Bebriitungsversnclie~i,”Zeit.f.Jl’iss. Zool., xxii., 1872, p. 220) seem to show that the hen’s egg does have tlic power to p a s tlirougli the first stages of development whether it is impregnated or not. The instalices of partlienogenesis which I have given shorn that this power inay be independently acquired by animals wliicli cannot possibly inherit it from it common source. In the vast majority of insects, and i n the mgjority of the crustacea, the egg does not show the sligliteJt’tendency to derelop before i t is fertilized. It is true that in the case of the crustacea the evidence for this statement is almost eiitirely of a negative character, for no one has ever shown by experiment on any consrdcrable number of species that the female cannot lay fertile eggs when the access of a male is prevented, but in niany insects we h o w from actual observation that the eggs die soon after they are laid, unless they are fertilized ; and we know enough of the breeding hubits of crustacea to feel confident that partlienogenesis is exceptional among them, just as it js among insects. We must, therefore, conclnde that if we could retrace the course of evolution of any parthenogenetic animal we should be led back to an ancestral form wliich never
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Heredity.
manifested any such power. I t is inipossiblc t o bclicl-e that Daphnia and the honey-bee have inlieritcil from D common parthenogenetic ancestor the power to prodncc fertile nnimpregnated eggs, for the one form is much more closely related to normal insects and tlie othcr t o nornial crustacea than they are to each olhcr. T o may therefore state with confidence that the power has been independently acquired by many animals. I n the second place, we must admit that partbenogenetic ova are true ova in every sense : they are developed in an ovary like other eggs, and in many cases, as in those butterflies wliicli arc occasionally parthenogenetic, the very eggs which usually reqnire imprcgnation may in rare instances develop without it. Wcismann has made rery carcful examination as to the origin of both kinds of eggs in Leptodora, swnter-flearelated to Daphnia (‘
Bi’story of the Theory of Heredity.
69
diffcrcnce. I n some cases, as in Dnplinia, all the partheuoTenetic eggs hatch into fenides ; in other cases, as i n bees, they give rise to males alone; while in still other cases, as in the gall-wasps, some of the unfertilized eggs produce males and some females. In many cases the animals which are thus produced are perfectly normal, iZnd have nothing to distingnish them from those born from impregnated eggs. They 1iaI-e the ordinary structure of tlieir species, and they are perfectly capable of propagating tlicir kind. I n some cases, as in tlie gallwasps, reproduction is preceded by the union of tlie sexes, and in other cases~ the animals born from parthenogenetic e w s are them6 selves parthenogenetic. There is possibly one difference between ordinary and parthenogenetic eggs,-the presence of polar globules in the one case and their absence in the other; and I ~ 1 ~ ~ 1 1 . discuss this difference soon. Except in this particular, the history of the development of the egg into the perfect .animal is the same, whether the egg is fertilized or not. TVeismann, wlio has studied the embryology of both parthenogenetic and fertilized eggs in insects (“ Beitrage ziir Kenntniss der ersten Entwicklnngsvor~lngein1 Insectenci ”), shows that all tlie minuter details in the process of building up tlie embryo are tlie same, wlietlier the egg is fertilized or not. We must therefore believe that an oriim 11as in itself tlic power to give rise to a new organism, and that although it does not usually manifest this power, unless the egg is fertilized, it may exhibit it under certain circiin~st:tnce~,DS parthenogenesis. Of the clmracter of the circnmstances which lead to parthenogenesis we know little, except that such circumstances have
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Heredity.
thus acted in many groiips of animals where tlie eggs ordinarily require to be fcrtilizcd. Certain authors liavc suggested that thcrc mny bc a connection between the extrusion of tlic “ polar globales” from the ovum and the need of impregnation by a m:de cell. The ripe ovarian ovum of an animal nsually contains a transprent central body, the germinative vesiclc, and When the egg is fully ripe the gcrminative vesicle approaches the surface and divides into two portions : OLIC of these is discharged from the egg, tlius forming the “polar globules.” Tliese take no part in the fornintion of the embryo. They become entirely scparatcd from the egg, and soon die and disappear. The remainder of the germinative vesicle remains in the egg, as tlie ‘6 female proti~~cleus,” which unites with thc ‘‘ male pronucleus” formed froin the male cell after impregnation, and thus builds up a compound body, the first “qrnentution nuclcus.” The formation of tlicse ‘‘polar globules” has been observed in all groups of the animal kingdom, except the rotifcm and arthropods, and tlicir functional significance is therefore a subject of tlie greatest interest. They obTTiously contain something wliicli is not ncedcd for the formation of tlie embryo, and they may be discharged from the egg before it is laid, or they may remain until it is laid, as seems to be the general rnlc, and may be discharged just beforc fcrtilizutiori t d x s place, as is the case’in the star-fiah, or they mtiy bc discharged immediately after the egg is imprcgnatccl. Within recent years an hypothesis regarding tlicir significance has excited considerable notice. This II~pothesis, which mas first advanced by the late Prof. JlcCrady, and which is stated at length in Ballfour’s Treatise on
History of the Theory of Neredity.
7’1
Comparnttve Rrnbryoloyy, is that each sexual element originally contaizis a male portioii and a female portion; the ripe male cell is tlie male lialf of the niale element, and the “polar globulca” contain the male substance of the ovum, whicli is discharged in order that i t may be replaced by tlie male element from tlie body of anotlier organism. Balfour says : “ I would suggest that in tlie formation of tlie polar cells part of the constituents of the germinal vesicle, wliich are requisite for its functions as a complete and intlependent nucleus, is removed to make room for the sxpply of the necessary parts to i t a g i u by the spermatic nucleus. M y view amounts to the following, viz., that after the formation of tlie polar cclls the remainder of the germinal vesicle within the ovum (tlie female pronuclcns) is incapable of further dcvclopmeiit without the addition of the nuclear part of tlie male element (spermatozoon), and that if polar cells were not formed partlieiiogencsis might normally occur. A strong support for this hypothesis would be afforded ere it to be definitely established that a polar body is not formed in the arthropoda and rotifera ; since the normal occurrence of parthenogenesis is confiued to tliesc two groups in which polar bodies have not so far been satisfactorily observed. . To the suggestion already made with reference to tlie fnnction of tlic polar cells, I will venture to add the further one, that the function of forming polar cells has been acquired by the owim for the express purpose of preTenting parthenogenesis. . . . There can be little doubt that the ovnm is potentially capable of developing, by itself, into a fresh individual, and therefore, unless the absence of sexual differentiation was very injurious t o the vigor of tlic progeny, p:irtlienogenesis would most certainly be a very constant occiirrence ; aid, on the
..
72
Heredity.
analogy of the arrangements in plants to prewnt selffertilization, we niiglit expect to find some contrimice both in animals alld plunts to prevent the ovum dei-eloping by itself without fertilization. . On my hypothesis tlie possibility of parthenogenesis, or a t any rate its,freyiieiicy in arthropoda and rotifera, is possibly due t o the absence of polar cells” (C‘0772p.Eqnb.., vol. i. p. 6 3 ) . The simplicity of this hypothesis renders it very fascinating, but even if it were possible t o accept it without qualification, it would not affect our argument, for it would still remain true that the ovum is potentially capable of developing, by itself, into a fresh individoal,” and mast tlierefore be yery different in function from the male cell, wliich under no circumstances exhibits a similar power. My reasons for doulsting the bypothesis are, first, that a failure to discover polar cells in the eggs of rotifera or of the artliropods may be dne to tlie fact tliat they are discliarged very early in the history of the ovarian ovum. We know that in some animals, as in lijdra, the polar cells are discharged wliilc tlie cgg is still contained in the ovary, and we also know tliat the eggs of many arthropods undergo in tlie ovary very peculiar changes, which greatly obscure their fundamental similarity to ordinary uncomplicated eggs, so that it is qnice possible that o ~ i failure r to discover the polar cells may be due to something else than to tlie fact tliat they are never formed. The eggs of insects especially are w r y peculiar, and Weismann says that ‘< nirgends im gnnzen Thierreich die Ontogenese so versclioben und coenogenetisch entartet ist” as it is among tlie insects. Tliis author has figured, in tlie fertilized egg of a species of Chironomus, certain bodies which are not present in the
..
((
History of the Theory of Heredity.
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partlleiiogenetic eggs of Rhodites, and lie suggests that these may be the long-sought polar cells, but he does not feel certain tliat this is the case, and examination of his p q x r will sliow that there is so much difference between the early stages of insect eggs and the corresponding stages of simpler and more typical eggs, that tlie identity of tliese bodies must remain open to some doubt, but there can be no doubt of the nature of the polar cells described by Grobben in the parthenogenetic eggs of Moinn. Tlicre is another objection to the hypothesis, which secms to nie to be entitled to great weight. According t o U,ilfotir’s statement we should expect that any egg whicli retained the polar cells might develop without impregnation. Observers have failed to diseover their extrusion in the eggs of ordinary arthropods, as well as in those which are parthenogenetic, and we should tlierefore expect all the artltropods to be parthenogenetic, bnt this is not the w e . I n many other animals, as in tlie oyster, they are not discliarged until the egg is fertilized, and the hypothesis would require us to believe that an unfertilized oyster egg contains a male element as well as a fcniale element; but when perfectly ripe oyster eggs are placed, without fertilization, under conditions wliich are perfectly favorable to development: they show no signs of life, and soon die and decoy. If a little male fluid is added, however, they quickly discharge their polar cells, and then rapidly pass through the changes wliicli build up the embryo. If the polar cell is really equivalent to a male cell, we certainly might expect these oyster eggs, which are perfectly ripe, and, according to the hypothesis, contain all that is necessary for development, to show some power to develop without impregnation. If tlie power to extrude polar cells “has been acquired by the ouum for the express picrpose of preveatiiig parthenoyen-
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Heredity.
esis,” we certainly should look for the occurrence of parthenogenesis in ripe ova which have not extruded these bodies. However this may be, the correctness or incorrectness of the polar-cell hypothesis has no bearing upon our present argument, for the phenomena of parthenogenesis show beyond qLiestion that an egg may develop without union with a male cell, and there is no evidence whatever that a male cell ever acts in a similar‘way.
Other reaRons for believing that the ouum and the male cell perform dgxered functions irc heredity. Even if the possibility of parthenogenesis did not show us that the part played in heredity by the ovum is different from that plajed by the male cell, there are many other reasons for believing that the difference in the form of the two sexual elements corresponds to some profound difference of function. I shall devote several chapters of this book to the extended discussion and proof of the facts which drive 11s t o this conclusion, and I shall show that the belief in the essential similarity of the fanctions of the reproductive elements cannot possibly be retained. When tlie niale of one species or variety is crossed with the female of another species or variety, the hybrid offspring is often very different from that which is produced when the female of the first species is crossed with the male of tlie second. If the fcriiction of the ovum is the same as that of the male cell, we should have exactly the same elements jn each case, and shoiild expect the same result. The fact that the result is not the same proves that the elements are not tlie same either. Iu many cases the male of one species will breed
Xistory of the Theory of Heredity.
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freely mi th tlie female of the second species, while absolute sterility follows the union of a male of the second species with a female of the first species. The offspring of a male hybrid :ind tlie female of a pure species is mncli more variable tlian the offspring of a female hybrid and the male of a pnre species. These facts are absolii tely inexplicable, if tlie two sexual elements play similar parts in heredity. A structure wliich is more developed or of more functional importance in the male parent than it is in tlie female parent is very much more apt to vary in the offspring than a part which is more developed or more important in the mother tlian it is in tlie father. These facts, and many otliers which will be mentioned f;irtlier on, comprl us to believe that tliere is some profound functional difference between the ovum and the mule cell. I t is, therefore, only reasonable to distrust the absolute correctness and completeness of any hypothesis of heredity, which, like Darwin’s Pangenesis liypothesis, recognizes xio such difference.
Surnnaary of last two Chapters. The phenomena of heredity are certainly among the greatest marvels of the material universe, but there is n o rcason to believe that they lie outside the province of legitimate scientific inquiry. Our present purpose is riot to trace them back to their origin or to show that they resnlt from the propcrties of matter, but simply accepting them as vital phenomena, to trace the second:rry laws to wliicli their present form is due. The Pact that the distinctive properties of the egg of any living species liare been gradually acquired during the evolution of the race through the action of influences which
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Heredity.
are, t o a certain extent, open to obserration arid study, gives us ground for believing tlint we may liopc to discover mliut i t is in tlic stroctnre of the egg, wliicli renders tIiese propertics possibIe. There Iiavc been insiiy attempts to do this, b u t i t is impossible to accept any hypotiiesis which Iins ever been advanced. The cvolution Iiypothesis, as advocated by Bonnet and Haller, is directly contradicted by the discoveries i n tlic modcrn scicnce of embryology, a n d i t is according~ynow rcgardcd as having only a n liistorical interest, biit the modern epigencsis hypothesis is no morc s:itisf;tctory, for tlie reseinblniice between tlic cvolntion of a species from an nniceilnlnr ancestor and the development of a n individual animal from ail nnicellnlar egg is only an aim1ogy. T h e efficient cause in the first case, the slow modification of the race by the natural selcction of tlic most favorable variations, is absent in the second case, aiid tliere is notliing wliatever t o t a l z its place. The p:imllelism between enibryology, or t h e ontogenetic devclopmeiit of the individual, and p1ijlogeny7 o r the evoliitioi~ of the race, is one of the most rcmnrl
History of the Theory of Heredity.
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~
dncc from two essentially similar eggs adult animals mliicli are so essentially dissimilar. We cannot attribute this rcsiilt to natural selection, for this law can only act on successive individudq; we cannot attribute it to the dircct action of external conditions, for we know t1i:it eggs m:ty give rise to very different aiiimals when pl:iccd noder idcuticd surrounding conditions. H w c k el’s st:ttcincnt that heredity is memory, contains a profuuiid trnth, as we 1i:ii-e already seen, but i t does not hcli) ns to understand heredity. Wc know memory only in conncctipn with organization, and if we bclicrc that an cgg contains the memory of all tlic p:tst esperience of the race, we musl believe that it contnins a complex organiz;ition to correspond to the complcsity of this past experience. So f;ir as Haeckcl’s hypotlicsis of pcrigenesis has any claim to be considered an explnizntion of heredity, it is an 11y pot h cs i s o f evol ti ti on, not of e pi gen esis . Jhger’s view that the oviim is a t first nnspecialized, and t1i:tt it gradually assimilates from its developing p;ucnt all tlic sl~cci:tliaationsof the structure of the ]attcr, fails to acconiit for reversion or for the transmission of adult characters by immature parents, and the author i J compelled to substitute for it an evolution Iiypothcsis wlicn hc comes to treat of reversion. There is no cscapc from the concl’usion that the ovum of a n animal actually contains in some form t h e potentiality of that particular animal, and Huxley acknowledges that the derelopment of an egg is in essence a process of cyolntion. TYe tlins find ourselves driven back from the modern Iiypotlirsis of epigenesis to the Iong abanrloned hypothesis of e~oIution,and me must tliercfore inquire whether O L I ~recent great advances in knowlcdge of the forces
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Heredity.
wliicli have produced the various forms of animal and vegetable life, mill guide us nearer to the truth tlian the speculations of tile last century. Bonnet and Iialler might fairly assnme that each species had been wbat it 1s iiow “from the beginning,” but we cannot nowaday make any aydi assumption, and me must believe that the structure of the germ, like the structnre of the adult animal, has been gradually acquired by natural selection. A modern hypothesis of evolution must therefore be it very different tlljng from the one which Bonnet fnrnishcd, and must account for the slow advancement of the germ from generation to generation. I n Darwin’s pangenesis hypothesis we I i a ~ en provisional explanation based upon the generaliza tions of modern science. I t is a true evolution hypotlieeis, for Dmwin belieyes that an ovum or a male cell is awonderfully complex structure, and that it contains gemmules to represent each feature in tlie organization of the adult. One essential difference between this hypotliesis and the original hypotliesis of evolution as stated by Bonnet, is that Darwin believes that the ovum contains, not the perfect animal in miniature, but a distinct germ for each distinct cell or structural element of the adult. Darwin’s hypothesis recognizes the gradual specialization of the ovum during the evolution of the race, for each cell of the body of the parent may a t any time transmit t o it new gemmnles. Most of the objections to it are based upon its complexity, and on tlie almost infinite nnmber of gemmules which it requires; but besides these objections me know from Galton’s experiments that it is impossible to accept it without modification. We also hnve, in the fact that the functions of the two sexual elements are not alike, a reason for believing that,
History of the Theory of Heredity.
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although it niay be an approximation to the truth, it cannot be regarded as a complete and satisfactory explanation. The object of this work is to present a new hypothesis which will be seen to bear a close resemblance to the one which has been advocated by Darwin, although careful examination will show that it is in reality very different. I hope to show that it is not open to the objections which are urged against the pangenesis bypothesis, while it contains all the features which give value to the latter.
CHPTER IV. A NEW THEORY OF HEREDITY.
The objection to tlie hypothesis of pangenesis would be almost entirely removed if it could be simplified-Statement of a new theory-Heredity is due to tlie properties of the egg-Each new character lias been impressed upon tlie egg by the transmission of gernmules-Tendency to form gemmules is clue to the direct action of exterual conditions-The ovuiii is the conservative element-The male cell is tlie progressive elementThis theory has features of resemblance to most of the liypotlieses which liave been noticed-It fills most of Mivart’s conditions also-It is not necessary to assume that tlie ovum is as complicated as the adult-Tliere are many race cliaracters wliicli are not congeuital-There are many congenital cliaracters wliicli are not hereditary-Direct action of external conditions-Our theory stands midway between Darwin’s theory of natural selection and Lnmarckiauism.
IF the hypothesis of pangenesis conld be so remodelled as to demand the transmission of only a few gemmules from tlie various parts of the body to the reproductive elements, instead of the countless numbers which are demanded by tlie hypothesis in its original form, we should escape many of the objections which have been urged against it. If it can be shown that these few gemmules are not necessarily present a t all times and in all parts of the body, but only occnsionally and in certnin regions, me shall escape the difficulty presented by Galton’s csprriments, and the presumption in favor of llie hypotliesis will be greatly increased. If the theory of lieredity, in its new form, agrees with
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all that me know of the fnnctions of tlic two sexual elements and if, besides f~irriisliingan explanation of all the plieiiomeiiawliicli are accounted for by other hypotheses, i t embraces new classes of facts as well, t h e presumption in its favor becomes still greater. Finally, if i t leads to t h e discovery of ncm and nnexpected relations between phenomena, and t o tlie estnblislinient of lams which group and interpret phenomena between which no connection h:td previously been recognized, its value uinst be aeknomlcdged. I venture, then, to advancc a new theory of hcredity, which, briefly stated, is as follows: The union of two sexual elements gives variability. Conjugation is the primitive form of sexual reproduction. IIere the fnnctions of the two elements are alike, and the union of parts derired from the bodies of two yareiits simplg insures variability in the offspring. In all multicellnlar orgnuisms t h e ovum and the male ccll liave gradually becoine specialized in diff ercnt direc-
ti oils. The ovnm is. a ccll mliicli has gradually acquired ,a corn 111icatetl orgnniz:ition, and wliicli con taiiis matcri:il puticlcs of some kind to correspond to each of thc hcrcditary characteristics of tlic spccies. The oviim, like otlier cells. is able to reproduce its like, and it not only givcs rise during its development t o the divergent cells of tlie organisni, b a t also to cells like itself. The ovarian ova of tlie offspring arc these latter cells, or their direct unmodified dcscend;ints. Each cell of the body is, in a niorpho1ogic:il sense, an iuclepcndcnt indiriilnal. Tt has the power to grow, t o give rise by division to similar cells, and to throw off minntcgerms. During the evolution of the species i t has
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Heredity.
by natural selection acquired distinctive properties or functions, which are adapted to the conditions under which it is placed, So long as these conditions remain unchanged it performs its proper function as a part of the body; but when, through a change in its eavironnient, its function is disturbed and its conditions of life become unfavorable, it throws off sniall particles which are the germs or “ gemmnles” of this particular cell. These germs may be carried to all parts of the body. They may penetrate to an ovarian ovum or to a bud, b a t the male cell has gradnally acquired, as its especial and distinctive fnnction, a peculiar power to gather and store up germs. When the ovum is fertilized eachgerm or “gemmule” unites with, conjugates with, or impregnates, that particle of the ovum which is destined to give rise in the offspring to the cell which corresponds to the one which produced the germ or gemniiile; or else it unites with a closely related particle, destined to give rise to a closely related cell. When this cell becomes dereloped in the body of the offspring it will be a hybrid, and it will therefore tend to vary. As the ovarian ova of the offspring share by direct inheritance all the properties of the fertilized ovum, the organisms to which they ultimately give rise will tend t o vary in the same way. A cell which has thus varied will continue to throw off gemmnles, and thus to transmit -mrial.,ility to the corresponding part in the bodies of successive generations of descendants until a favorable variation is seized upon by natural selection. As the ovum which produced the organism thus selected will transmit the same variation to its ovarian
A New Theory of Heredity.
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ova by direct inheritance, the characteristic will be estddiahed as an hereditary race characteristic, and will bc perpetuated and transmitted, by the selected individuals and their descendants, witliont gemmules. According to this view, the origin of a new variation is neithcr purely fortuitous nor due to the direct and definite modifying influence of changed conditions. A change in thc environment of a cell causes it to throw off gemmules, and thus to transmit to descendants a tendency to vary in tlie part whicli is affected by tlie change. The occurrence of a variation is due to the direct action of external conditions, but its precise character is not. My view of the cause of variation is tlins seen to be midway between that accepted by Darwin and that advocated by Seniper and other Lamarkians. Many naturalists liave giren reasons for believing that tlic transmutation of species is not always gradual, but that a form which has long persisted without change may suddenly vary greatly, and tlins Five rise to a stronglyniarked race of descendants. Mivart has discussed this snbject at considerable length, and he quotes Professor HuxIeJ"s opinion that "we greatly suspect that Nature does make considerable jumps in tho may of rariation now and then, and that these saltatioiis give rise to some of the gnps which appear to exist in the series of known forms;" and Dall has proposed the term saltatory evolut h z for abrupt change of this kind. According to the theory here advanced, variation must tend to accumulate or culminate, and one variation must cause others; for when any particular cell changes, the harmonions adjustment between it and adjacent or related cells will be disturbed, and all the cells which are thus affected will tend t o throw off gemmules, and thus to indnce variability in the same cclls of succeeding generations. Then, too, a
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genimnle may unite or conjugate in tlie ovum villi particles which :ire not perfectly equivalent 1 o it, b u t only very closcly related to it. Tlins a v:iriation may affect a considerable number of related cells at tlie same time, or a variation in any part m:iy c:iuse in sncceeding gcnerntions the vari:btion of Iiomologoys p r t s , tlins producing what Darwiii has called cor*?.eZnted unrintioii. V e can d s o underst:ind liovv i t is tliat mlien nn~7pait of a complicated organ wries, vxri:itioiis i n otlicr parts of i t are also soon lresentcd for tlie action of natural ~ e l c c tion, so that an harnionioas rcadjastmcnt is 6oon established. According to this ricw we must bclicvc that a11 t h e char:ic t cri s t ics \;.h icli are cs tablished as true raceclinractcristics, as licrcditary pecnliaritics of the spccieP, are traiismittcd by the ornm, which lins i n itsclf tlie p o m r to develop, when escitcd by a proper stimulus which may or may not be dne to impregnation, into a ncw individual of the parent form. New variations, on tlic other Iinnd, arc ~~ r o d i i c e d through the agcncy of gemmnles thrown off from cclls like those i n which the v:iriation appears. Gcmmulcs may peiietratc to all parts of the body, and they may thus give rise to bud-variation and to analogoris cbangcs; or they may pcnetratc to an or:irian o n i m and give rise to variation without fertilizntion: but as these phenomena depend upon chance, they are comparatively rarc, mliile tlie aggregation of the gcmmulcs in the male cell and tlieir transmission by impregnation are normal proccsses. According to this view, the male clement is the originating and the fem:ile the perpetuating factor; t h o ovum is conservaave; the male cell progressive. IIercdity or adherence to type is brought about by tlie ovum;
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variation and adnjkition tliron,rrh the male element; and the ovnni is tlic essential, the male cell the secondary, factor in heredity. Tlic various hypotheses wliicli wc lime noticed have little in common, and it is therefore interesting to note that t h y all present points of reseniblancc to the one wliicli is here advanced, and that this alone has features in common with tlieni all. Like Aristotle and tlic ancients, me must believe that the two rcprodnctivc elements play widely different parts. Like Bonnet and Hdler, vte see that the strncturc of the xdalt is latent in the egg. The modc of origin and transmission of the gemmules is essentially lilie Dmvin’s conception, and we must acknowledge that Bnffon’s view of the part played by liis org;inic molecules mas very ncar the truth. Tlic analogy upon wliicli IIaeckcl lays so much stress is readily explicable by our theory, for sincc each stage in tlic evolution of the species has been impressed by gemmules upon tlie egg, i t is, in truth, only ~zntzimlthat the developing organism sliould mirror the ancestral history of its species; and, finally, our view of tlie origin of the properties of the omTim egg is identical with thai given by Jager in his explanation of reversion. An honest attempt to reason from the phenomena of nature can hardly fail to result in the discovery of 6ome little truth, and I think we may hope that all these points of agreement with Iiypothcses wliicli are manifcstly inadcquate can only bc duc to the presence in tlieni all of some portion of the trim light of nature. Jlivart, who believes with Darwin that natural selection lias been agreat but not the exclusive means tliroiigh mliich orgnnisms have been modified, has attempted i n Chapter xi. of his book on the Genesis of Species t o
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indicate some of the requisites of a triie theory of the origin of specics. This valuable and instructive book is well worthy of careful study, and most students will find in i t much material for reflection. Mivart has no explanation of his own to offer, and some of the chnracteristics of the explanation which he believes in, but does not furnish, are conspicnously absent in the present attempt as we11 as in Darwin's work; but it is interesting to note that many of the conditions mhich he cnnmerates are complied with by our theory of hcrcdity, and by n o other explanation which has ever been proposed. Thus he says (p. 244) that " I t is quite conceiruble that the material orginic world. may be so constituted that the simultaneous action upon it of all known forces, niechan i cal, physical, cliemical, m agn c ti c, terrcstrial and cosmical, together with otlicr as yet unknown forces which probably exist, may result in changes which are harmonious and symmetrical, just as the internal nature of vibrating plates causes particles of sand scattered over them to assume definite and qmmetric:il fignizs when made to oscilltlte in djfferent ivays by the bow of a violin being drawn along their edges. The results of tliese combined internal powers and external influcnccs might be represented under the symbol of complex series of vibrations (analogous to those of sound and light) forming a most complex harmony or a display of most varied colors. "In such a may the reperation of local injnries might be symbolized as a filling up and completion of an interrupted rhythm. Thus also monstrous aberrations from typical structure might correspond to a discord, and sterility from crossing be compared with the darkness resulting from the interference of waves of light. ('Such symbolism will harmonize with t11c peculiar
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reproduction, before mentioned, of heads in tlie body of certain annelids, with the facts of serial homology, as well as those of biliiternl and vertical symmetry. Also as the atoms of a resonant body may be made to give out sowid by the juxtaposition of a vibrating tuningfork, so it is conceivable that the physiological uitits of a liuiiiy orgaiais~n,may 6e so iiq'ikenced 6y surrozcncZiny c o i d i t i o m (organic and other) that the accumulation of these coiiditioiis may qvset the previous rhythm of such qtnits, producing mod$cati'ons in them-a fresh chord i ~ tAe ILnrmoiiy of Nature-a nezu species. It seems probable, thercfore, that new species may arise from some constitutional affection of parental forms-an affection mainly if not exclusively of their generative system." Accordiug to the view which I have presented a new variation is cansed in essentially the manner which Mivart suggests as probable. The accumulated influence of surrounding conditions, organic and inorganic, does upset the previous rhythm of the physiological units of tlie living organism, and causes them to give rise to gemniules, and the tendency of the corresponding nnits of the offspring to vary, is directly due to this constitutional affection of the parental forms. I liavc spoken of the egg as containing material particles of some kind to represent each of the hereditary congenital pecnliaritics of tlie race. According to this view tlic egg of one of the higher animals must be a wonderfully complex structure. At first sight it would seem as if it must be as complicated as the adult animal, but a little thought will show that this is by no means tlie case. I n the first place, there are many structures which enter into ,the formation of the body without being part of its actual living substance. Nearly every living
...
2
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body consists in part of striicturcs which are in no Eense alive, but wliicl: are built up by tlie formative activity of tlielivirigl~rotoplasrn. The shell of a snail or of an oyster is purely inorganic, and although it is built u p Ly tlie animal, and is necessary to its existeiice, i t is no more D part of t h e living substance of the animal tlian the sliell wliich is picked u p and inhabited by a hermit cr:tb. It is true that t h e oyster’s shell is formed by the animd, as part of itself, but the shell does not grow, like liiiiig tissues, by the absorption and transformation of nutriment, but by tho crystallization of the amorplions mineral matter which is poured out by the living cells of t h e mantie; and microscopic esaniination shows tliat i t is not a n organized tissue made iip of cells, but a n aggregate of purely niincr:tl crjstnls. Siiicc this is the case i t is clear that i t is not the slicll itself, b u t a tendency to build the shell, which is lieredit:iry, mid is contained i n t h e egg; a n d an illnstration will Eerve to show that the inheritance of tlie tcndciicy involves mnch less complexity in the structure of the egg than the inheritance of the thing itself would imply. A bec inherits a tendency to build ~ i ap comb of wax, and to fill the cells of this comb with honey. The comb and tlic honey arc due t o the vital actiF-ity of the bee, just as tlie shell is t h e result of t h e vit:tl activity of t h e oyster; but the statement that the bee’s egg contains sometliing mliich corresponds to tlic structural organization to which t h e tendency is due, is certrinly not eqoivalent to a statement that tlie actiial comb, filled with honey, is rcprcsented in tlic egg. This is just as true of structures wliich are hiiilt 1113, inside tlie body, by its vital activity, as i t is of those which are built up in the same way outside the body. When we take into account a11 structures of this kind
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whicli are not parts of tlie living substance of the org:rnisni, but which simply owe their existence to the properties of its living substance, we can readily nndcrstand that tlic complexity of an adult animltl may be vastly greater than the complexity of tlic egg. I n the second place we must recollect that there are ninny race characteristics which are of constant occurrence without being hercditary. O r p i i s m s are often greatly modified by the direct actioii of external conditions; for instance, a tree may be dwarfed by insufficicnt food, or the muscles of a limb m:iy be greatly enlarged by unusual work. If all the individnals of a species are similarly exposed to conditions of tliis sort, tliey will all be acted hpon in the same may, and the modification whicli is thus produced will be cliaracteristic of the species, without being hereditary. To take one of the simplest cases: Trees which grow upon mountain-tops, where they are exposed to extreme changes of climate, and to constant and violent winds, haw a very characteristic appearance, which is familiar to ill1 mountain climbers. In some cases this peculiar form is hereditary, and persists in seedlings which are grown in more favored regions, but in other species the tra~isl?li~~ited trees shorn, by losing their peculiarities, that tliesc arc due to direct modification. If a certain species occurs naturally nowhere except in such situations, this species mill be characterized by its dwarfed size and by its twisted and distorted branches; but if i 11d i vi d LIals reared ti n der fiLvorable influences grow and flonrish and become regular and symmetrical, we may conclude that tlie characteristics of each wild indivicln:il are caused by its scanty food and constant exl~osnre,and that they are not represented in the egg, and are not congenital.
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If this experiment is impossible, if all the transplanted trees die, and if the seeds fail to germinate in fertile ground, there will be no may to show whether the pccnliar characteristics of the species arc or are not hereditary. We know that organisms may be modified in many ways by the direct action of external condicions, but a few illustrations mill not be out of place. Hemp-seed causes bullfinches and certain other birds to become black, and me know from the observations of many naturalists that caterpillars which are fed on clifferent kinds of food either themselves acquire a different color, or they may produce moths which differ in color. Many curious cases of this kind have been noticed in birds and insects, and if unnatural food causes deviations froni the natural color of a species, i t is quite possible that the nornial color may in many cases be due directly to the action of the normal or natural food. Darwin gives many instances of plants wliicli tire characterized by a certain peculiarity in one coiintry, while in another couotry this peculiarity is almost or entirely lacking. Thus when the American sassafras tree is grown in Europe, it loses its aromatic flavor. I n India the fibres of flax and hemp are brittle and useless, and the latter plant yields a resinous narcotic substance, hasheesh, which is used as an intoxicating drug, but in England this property is lost and the fibre becomes long and tough. Large, fincly-fiar.ored, and brjgljtly-colored American apples, when reared in England, produce fruit of a dull color and poor quality. I n these cases we are unable to state what the determining conditions are, but the fact that peculiarities are made to disappear by a change from one country to another shows that they are not congenital but are due
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to something outside the plant, which is present i n one country but absent in another. The following inst:tnce, which is given by Darwin, is most interesting 6. Mr. Salter, who is well lii~ownfor liis siiccess in culti vating variegated plants, iiiEornis me that rows of straw. berries were plsntcd in his garden in 1859, in the usual way; and at various disctances in one row several plants simultaneously became variegated, and what made the Case more extraordinary, all were varicgated in lrecisely the same manner. These plants were removed, but during the tliree snceecding years otlicr plants in the same row became variegated, a i d in no instaiice were the p1ants in any adjoining row affected.” H e alsosays that in certain parts of India the turkey beconics reduced in size with the pendulous appendages over the liead enorm onsly d eve1oped. In these cases i t is dificult to determine what has canscd the change, but in other instances this is more obvious. Thns Darwin states that good :inthorities assert that horses kcpt during several years in the deep coal mines of Belgium become covered with velvety hair almost like that of the mole, and he quotes from Dr. Falconer the statement that the Tliibet mastiff and goat when brought down from the Himalayas to Kasliniir lose their fine wool. These are only a few of the cases which Darwin gives, and many more might be added from ot1:er authorities, h i t I have given enough to sliom that external conditions of life may act in one country to cause certain modifications wliich are entirely absent in another country. The change of Artemia into Branchippus, by rearing it i n fresh water, is one of the most remarkable instances of definite modification due to it change of external
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conditioiis. Artemia salina is a small crnstacean, found in tlie d t lakes of America, Enrope, and Africa. When tliis species is kept in water in wliich tlic quantity of salt is gradually diminished, it becomes transformed, in a few generations, into wliat has been described as a distinct species-Arteniia Milhausenii-and if tlie proccss of diluting with fresh water is continned until it finally beconies perfectly fresh, the Arteniia becomes changed into the well-known fresb-water form Brancliippns, mliicli has always becn considered a distinct gciius. Seniper has sliown (Animal Life, p. 1Gl) tliat certain definite clianges in the sizc of tlic fresh-water snail Lymnza are produced i n a short time by conGning if in a small quantity of water. These are a few of the cases where we are able to show, by experiment, that certain race-cliaracteiistics are not congenital, but are due to external influences, and we hare every reason to believe tliat the same tliing is true in many cases which have nei-er bccii made tlie subject of experiment, and i n many more where expcrimcnt is impossible, since the cliange wonld cause death rather than modification. Tile possibility that sfructures of tlie greatest coiistancy and importance may not redly be hereditary is well illustrated by Huntcr’s well-known experiments on the sea-gull. In pigeons, and i n most birds wliicli feed upon grain, the muscular wall of a portion of tlic stomacli is greatly developed, to form the crnsliing nncl grinding gizzard, which is lined with a covering of tough mcmbrane, wliile the stomach of the gull and of most fleshfeeding birds is soft, and the mnscnlar laycr littlc devcloped. Enntcr fed a sea-gall for a year on grain, and he thus sncceeded in hardening the inner coat of the bird’s stomach, thns forming a true gizzard; and Dar-
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win quotes from Dr. Edmonston t h e statement that 8 similar change o c c ~ ~twice r s a year i n t h e stomach of another sc:t-gnll in t h e Rhctltind Islands, where this bird frcqnents the corn-fields and fcedv on seeds i n t h e spring, but catches fish daring the rest of the year. This observer has noticed a great change in the stomach of awren which had long been fed 011 vegetable food; and Menetries states that wlien a n owl was similarly treated the form of the stomach was changed, and the inner coat, becnme leathery, while the liver iiicreased in size. Scmper states that Dr. IIolnigrin has been able to transform the gizzard of a pigeon into a carnivorous stomach by feeding tlic bird on meat for a long time. Tlicre is 110 reason for believing that the few cases known to us arc all which are due to the direct action of e x t c r n d conditions, and we must acknowledge that there may posbibly be many structural characteristics of animals and p h t s which arc not hereditary, but are constant simply because the conditions wliich cause them are constant, :tiid as me are only compelled to attribntc to the ovnrn relmsentatives of all the hereditary racc characteristics, i t will be seen that the s t r n c t u r d complexity of t h e egg may be vastly less than that of the dcveloped organism. This is not all, homerer. There may be many congenit 11 race characteristics which are not hereditary. r 1 l l i c various parts of a developing organism are exposcd in cnnntless ways to the inflnence of othcr parts. The simplest illustration of this fact is t h e mechanical presSUIT exerted u1mi each other by t h e dcveloping viscera. This is a subject mhicli is almost outside the proi-ince of experiment, for me cnnnot shut out the influcncc of any imticular organ without removing t h e organ itself, and the removal of any organ of consideraLle size is
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more likely to came death than to came modification. The fcaturcs of microcophalous idiots sliow us, ho\w.;er, that tlic shape of the skid1 a i d of the face is only due, in part, to heredity, and is, in part a t least, diie to the size and s1i:tpe of the brain. In lop-eared rabbits the whole conforination of tlie skall is altered by the meclianical prcssnrc of the drooping ears, and it is stated that certaiti monstrosities in tlic shupe of snail-shells are due to the arrested devclopment of the reproductive organs. bioq nin-Tanden re marks that mi tli plants the axis cannot become monstrous without in some way affecting the organs snbsequcntly produced from it. We can see, from tlie study of domesticated pigeons, that an incrcasc or a decrease in certain o~gaiisis ;L direct cause of modification in otlicr parts. Pouter pigeons have becn selected for length of body, and tlic establishment of a long-bodied race lins increased the number of their rertcbixx and the breadth of tlieir ribs. Tnmblcrs hare been selected for tlieir small size, and the number of ribs and of primary wing-fcatIicrs his thus been rcilnced. Fantails h v e bccn sclcctcd for tlieir large widcly-expanded tails, with nomerow tail-fcatliers, mid tho size and number of tlic candul y e i t e b w lia.;~ tlins been increased, and the selection of l o n g - b c d d cxrriers has increased the length of their tongues. Cline states that the s l d l of a ram with horns weighs four times as much as that of a liornlcss ram of the s:me age, and Youatt states that in hornless cattle the frontal bones are matcriallg diminislied in breadth towards tlie poll, and the cmitics between the bony plntcs are not so deep, iior do they extend beyond the fron tals. The kidnepsof different birds differ mnch in size, and St. Ange believes that this is determined by the size of the pelvis.
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I t is plain that if the character of important parts can bc thus changed by changes in otlier parts, the typical or ciiaracteristic form of these parts may be clue oiily partially to heredity. TVe see then that the structnral complcxity of an adult animal is due in part to the formation of strnctures wliicli are not alive, in part to the direct modifying influence of external conditions of life, and in part to the action of one organ of the body upon another, so that tile number of features wllicli are directly inherited is very much less than tlie number wliich are constant in and charswteristic of tho species. I t is impossible for us to state at present how many features must be subtracted from the race characteristics of an mima1 in order to give ns the total number of liereclitiiry congenital characteristics. The observations and experiments which are recorded are few in number, but they are sufficient to show us that, in all tlie higher aiiimiils, very considerable dednction must be made, and ivc nuy bc sure that the mature animal is vastly more complex tliau tlie egg. There is still another lirnitiug circnnistaucc whicli has not yet been mentioned. Many of the parts of an organism are due to indefinite multiplication of a single clement. The simplest illustrntinns of this fact are the blood corpuscles of vertebrntcs and the leaves of plants. I t is clearly unnecessary to suppose that each vertetrate ovum contains scparntc particles for all the blood corpuscles, or that each sccd contaiiis separate particles for all the leaves whicli the p h t is to produce. All that is necessary is to assume that it contains particles which arc capable of prodiicing a single one of these structures, with a capacity for indefinite multiplication, and that Eurrounding conditions determine how far, and in what places,
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tliis power of multiplication shall manifest itself. Most of the organs of the body contain great numbers of ecIf3 which are alike both in structure and fnnction, and as it is usually qnite impossible to say how far the size of :in organ is truly hereditary, and how far it is determined by surrounding conditions, i t is, of course, impossible to 8ay to what extent its mature strncture is represented in the ovum, but as we know that the size of most organs varies, and may be increased or diminished by external influences, me may be quite certain that the number of independent cclls which make iip the tissues and organs of a mature organism, is w r y much greater than the number represented by distinct particles in the ovum. I t is not even necessary to suppose that all classes of cells which are present in the adult arc represented i n the ovum. I n a mammal, for instance, certain epithelial cells become converted into hairs, ~ h i l others e become converted into glands or other specialized epitlielial structures. I t is not necessary to assume that all of these spccialization,o are represented in the oviim, for we know that ordinary epithe1i:tl cells, i n a part of the body where 1 1 0 hair is normally developed, may, when inflamed, give rise t o hairs. It is therefore quite possible that cadi epithelial cell may, when excited by the propcr influence, tend to become converted into a hair cell. Ench cell of the body may possess the tendency to manifest certain properties under certain conditions, and to manifest certain other properties under other conditions, and the descendants of a single cell may thns become modified in several divergent directions, and each modification may be perfectly constant and characteristic of the race without being hereditary; that is, without being rcpresented in the ovum by a particle with the same specialization.
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I t may seem to some that the assumption that the egg contains particles capable of producing an unspecialized epitlielial cell which shall have the power to give rise to all the specialized sorts of epithelial cells, involr-es just as mucli complexity of structure as the assumption that each kind of cell is represented in the ovum, but I think an illustration mill show that this is not the case. Training of a certain kind mill dewlop a boy into a good pedeshian, while another sort of training will make him a good shoemaker; but it is surelysimpler to assume that he is born with a pomer to devclop the characteristics of a shoemaker nnder the influence of certain conditions, and those of a pedestrian nnder other coiiditions, than to assume that he is born with all the peculinritics of both latent i n his organiz a t'ion. The direct modifying influence of surrounding conditions is a subject upon which very much remains to be done, but me know enough about it already t o state that many of the constant characteristics of organisms are duo to exposure to constant and nuiform conditions rather than to heredity. TOwhat extent this is true we are qnite unab!., to determine, but me can be sure that the organization of the ovum is simpler, and in all probability vastly simpler, than that of the dereloped org n n i sm After all these deductions are made the number of strictly hereditary features is very great indeed, and the egg of one of the higher animals must be a tmrvellous stractare, for we know that, after all, most of the characteristics of an orgaaism are not due to the ioflnence of its conditions of life, bat to the past history of tlic race; and Darwin has shown us that the sncccssite changes which have resulted in the evolntion of any organism do not, usually, owe their existence t o the direct niodify-
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ing effect of external influences, but to the natural selection of congenital variations. The fact that our theory requires 11s to be1ieT.e tliat the egg of one of the higher animals is complex beyond our poivers of conception, mnst not be regarded as an argument against the theory, for me are compelled to believe this in any case. The difference between our theory and other attempts to explain the phenomena of heredity, is that it does what no other hypothesis attempts. I t furnishes a simple explanation of the manner in which the ovum has acquired its present complexity. I n the following chapters I shall give some of the reasons for believing that the difference between the functions of the sexual elements whi& the theory requires does actually exist, but even in the absence of this proof it mould be natural to conclude that if race niodification could be furthered and aided by the dirergent specialization of the functions of the two reproductive elements, natural selection would, in all probability, have acted so as to bring such a specialization about. We know that the influence of natural sclection is constantly exerted to seize upon and perpetuate any tendency to division of labor among the organs and tissucs and cells of the body, and it is only natural that the successive stages in the specialization of the sexual elements should have been perpetuated like any other useful specialization.
CEIAPTER V. ON THE OPINION THAT E d C H SEX YAY TRANSMIT ANY CHARACTERISTIC WHATEVER.
The argument from hybrids-This
argument is inconclusiveThe argument from the homology between the ovum and the male cell-Hon>ology does not iiivolve functional similarityThe argument from the dual personality of each individual; from reversion ; and from polymorphism-These phenomena admit of a simpler explanation-Summary of chapter.
The Arpmeizt from Bybrids. According to the view to be presented in this work, the functions of the two sexual clements, in inheritance, arc not alike. Tlic proof of this mill be presented rnrther on, when tlic snbjcct is reached in the logical course of the dcvelopni en t of o 11r argument. Some of thc very liigliest autliorities have been led to a view wliicli is directly opposite, and lime lield that citlicr parent may transmit to the offspring any characteristic whatever. Lest any reader should assume, a t the beginning of this book, that the work involves an absurdity, and that my conclusion is already d i s p r o d , it seems best to at oncc examine the reasons for the opposite view. If I can show that these reasons are inconclusive, and tliat tliere is and can be no proof for tlic statement that each sexual element transmits to the offspring every characteristic of the parent, we can then enter into the subject without prejudice, and can wait
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for the proper time to present tlie proof of tlre opposite fview, tliat tlie two sexual elements play different parts i n heredity. If tlic authority of great names counted for anything Tvlmtever in sciciice, the case against me would be w r y strong, but where an appeal to natiirc is possible, authority counts for nothing. Darwin’s place among the students of heredity is certainly the highest, and lie takes very strong ground indeed upon this subject. Thus lie says (Variation of Animals and Plaids, Vol. ii. p. 8 8 ) : “I am aware that such cases (of prepotency) have been ascribed by various authors to snch rules as that the father influences the exteriial characters, and tlie motlicr the internal cliaracters. “ But tlie great diyersity of tlie rules given by various authors almost proves their falseness. Dr. Prosper Lucas lias fully discussed this point, and has shown tlii~tnone of tlic rnles (and I could :tdd others to those quotccl by him) apply to ;ill animals. Similar rules have been announced for plants mid have been proved by Giiirtucr t o be all erroneous.” In the Atiatoiiiy of Inverte6rnted Aniiiinls, 11 30, IIuxlcy states tliut “ n o structural modification is so slight, and n o fonctional pcca1i:trity is so insignificant i i eitherparent, ~ tliat i t may not malie its appearaim in the offspring.” D ~ r w i n ,in many parts of his mritings, is still more explicit. Thus he says (Vcci.iatioit of Aizin~uls nqzd Platrts, Vol. ii. 13. 431): “Ovules and tlic male clement, before tbcy become united, have, lihc bnds, :ti1 indcpcndent existence. Both have tlic p o ~ ufr t i x i i s m i ttiiig every sinyZc character possessed by the parent fol ni. We see this clearly when hybrids are paired inter se, for
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the characters of either grandparcnt often reappear, citlier perfectly or by segnicnts, in the progeny. It is I r i L error t o sappose that the iiiale traiisnzits certain charac. t p r s mail the jEinnZe other chni*acfers.” I tliiiik a little cxarnination will show clearly the impossibility of proving this statement from the phcnomeI n order to brecd together animals must i i i t of crossing. I)c closcly related; they must belong to the samc species or to two closcly allied species. Since the individuals which belong to two closcly related species are the dosccudants ‘of a common, and not very remote, anccstral spccies, it is clear that almost the wliole course of their evolntion has bccn shared by tficm in common; all their gcneric characteristics being inherited from tliis ancestor. Only the slight differcnccs i n minor points, ~vliich distinguish one species of a genus from another, liare been acquired since the two diverged, and not even all of these slight differences, for a diffcrence brtmcen two allied specics may be due to the fact that wliile one has been modified the otlier has retained, unmodified, certain resemblances to their commoIi ancestor. We know that thc dtiration of even the moat persistcnt species is only an infinitesinial part of the wholc history of their evolution, and it is clear that the common cliaracteristics of two allied species must outnumber, thousands of times, tlie differences between them. It follows that the parents of any possible hybrid must be alike in thousands of features for one in which they differ. I t is therefore out of the question to attempt to prove, from the phenomena of crossing, that each parent can transmit to tlie child all its characteristics. Crossing simply results in tlie formation of a germ by the union of a male and a female element derived from two essentially similar parents, with at most only a few secondary
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and comparatively slight differences, all of which have been recently acquired. If a perfect animal couId be developed from the spermatozoon of a male parent, as it can be, in cases of parthenogenesis, from the ovum of a female parent, we should have a means of proving that each sex transmits its entire organization to its offspring. The phenomena of parthenogenesis prove that the female does actually thus transmit its entire organization, but there is nothing to show that the male parent docs also, for it is clear that, from the nature of the case, the phenomena of crossing are incompetent to prove it.
The Argument from the Homology of the X a l e aitd Female Sexual Elenients. Many authors have gone much further than the statement that any charactcristic whatever may be transmitted by either parent, and have held that the offspring is actually a dual personality, made up of a complete organization or indjvidualitg inherited from the father, and another, equally complete, inherited from tlic mother. This view has found favor with a number of modern writers, and frequently makes its appearance in tlic literature of the subject. Thus Huxley says (Em~cZoop. Brit., Art. Evohitioii), “It is conceivable, and indeed probable, tIiat cvcry part of the adult contains molecules derived from the malc and from the female parent; and that, regarded as B mass of molecules, the entire organism may be compared to a web, of which the warp is derived from the female, and the woof from the male. And each of these may constitute an individuality in the same sense as the whole organism is one individual, although the matter of the organism has been continually changing.”
Qarious Opinions on Heredity.
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+
I t will be found, on examination, that there is niuch to be said in support of this view, although I believe that there is a much simpler explanation of the facts which seem to favor it. The only reason given by Huxley, in the article above quoted, is the homology between the ovum and the spermatozoon; the fact that in all the higher animals and plants the germ is formed by the unioii of one nucleated cell, the ovum, with another more or less modified nucleated cell, tlie male cell, and that the structural components of the body of the embryo are all derived, by a process of division, from the coalesced male and female germs. I n answer to this we may point out that while the hypothesis requires that a wasp born from a fcrtilize& egg should differ essentially from one born from a parthenogenetic egg, the one being a dual person and the other a unit, we do not find any obvious difference corresponding to tlie supposed molecular difference. We should not expect a wasp with a dual personality to be, to all appearances, exactly like one with a single personali ty. A fatal objection to Huxley’s argument, above given, is that, at bottom, it is simply an assumption that the homology or morphological equivalence of the ovum and male cell proves their functional equivalence. The fallacy of this assumption hardly needs notice, since it is well linomn that hotnology is no evidence whatever of functional resemblance. The quill feathers which fit a bird’s wing for flight are homologous with the scales whicli cover and protect the arms. and fingers of a crocodile, but we conld hardly name two structures wliich scrve more different purposes. The homology between them simply indicates that, at some time in their his-
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Berediiy.
tory, both scales and feathers have had a common origin i n an epidermic structure, which has gradually Become specialized into these organs. While tlie homology between the ovum and the male cell is no reason for assuming that their fnnctions are now dike, the constant differences between them, througliout almost all of the organic world, seem to afford a very convincing reason for believing that their functions have been specialized in two divergent dircctions. If we can sliow that good might have resulted to the organism from such specialization, and from the rostriction of certain parts of the rcproductive fnnction to one element, and tlie restriction of otliers to the other, we may feel confident that, provided rariations i n tliese directions have a t any time arisen, natural selection would ham seized upon and yerpetnated them. I h o p to show the great usefulness of a specialization of this sort, and if I can do so, it is clear that the known differences between tlic ovum nad the ~perrnatozoonare reasons for a belief in its existence, while the only conclusion mliicli can be drawn from tlie homology bctween them is, that at one time tlieir functions were alike.
The Arguments .frona the Transmission of Latent S'ezua2 C?baracteristics; from Beaersion, und from AItermt i o n of Generulions. I n addition to tlie reason given by Xuxley for a belief in the dual natiire of each orginism, lie might have adduced the fact that tlie cliuracteristics of each sex are potential and latcnt iu the organism of t h e opposite sex, as is proloved by the trmsmision by ,a falber to his d:llrghter of characteristics inherited from his grandmother. The fact that the characteristics of one sex are latent
Various Opinions o n Heredity.
105
in the organism of the other is proved by countless wellknown illustrations, and i t seems, at first sight, t o afford evidence of the dual persopzlity of each animal. The fact in itself is so interesting that, while I believe in tlie possibility of a much simpler and more satisfactory explanation, it will not be out of place to devote a little space to the subject. ‘‘In every fcmnle all the secondary male characters, and in every male all the secondary female characters, apparently exist in a latent state, ready to be evolved nnder certain conditions” (Darwin, Variation. VoZ. ii.
p. 68). A perfect beard often begins to grow upon the face of a woman after the power of reproduction is lost by age or disease. Such women are often alluded to by Roman authors nnder the name of “ viragines,” and Hippocrates (De iI401-b. PuZg., Lib. vi. 55-56) has left us the description of two well-marked instances. Aristotle (Hist. Aizimal, ix. cap. 36) gives an account of a hen which had ceased laying, and assumed the characteristics of the male bird, and similar change i n female birds has been recorded by many writers. It has been observed in tlie hen, common pheasant, golden phcasant, silver pheasant, turkey, pea-hen, partridge, bnstard, pelican, various ducks, cuckoo, cotinga, chaffinch, bunting, and other birds. The change may be produced by age, by disease of the ovaries, removal of the ovaries, and even (Yarrel, Phil. Tran,?, 182‘;: ii. p. 268) by r e m o d of part of the oviduct. Old liens which have stopped laying often acquire a comb, wattles, spurs, tho brightly-colored plumage and long tail-feathers of the cock, assume the habits of the malc, and w e n learn to crow. The bad character, as lajers, of crowing hens, has even given rise to a proverb.
106
Heredity.
According to Darwin, Waterton gives a curious case of it hen which had ceased laying, and had assumed the plumage, voice, spurs and warlike disposition of the cock: when opposed to an enemy she mould erect her hackels and show fight. Female deer often acquire the horns, peculiar hair, ears, odor, and sexual desire of the males. On the other hand, it is well known that the secondary sexual Characteristics of male animals are more or less completely lost when they are subjected to castration. Darwin states, on the authority of Yarrell, that if t h e operation be performed on a young cock, he never crows again; the comb, wattles and spurs do not grow to their full size, and the hackels assume an intcrmcdiate appearance between the true hackels and the feathers of the hen. Similar results are said to be produced by confinement. Buffon states (Hist. Nat., Tom. vi. p. 80) that the horns of a stag castrated during the rutting season become permanent, but that new horns do not usually appear if i t is castrated when out of heat. Simpsonsays (Cyc. qfAnat., 702. ii. p. 71’7), “From the frequency with which castration is performed, the effect of the testes in evolving the general sexual peculiarities of the male have been more accurately ascertained than that of the ovaries upon the female constitution. These effects vary according to the age a t which the removal takes place. When an animal is castrated some time before it reaches the term of p b e r t y , the distil;ctive characteristics of the male are in general nemr devcloped; and the total absence of these cl~aractcrs,togcthcr with the softness of their-tissues, the contonr of their form, the tone of their voice, and their want of encrgy and vigor, assimilate them more in appearancc and
Various Opinions o n Heredity.
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habits to tlic female than to the male type. If the testicles are rcnioved nearer the period of puberty, or a t any time after that term lias occurred, and when tlie various male sexual l~cculiaritieshave been already developed, the effect is seldom so striking: tlie sexual instinct of tlie animals, and the energy of cliaracter wliicli these instincts impart, are certxirily more or less completely destroyed, and tlic tone of the voice is sometimes changed to tliilt of puberty, but the general male cliaracter of form, siicli as tlic beard in man, and the liorns of ruminants, gciicrnlly continue to grow.” D;1rivin, after reviewing these facts, concludes as follows: 66 We thus see that in many, probably in all cases, tlic secondary sexual characters of each sex lie dormant or latent in the opposite sex, ready to be evolved under peculiar circumstances. “ We can thus undcrstand horn, for instance, it is posz:lsle for a good milking cow to transmit her good niilking qualities through her male offspring to future gcncrations, for we may confidently believe that ‘these qualities are present, thougli latent, in the males of each generation. So it is with the game-cock, mlio can transmit his snpcriority in courage and vigor through his female to his male offspring; and with man it is known that diseases neccssarily confined to the male sex can be transmittcd through tlic female to the grandson. Such Eascs are iiitelligiblc on the belief that characters common to the grandparent and tlie grandchild of the same sex are present, though latent, in the intermediate paren t of tlic opposite sex.” Facts of this sort certainly seem, a t first sight, to show the existence in each individnal of two complete individualities, one from each parent; and the presence in each
...
108
Neredit y.
sex, in a latent condition, of the organization of the other sex ;but i t is not difficult to show that the plienome m in question adniit of a much simpler explanation. I n most cases when the sexes differ from each other i n what are known as secondary sexual characteristics, that is, features which are not directly concerned in the reproductive function, the mature male is more different than the maturefemale from the young. I shall discnss this subject more fully in another place, so I shall give only a few illustrations a t present. I t will be sufficient to call attention to the resernblance betmcen tIic smooth face of a woman and the face of either a boy or a girl, as contrasted with the bearded face of a miin. The voice of a wonian, the voice of a girl, and that of a boy, a11 resemble each othcr, and all differ from the voice of st man in tlie same, or nearly tlie same, respects. I n fowls tlic young of both sexes are much like the adult female i n form and color. These familiar instances are enough for our present purpose, and they show that, so far as the secondary sexuil characteristics are concerned, the female is, as a rule, distinguished from the male by her failure to acquirc tlic fully developed clinracteristies of the race. I n these respects the female j s an arrcstecl male, and this is me11 shown by that fact that while the females and young of two closely related species of mild animals may be so mncli alike tlint they can Iiardly be rlistingriished, thc adult males may bc very different from each other. All we need t o m u m e , then, i n ordcr to each a simplc explanation of the sccondayy sexual diff crenccs betwccii tlic sexes, is that each ovum has the pomer to derelop into an organism with all the characteristics of the species, but that the fernale function acts, in some
Various Opinions on Heredity.
109
J\':I?, to :wrest the general organization somcwhat short of f u l l lwfection. Jvc can also mdcrstand that the power to develop perfccily :ind to assnmc the charactcristics of the species niiglit rcmain Iatcnt in tlic female, and might come into action aftcr the loss of rcprodnctive power. Accorcling to this view, thc possession of a beard must be rcgmlcd as a general cliaracteristic of our race, inhcritod by all children, girls as well :is boys. The derclopmcnt, in the girl, of the fenide rcprodactive fnnction, or the 1;iclr of the stimulus which comes, in the male, from the dcvclopmeiit of tlie malc fnnction, arrests the dcrelopmciit of tlie beard, :iltho~igliits po\vcr for growth may rcninin htcnt, and may come into more or less perfect activity aftcr the period of reproduction is past. A careful cxamination of tlic examples given above mill bring out the interesting fact that when a female, from disease or mutilation or old age, assumes a resemblance to tlic male, the cliange is an advance, nnd consists in t he acqnjsition of structures not usually present i n the female. When, on the otlicr hand, the male, from castration or confinement, comes to resemble the fcmJC, the resemblance is due, in most cases, to arrest, or a failnre of the malc to acquire the adult male characteristics of the species. Sinipson (Hermaplwoditism, Cyc. of Annt. and Phys., Val. ii. 11. '719) gives the following summary of the subjcct: " The consideration of the rnrious facts that we have now stntcd inclines us to the belief that the natural history c1i:irnctcristics of any species of animal are certainly nct to bc songlit for solely either in the system of the mclc or in that of the female; bnt as Mr. Hunter pointed out, they are to be found in those properties that are
110
Heredity .
common to both sexes, and which w Iiaw occasionally seen combined togetlicr by m ~ t u r cupon the bodies of hermnplirodites, or evolved from tlic intcrfercnce of art upon a castrated male or a spaled female. “ I n assuming at tlic age of puberty tlic distinctire secondary peculiarities of his sex, tlie male, so f:ir as regards tliese secondary peculiarities, CT idently passes iiito a higher degree of dcvelopmcnt tlian tlic fcm:ilc, and lcavcs her more in poswssion of those cliamcters that arc common to thc young of both sexes, and which he himself never loses when his testicles are early removed. Tlicse and other facts connected with thc CTOlntion of both the primary and the secondary pccnliarities of the sexes farther appear t o us t o show that, ph~siologicallyat least, we ought to consider tlie male type of orgnnization to be tlie more perfect, as respects the individual, and the female as respects the species. Hence we find that, when the femalc is malformed in the sexnal parts so as to resemble the male, the m d formation is almost always one of excessive development, and, on the other hand, wlien tlie male organs are malformed in sncC a manner as to simulate the female, tlie abnormal appesrance is generally to be traced to a defect of development. I n tlie s:bme WRY, alien tlie female assnmes the secondary characters of the malc it is either, first, when by original malformation its own ovaries and sexnaI organs are so defective in structure as not to be capable of taking a part in the function of reproduction, and of exercising tliat inflnencc over the general orgnnization which this faculty imparts to them; or, secondly, when in tlie eoiiree of Rgc the ovaries hare ceased to be capable of pcrforming the action allotted to them in the reproductive process. I n both of tliese cases we obserre the powers of the female organization, now that its
Various Opinions o n Heredity.
111
capabilities for performing its particular office in the continuation of tlic species are wanting or lost, expend themselves in perfecting its own individual system, and herice the aninial gradually assumes more or fewer of the sceondary sexual characters tlirit belong to the male.” It is true that, in a few instances, the male has been knowii to acquire true feminine characteristics, foreign to normal males. Thus, according to Darwin, “characteristics properly corifined to the female are likewise acquired: the capon takes to sitting on eggs, and will briug u p chickens; and what is more curious, the utterly sterile male hybrids from the pheasant and fowl act in the same manner, their delight being to watch when the hen 1e:tves the nest, and to take on themselves the office of a sitter. Many male birds normally sit, and hatch the eggs, and there are reasons for believing that the incubating habit was originally shared by both sexes, and I am therefore inclined to attribute such cases as this to reversion to a remote male ancestor, rather than to the acquisition by tlic male of a female characteristic. We may conclude, then, that the transmission by one sex, in a latent condition, of the secondary characteristics of the opposite sex, does not compel us to believe in the dual sexual personality of each individual, since we have a much simpler explanation in the view that each embryo inherits the power to derclop all the characteristics of the species, but that this power does not fttlly manifest itself in the female. It may seem di6cnlt to explain in this way the transmission by a bull of the good milking qualities of his mother, or the capacity occasionally shown by male ma!nmnls of yielding milk, but it is surely simpler to a ~ n i i i c that each male inherits, like the females, the
112 power of devclopiag perfect functional mammze, and that this power is arrested in the male, than to assume that each male animal includes in itself a complete female duplicate. An illustrationmay make the subject more clear. Certain embryo bees, when exposed to certain conditions, develop into sterile workers, but when exposed to another set of conditions they become fertile females. The differences between the workers and the queens are not confined to the reproductive organs, but extend to the shape and size of the body, the general organization, and to the instincts of the animals. These differences are not due to the direct action of the conditions to wliicli the young are exposed, but are truly hereditary, as we see from the fact that the workers of different species are as distinct and as characteristic of their species as the maIe or the fertile females. Now which is simplest, to assume that each female embryo has a complete worker organization and a complete queen organization, or to hold that it has the power to develop a11 the characteristics common to both, and also the distinctive characteristics of each; that one set of conditions suppresses the distinctive characteristics of a perfect queen, while another set of conditions arrests those of a perfect worker ? The argument in favor of the multiple personality of individuals which is furnished by polymorphic comniunities is at least as strong as that furnished by the latent transmission of secondary sexual characteristics. I n the case of the polymorphic hjdroids an egg-embryo may give rise, by budding, t o certain descendants with fully developed digestive organs, but with no organs of locomotion or reprodiictive organs, to other descendants with organs of locomotion, but without diges-
Various Opinions o n Heredity.
113
tive organs or reprodnctive organs, and to still others with rcprodnctive orgalls, but with no organs of digestion or locomotion. All these forms are hereditary and arc characteristic of the species, so there is no escape from tlie conclusion that they a11 are present in some form i n the egg-embryo, and i t is certainly natural to snspect that the entire organization of each one of them is latent in this embryo, but the explanation which I 1iay-eproposcd to account for tlie transmission of secondary sexual characteristics, applies to such cases as this jnst as well. The hypothesis that the egg-embryo inherits and transmits to each of its descendants, those produced asexually as well as those produced sexually, all the characteristics of the species, and that it also inherits and transmits to each of them a tendency to suppress certain of these cliaracteristics under certain conditions, seems to furnish a simple and satisfactory esplanatioil of all tlie facts. According to this view the feeding zooids of a polym orpli i c Si 1’11on oph ore are in d i v i d n a1s mli i ch h ave i nheritcd in full all the cliaracteristics of the race, but which do not attain to perfcct development in all respects. The swimming zooids are similar individuals, with other characteristics suppressed, and so on. This explanation seems mnch more satisfactory than the snpposi tion that the egg-embryo contains one complete personality for feeding zooids, one for locomotor zooids and one for reproductive zooids, and J hope that this case will make clearer the lack of necessity for asassuming the dual personality of each malc or female animal, so long as we liave a mnch simpler explanation in tlic liypothesis that each embryo lias the power to develop all the clinracteristics of the species, together with a tendency to suppress certain ones in each sex.
114
Heredity.
A little thought will show that if there were no explanation of the transmission of latent sexual characteristics more simple than the hypothesis of a dual personality, this hypothesis would then be too simple, and would nced to be made much more complicated. The characteristics of the opposite scxare not the only ones which may be latent, aiid in cases of reversion it parent may transmit to children characteristics which were exhibited by neither parent nor grandparent, and whic!: may have remained latent for many generations. If we must assume the existence of a dual personality t o acconnt for the latent transmission of the charncterietics of the grandparent of the opposite sex, we must assume still other personalities to account for rel-ersion t o more remote ancestors, and Darwin has not hesitated to carry the hypothesis to this, its logical conclasion. He says (Variatior~,ii. 65), " Smeral a u t h o ~ sl m e maintained that hybrids and mongrels include all the characteristics of both parents, not fused togetlier but merely mingled in different proportions in different parts of the body; or, as Naudin has exprcsscd it, a Iiybrid is a ]iring mosaic work, in which the cye cannot distinguish the discordant elements, so completely are they intermingled. We can Iiardly doubt that, i n a certain sense, this is true, as when we behold in a hybrid the elements of both species segregating thcmselocs into segment in the same flower or fruit-bya process of selfattraction or self-affinity-this segregation taking place either by seminal or by bud propagation. Naudin further believes that the segregation of two specific elements or essences is eminently liable to occur in the male and fenmle reproductire matter, and he thus explains the almost universal tendency to reversion in successive hybrid generations. . But i t would, I suspect,
. .
Various Opinions
on
Heredity.
115
be more correct to say that tlie elements of both parent species cxist in every hybrid in a double state, namely, bIcnded together and completely separated.” In anotlier place (J’hrintion, ii. p. 80) he says: “ O n tllc doctrine of reversion, as given in this chapter, the germ becomes a far more marvellous object, for besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characteristics, proper to both sexes, to both the riglit and left sides of the body, m d to a long line of ancestors, male and fcmale, separated by hundreds or cveii thousands of generations from the present time, aud these characters, like those written on paper with invisible ink, all lie ready t o be cvolrcd under certain known or unknown conditions. ’’ I shall discuss the phenomena of reversion somemhat at length i n another place, mid wish to simply call attention a t present to the fact that here, as in the case of secondary sexual characters, me have a much simpler explmntion in tlie liypotliesis of arrc-t, and tlierefore do not necd to call in an iiiiknown factor, such as the multiple 1~ersonalityof each individual. I tliitik that the plienomcna of alternation of generations favor this latter supposition even more than the facts of reversion. The egg embryo of a hydro-medusa may give rise by budding to an indefinite number of hydroids like itself, arid each of these may give rise to other hydroids, and so on indefinitely. Em11 one of thcse may also, under certain conditions, give rise to medusa quite different from the hydroids and like tlic original medusa. As the medusa which are thus produced inherit through a long series of hydra ancestors all the specific characteristics of the origi-
116
Heredity.
we are forced to conclude that e:ich 117 clrciicl ~oiitaiiis,i n a latent state, the 1)ower to rcprodnc~a dcGnite specific medosn. As the hydra and its rncdusn differ from cach otlrcr rcry milch more than :Lmalc and :t female ni:iiiim:il. xntl have little i n common except the general plan of tlicir org:inization, there seems :kt first to be 110 r e c a p ft 0111 tlic coticlusion that the mcdasa structnrc exists side L? side with the hjdra striictiire, in each lijdruicl, as :i sccond personality. I hope to show, in the chapter on asexual rcprotliiction that alternation of gcncriitions is a seco1iii:ii.y condition of things, and tliat i t lias been bronght abutit Iiy a modification of ordinary nietamorpliosis. I think there is every reasoil to Lelicve t h t a t one time the hydra-larva wliich liutched from a rnetltis:i c ~ bcg came rnetamorphoscd, by a gmdual cliauge during gron tii, into a medusa. If this mere the case now, there mould be 110 more reason for believing in D hgdra pcrhoti:ility nud n rncdus:~ personality than thcre is for belie\ing tliiit a 11li111i~t1 child contains a distinct adnlt personality. Now me can understand that if snch D 1nrr-s slioirl~l give rise by budding to otlicr hydroids like itself, tlioy also wotild have the power t o grow into matnre medusae. We can also understand that circumstances might wise to cause the later stages in the development of some of these hydra-larv2e to become latent. We should tlrrn have two generations-hydroids without a nicdtisn st .get and hydroids with a medusa stage. The suppression of tho ligdra features of tlic l#ittcr would tlien gire 11s a generation of niedosre w i t h no liydra stage, giving birth t o a generation of liydruids with no medusa stage, and these iu turn producirig a
11111 medusa,
Various Opinions o n Heredity.
137
generation of mednsz with no hydra stage. We should then linvc a case of alternation like that which is presented by ordinary hydro-niednsae.
Szwnmwy of Chapter. A careful review of the reasons mhicli have induced various authors to believe that either sexnal element may transmit any cliaracteristic whatever, leads to the coiiclusion that its truth is not proven. It is impossible to prove i t by the phenomena of crossing, since the only animals mliicli can be made to cross are essentially alike, and differ only in minor points. The homology bet&en the ovum and the male ccll is no reason for supposing that their functions are similar, and the differences between them should lead us to believe that their functions are not alike. There is no reason for assuming that each sex transmits its entire organization to the offspring, in order to acconnt for the latent transmission of secondary sexnal eh:ir:~eteristics, since this triznsmission can be more simply explained by assuming that each embryo inherits but docs not necessarily develop all the characteristics of its Fl’ccies. Reversion and alternation of generations admit of a similar explan at’ion. We may therefore conclude that there is and can be no proof that each sexual element transmits d l the characteristics of the parent, and that there is no a priori absurdity in the liypothesis that the male and female reproductive elements are unlike i n function, and are specialized in different directions. We can therefore enter without prejudice into an examination of the evidence for this latter view.
.
CEIAPTER V. ON THE OPINION THAT E d C H SEX YAY TRANSMIT ANY CHARACTERISTIC WHATEVER.
The argument from hybrids-This
argument is inconclusiveThe argument from the homology between the ovum and the male cell-Hon>ology does not iiivolve functional similarityThe argument from the dual personality of each individual; from reversion ; and from polymorphism-These phenomena admit of a simpler explanation-Summary of chapter.
The Arpmeizt from Bybrids. According to the view to be presented in this work, the functions of the two sexual clements, in inheritance, arc not alike. Tlic proof of this mill be presented rnrther on, when tlic snbjcct is reached in the logical course of the dcvelopni en t of o 11r argument. Some of thc very liigliest autliorities have been led to a view wliicli is directly opposite, and lime lield that citlicr parent may transmit to the offspring any characteristic whatever. Lest any reader should assume, a t the beginning of this book, that the work involves an absurdity, and that my conclusion is already d i s p r o d , it seems best to at oncc examine the reasons for the opposite view. If I can show that these reasons are inconclusive, and tliat tliere is and can be no proof for tlic statement that each sexual element transmits to the offspring every characteristic of the parent, we can then enter into the subject without prejudice, and can wait
100
Heredity.
for the proper time to present tlie proof of tlre opposite fview, tliat tlie two sexual elements play different parts i n heredity. If tlic authority of great names counted for anything Tvlmtever in sciciice, the case against me would be w r y strong, but where an appeal to natiirc is possible, authority counts for nothing. Darwin’s place among the students of heredity is certainly the highest, and lie takes very strong ground indeed upon this subject. Thus lie says (Variation of Animals and Plaids, Vol. ii. p. 8 8 ) : “I am aware that such cases (of prepotency) have been ascribed by various authors to snch rules as that the father influences the exteriial characters, and tlie motlicr the internal cliaracters. “ But tlie great diyersity of tlie rules given by various authors almost proves their falseness. Dr. Prosper Lucas lias fully discussed this point, and has shown tlii~tnone of tlic rnles (and I could :tdd others to those quotccl by him) apply to ;ill animals. Similar rules have been announced for plants mid have been proved by Giiirtucr t o be all erroneous.” In the Atiatoiiiy of Inverte6rnted Aniiiinls, 11 30, IIuxlcy states tliut “ n o structural modification is so slight, and n o fonctional pcca1i:trity is so insignificant i i eitherparent, ~ tliat i t may not malie its appearaim in the offspring.” D ~ r w i n ,in many parts of his mritings, is still more explicit. Thus he says (Vcci.iatioit of Aizin~uls nqzd Platrts, Vol. ii. 13. 431): “Ovules and tlic male clement, before tbcy become united, have, lihc bnds, :ti1 indcpcndent existence. Both have tlic p o ~ ufr t i x i i s m i ttiiig every sinyZc character possessed by the parent fol ni. We see this clearly when hybrids are paired inter se, for
Various Opinions o n Xeredity.
101
c
the characters of either grandparcnt often reappear, citlier perfectly or by segnicnts, in the progeny. It is I r i L error t o sappose that the iiiale traiisnzits certain charac. t p r s mail the jEinnZe other chni*acfers.” I tliiiik a little cxarnination will show clearly the impossibility of proving this statement from the phcnomeI n order to brecd together animals must i i i t of crossing. I)c closcly related; they must belong to the samc species or to two closcly allied species. Since the individuals which belong to two closcly related species are the dosccudants ‘of a common, and not very remote, anccstral spccies, it is clear that almost the wliole course of their evolntion has bccn shared by tficm in common; all their gcneric characteristics being inherited from tliis ancestor. Only the slight differcnccs i n minor points, ~vliich distinguish one species of a genus from another, liare been acquired since the two diverged, and not even all of these slight differences, for a diffcrence brtmcen two allied specics may be due to the fact that wliile one has been modified the otlier has retained, unmodified, certain resemblances to their commoIi ancestor. We know that thc dtiration of even the moat persistcnt species is only an infinitesinial part of the wholc history of their evolution, and it is clear that the common cliaracteristics of two allied species must outnumber, thousands of times, tlie differences between them. It follows that the parents of any possible hybrid must be alike in thousands of features for one in which they differ. I t is therefore out of the question to attempt to prove, from the phenomena of crossing, that each parent can transmit to tlie child all its characteristics. Crossing simply results in tlie formation of a germ by the union of a male and a female element derived from two essentially similar parents, with at most only a few secondary
102
HerecZity.
and comparatively slight differences, all of which have been recently acquired. If a perfect animal couId be developed from the spermatozoon of a male parent, as it can be, in cases of parthenogenesis, from the ovum of a female parent, we should have a means of proving that each sex transmits its entire organization to its offspring. The phenomena of parthenogenesis prove that the female does actually thus transmit its entire organization, but there is nothing to show that the male parent docs also, for it is clear that, from the nature of the case, the phenomena of crossing are incompetent to prove it.
The Argument from the Homology of the X a l e aitd Female Sexual Elenients. Many authors have gone much further than the statement that any charactcristic whatever may be transmitted by either parent, and have held that the offspring is actually a dual personality, made up of a complete organization or indjvidualitg inherited from the father, and another, equally complete, inherited from tlic mother. This view has found favor with a number of modern writers, and frequently makes its appearance in tlic literature of the subject. Thus Huxley says (Em~cZoop. Brit., Art. Evohitioii), “It is conceivable, and indeed probable, tIiat cvcry part of the adult contains molecules derived from the malc and from the female parent; and that, regarded as B mass of molecules, the entire organism may be compared to a web, of which the warp is derived from the female, and the woof from the male. And each of these may constitute an individuality in the same sense as the whole organism is one individual, although the matter of the organism has been continually changing.”
Qarious Opinions on Heredity.
103
+
I t will be found, on examination, that there is niuch to be said in support of this view, although I believe that there is a much simpler explanation of the facts which seem to favor it. The only reason given by Huxley, in the article above quoted, is the homology between the ovum and the spermatozoon; the fact that in all the higher animals and plants the germ is formed by the unioii of one nucleated cell, the ovum, with another more or less modified nucleated cell, tlie male cell, and that the structural components of the body of the embryo are all derived, by a process of division, from the coalesced male and female germs. I n answer to this we may point out that while the hypothesis requires that a wasp born from a fcrtilize& egg should differ essentially from one born from a parthenogenetic egg, the one being a dual person and the other a unit, we do not find any obvious difference corresponding to tlie supposed molecular difference. We should not expect a wasp with a dual personality to be, to all appearances, exactly like one with a single personali ty. A fatal objection to Huxley’s argument, above given, is that, at bottom, it is simply an assumption that the homology or morphological equivalence of the ovum and male cell proves their functional equivalence. The fallacy of this assumption hardly needs notice, since it is well linomn that hotnology is no evidence whatever of functional resemblance. The quill feathers which fit a bird’s wing for flight are homologous with the scales whicli cover and protect the arms. and fingers of a crocodile, but we conld hardly name two structures wliich scrve more different purposes. The homology between them simply indicates that, at some time in their his-
104
Berediiy.
tory, both scales and feathers have had a common origin i n an epidermic structure, which has gradually Become specialized into these organs. While tlie homology between the ovum and the male cell is no reason for assuming that their fnnctions are now dike, the constant differences between them, througliout almost all of the organic world, seem to afford a very convincing reason for believing that their functions have been specialized in two divergent dircctions. If we can sliow that good might have resulted to the organism from such specialization, and from the rostriction of certain parts of the rcproductive fnnction to one element, and tlie restriction of otliers to the other, we may feel confident that, provided rariations i n tliese directions have a t any time arisen, natural selection would ham seized upon and yerpetnated them. I h o p to show the great usefulness of a specialization of this sort, and if I can do so, it is clear that the known differences between tlic ovum nad the ~perrnatozoonare reasons for a belief in its existence, while the only conclusion mliicli can be drawn from tlie homology bctween them is, that at one time tlieir functions were alike.
The Arguments .frona the Transmission of Latent S'ezua2 C?baracteristics; from Beaersion, und from AItermt i o n of Generulions. I n addition to tlie reason given by Xuxley for a belief in the dual natiire of each orginism, lie might have adduced the fact that tlie cliuracteristics of each sex are potential and latcnt iu the organism of t h e opposite sex, as is proloved by the trmsmision by ,a falber to his d:llrghter of characteristics inherited from his grandmother. The fact that the characteristics of one sex are latent
Various Opinions o n Heredity.
105
in the organism of the other is proved by countless wellknown illustrations, and i t seems, at first sight, t o afford evidence of the dual persopzlity of each animal. The fact in itself is so interesting that, while I believe in tlie possibility of a much simpler and more satisfactory explanation, it will not be out of place to devote a little space to the subject. ‘‘In every fcmnle all the secondary male characters, and in every male all the secondary female characters, apparently exist in a latent state, ready to be evolved nnder certain conditions” (Darwin, Variation. VoZ. ii.
p. 68). A perfect beard often begins to grow upon the face of a woman after the power of reproduction is lost by age or disease. Such women are often alluded to by Roman authors nnder the name of “ viragines,” and Hippocrates (De iI401-b. PuZg., Lib. vi. 55-56) has left us the description of two well-marked instances. Aristotle (Hist. Aizimal, ix. cap. 36) gives an account of a hen which had ceased laying, and assumed the characteristics of the male bird, and similar change i n female birds has been recorded by many writers. It has been observed in tlie hen, common pheasant, golden phcasant, silver pheasant, turkey, pea-hen, partridge, bnstard, pelican, various ducks, cuckoo, cotinga, chaffinch, bunting, and other birds. The change may be produced by age, by disease of the ovaries, removal of the ovaries, and even (Yarrel, Phil. Tran,?, 182‘;: ii. p. 268) by r e m o d of part of the oviduct. Old liens which have stopped laying often acquire a comb, wattles, spurs, tho brightly-colored plumage and long tail-feathers of the cock, assume the habits of the malc, and w e n learn to crow. The bad character, as lajers, of crowing hens, has even given rise to a proverb.
106
Heredity.
According to Darwin, Waterton gives a curious case of it hen which had ceased laying, and had assumed the plumage, voice, spurs and warlike disposition of the cock: when opposed to an enemy she mould erect her hackels and show fight. Female deer often acquire the horns, peculiar hair, ears, odor, and sexual desire of the males. On the other hand, it is well known that the secondary sexual Characteristics of male animals are more or less completely lost when they are subjected to castration. Darwin states, on the authority of Yarrell, that if t h e operation be performed on a young cock, he never crows again; the comb, wattles and spurs do not grow to their full size, and the hackels assume an intcrmcdiate appearance between the true hackels and the feathers of the hen. Similar results are said to be produced by confinement. Buffon states (Hist. Nat., Tom. vi. p. 80) that the horns of a stag castrated during the rutting season become permanent, but that new horns do not usually appear if i t is castrated when out of heat. Simpsonsays (Cyc. qfAnat., 702. ii. p. 71’7), “From the frequency with which castration is performed, the effect of the testes in evolving the general sexual peculiarities of the male have been more accurately ascertained than that of the ovaries upon the female constitution. These effects vary according to the age a t which the removal takes place. When an animal is castrated some time before it reaches the term of p b e r t y , the distil;ctive characteristics of the male are in general nemr devcloped; and the total absence of these cl~aractcrs,togcthcr with the softness of their-tissues, the contonr of their form, the tone of their voice, and their want of encrgy and vigor, assimilate them more in appearancc and
Various Opinions o n Heredity.
107
habits to tlic female than to the male type. If the testicles are rcnioved nearer the period of puberty, or a t any time after that term lias occurred, and when tlie various male sexual l~cculiaritieshave been already developed, the effect is seldom so striking: tlie sexual instinct of tlie animals, and the energy of cliaracter wliicli these instincts impart, are certxirily more or less completely destroyed, and tlic tone of the voice is sometimes changed to tliilt of puberty, but the general male cliaracter of form, siicli as tlic beard in man, and the liorns of ruminants, gciicrnlly continue to grow.” D;1rivin, after reviewing these facts, concludes as follows: 66 We thus see that in many, probably in all cases, tlic secondary sexual characters of each sex lie dormant or latent in the opposite sex, ready to be evolved under peculiar circumstances. “ We can thus undcrstand horn, for instance, it is posz:lsle for a good milking cow to transmit her good niilking qualities through her male offspring to future gcncrations, for we may confidently believe that ‘these qualities are present, thougli latent, in the males of each generation. So it is with the game-cock, mlio can transmit his snpcriority in courage and vigor through his female to his male offspring; and with man it is known that diseases neccssarily confined to the male sex can be transmittcd through tlic female to the grandson. Such Eascs are iiitelligiblc on the belief that characters common to the grandparent and tlie grandchild of the same sex are present, though latent, in the intermediate paren t of tlic opposite sex.” Facts of this sort certainly seem, a t first sight, to show the existence in each individnal of two complete individualities, one from each parent; and the presence in each
...
108
Neredit y.
sex, in a latent condition, of the organization of the other sex ;but i t is not difficult to show that the plienome m in question adniit of a much simpler explanation. I n most cases when the sexes differ from each other i n what are known as secondary sexual characteristics, that is, features which are not directly concerned in the reproductive function, the mature male is more different than the maturefemale from the young. I shall discnss this subject more fully in another place, so I shall give only a few illustrations a t present. I t will be sufficient to call attention to the resernblance betmcen tIic smooth face of a woman and the face of either a boy or a girl, as contrasted with the bearded face of a miin. The voice of a wonian, the voice of a girl, and that of a boy, a11 resemble each othcr, and all differ from the voice of st man in tlie same, or nearly tlie same, respects. I n fowls tlic young of both sexes are much like the adult female i n form and color. These familiar instances are enough for our present purpose, and they show that, so far as the secondary sexuil characteristics are concerned, the female is, as a rule, distinguished from the male by her failure to acquirc tlic fully developed clinracteristies of the race. I n these respects the female j s an arrcstecl male, and this is me11 shown by that fact that while the females and young of two closely related species of mild animals may be so mncli alike tlint they can Iiardly be rlistingriished, thc adult males may bc very different from each other. All we need t o m u m e , then, i n ordcr to each a simplc explanation of the sccondayy sexual diff crenccs betwccii tlic sexes, is that each ovum has the pomer to derelop into an organism with all the characteristics of the species, but that the fernale function acts, in some
Various Opinions on Heredity.
109
J\':I?, to :wrest the general organization somcwhat short of f u l l lwfection. Jvc can also mdcrstand that the power to develop perfccily :ind to assnmc the charactcristics of the species niiglit rcmain Iatcnt in tlic female, and might come into action aftcr the loss of rcprodnctive power. Accorcling to this view, thc possession of a beard must be rcgmlcd as a general cliaracteristic of our race, inhcritod by all children, girls as well :is boys. The derclopmcnt, in the girl, of the fenide rcprodactive fnnction, or the 1;iclr of the stimulus which comes, in the male, from the dcvclopmeiit of tlie malc fnnction, arrests the dcrelopmciit of tlie beard, :iltho~igliits po\vcr for growth may rcninin htcnt, and may come into more or less perfect activity aftcr the period of reproduction is past. A careful cxamination of tlic examples given above mill bring out the interesting fact that when a female, from disease or mutilation or old age, assumes a resemblance to tlic male, the cliange is an advance, nnd consists in t he acqnjsition of structures not usually present i n the female. When, on the otlicr hand, the male, from castration or confinement, comes to resemble the fcmJC, the resemblance is due, in most cases, to arrest, or a failnre of the malc to acquire the adult male characteristics of the species. Sinipson (Hermaplwoditism, Cyc. of Annt. and Phys., Val. ii. 11. '719) gives the following summary of the subjcct: " The consideration of the rnrious facts that we have now stntcd inclines us to the belief that the natural history c1i:irnctcristics of any species of animal are certainly nct to bc songlit for solely either in the system of the mclc or in that of the female; bnt as Mr. Hunter pointed out, they are to be found in those properties that are
110
Heredity .
common to both sexes, and which w Iiaw occasionally seen combined togetlicr by m ~ t u r cupon the bodies of hermnplirodites, or evolved from tlic intcrfercnce of art upon a castrated male or a spaled female. “ I n assuming at tlic age of puberty tlic distinctire secondary peculiarities of his sex, tlie male, so f:ir as regards tliese secondary peculiarities, CT idently passes iiito a higher degree of dcvelopmcnt tlian tlic fcm:ilc, and lcavcs her more in poswssion of those cliamcters that arc common to thc young of both sexes, and which he himself never loses when his testicles are early removed. Tlicse and other facts connected with thc CTOlntion of both the primary and the secondary pccnliarities of the sexes farther appear t o us t o show that, ph~siologicallyat least, we ought to consider tlie male type of orgnnization to be tlie more perfect, as respects the individual, and the female as respects the species. Hence we find that, when the femalc is malformed in the sexnal parts so as to resemble the male, the m d formation is almost always one of excessive development, and, on the other hand, wlien tlie male organs are malformed in sncC a manner as to simulate the female, tlie abnormal appesrance is generally to be traced to a defect of development. I n tlie s:bme WRY, alien tlie female assnmes the secondary characters of the malc it is either, first, when by original malformation its own ovaries and sexnaI organs are so defective in structure as not to be capable of taking a part in the function of reproduction, and of exercising tliat inflnencc over the general orgnnization which this faculty imparts to them; or, secondly, when in tlie eoiiree of Rgc the ovaries hare ceased to be capable of pcrforming the action allotted to them in the reproductive process. I n both of tliese cases we obserre the powers of the female organization, now that its
Various Opinions o n Heredity.
111
capabilities for performing its particular office in the continuation of tlic species are wanting or lost, expend themselves in perfecting its own individual system, and herice the aninial gradually assumes more or fewer of the sceondary sexual characters tlirit belong to the male.” It is true that, in a few instances, the male has been knowii to acquire true feminine characteristics, foreign to normal males. Thus, according to Darwin, “characteristics properly corifined to the female are likewise acquired: the capon takes to sitting on eggs, and will briug u p chickens; and what is more curious, the utterly sterile male hybrids from the pheasant and fowl act in the same manner, their delight being to watch when the hen 1e:tves the nest, and to take on themselves the office of a sitter. Many male birds normally sit, and hatch the eggs, and there are reasons for believing that the incubating habit was originally shared by both sexes, and I am therefore inclined to attribute such cases as this to reversion to a remote male ancestor, rather than to the acquisition by tlic male of a female characteristic. We may conclude, then, that the transmission by one sex, in a latent condition, of the secondary characteristics of the opposite sex, does not compel us to believe in the dual sexual personality of each individual, since we have a much simpler explanation in the view that each embryo inherits the power to derclop all the characteristics of the species, but that this power does not fttlly manifest itself in the female. It may seem di6cnlt to explain in this way the transmission by a bull of the good milking qualities of his mother, or the capacity occasionally shown by male ma!nmnls of yielding milk, but it is surely simpler to a ~ n i i i c that each male inherits, like the females, the
112 power of devclopiag perfect functional mammze, and that this power is arrested in the male, than to assume that each male animal includes in itself a complete female duplicate. An illustrationmay make the subject more clear. Certain embryo bees, when exposed to certain conditions, develop into sterile workers, but when exposed to another set of conditions they become fertile females. The differences between the workers and the queens are not confined to the reproductive organs, but extend to the shape and size of the body, the general organization, and to the instincts of the animals. These differences are not due to the direct action of the conditions to wliicli the young are exposed, but are truly hereditary, as we see from the fact that the workers of different species are as distinct and as characteristic of their species as the maIe or the fertile females. Now which is simplest, to assume that each female embryo has a complete worker organization and a complete queen organization, or to hold that it has the power to develop a11 the characteristics common to both, and also the distinctive characteristics of each; that one set of conditions suppresses the distinctive characteristics of a perfect queen, while another set of conditions arrests those of a perfect worker ? The argument in favor of the multiple personality of individuals which is furnished by polymorphic comniunities is at least as strong as that furnished by the latent transmission of secondary sexual characteristics. I n the case of the polymorphic hjdroids an egg-embryo may give rise, by budding, t o certain descendants with fully developed digestive organs, but with no organs of locomotion or reprodiictive organs, to other descendants with organs of locomotion, but without diges-
Various Opinions o n Heredity.
113
tive organs or reprodnctive organs, and to still others with rcprodnctive orgalls, but with no organs of digestion or locomotion. All these forms are hereditary and arc characteristic of the species, so there is no escape from tlie conclusion that they a11 are present in some form i n the egg-embryo, and i t is certainly natural to snspect that the entire organization of each one of them is latent in this embryo, but the explanation which I 1iay-eproposcd to account for tlie transmission of secondary sexual characteristics, applies to such cases as this jnst as well. The hypothesis that the egg-embryo inherits and transmits to each of its descendants, those produced asexually as well as those produced sexually, all the characteristics of the species, and that it also inherits and transmits to each of them a tendency to suppress certain of these cliaracteristics under certain conditions, seems to furnish a simple and satisfactory esplanatioil of all tlie facts. According to this view the feeding zooids of a polym orpli i c Si 1’11on oph ore are in d i v i d n a1s mli i ch h ave i nheritcd in full all the cliaracteristics of the race, but which do not attain to perfcct development in all respects. The swimming zooids are similar individuals, with other characteristics suppressed, and so on. This explanation seems mnch more satisfactory than the snpposi tion that the egg-embryo contains one complete personality for feeding zooids, one for locomotor zooids and one for reproductive zooids, and J hope that this case will make clearer the lack of necessity for asassuming the dual personality of each malc or female animal, so long as we liave a mnch simpler explanation in tlic liypothesis that each embryo lias the power to develop all the clinracteristics of the species, together with a tendency to suppress certain ones in each sex.
114
Heredity.
A little thought will show that if there were no explanation of the transmission of latent sexual characteristics more simple than the hypothesis of a dual personality, this hypothesis would then be too simple, and would nced to be made much more complicated. The characteristics of the opposite scxare not the only ones which may be latent, aiid in cases of reversion it parent may transmit to children characteristics which were exhibited by neither parent nor grandparent, and whic!: may have remained latent for many generations. If we must assume the existence of a dual personality t o acconnt for the latent transmission of the charncterietics of the grandparent of the opposite sex, we must assume still other personalities to account for rel-ersion t o more remote ancestors, and Darwin has not hesitated to carry the hypothesis to this, its logical conclasion. He says (Variatior~,ii. 65), " Smeral a u t h o ~ sl m e maintained that hybrids and mongrels include all the characteristics of both parents, not fused togetlier but merely mingled in different proportions in different parts of the body; or, as Naudin has exprcsscd it, a Iiybrid is a ]iring mosaic work, in which the cye cannot distinguish the discordant elements, so completely are they intermingled. We can Iiardly doubt that, i n a certain sense, this is true, as when we behold in a hybrid the elements of both species segregating thcmselocs into segment in the same flower or fruit-bya process of selfattraction or self-affinity-this segregation taking place either by seminal or by bud propagation. Naudin further believes that the segregation of two specific elements or essences is eminently liable to occur in the male and fenmle reproductire matter, and he thus explains the almost universal tendency to reversion in successive hybrid generations. . But i t would, I suspect,
. .
Various Opinions
on
Heredity.
115
be more correct to say that tlie elements of both parent species cxist in every hybrid in a double state, namely, bIcnded together and completely separated.” In anotlier place (J’hrintion, ii. p. 80) he says: “ O n tllc doctrine of reversion, as given in this chapter, the germ becomes a far more marvellous object, for besides the visible changes to which it is subjected, we must believe that it is crowded with invisible characteristics, proper to both sexes, to both the riglit and left sides of the body, m d to a long line of ancestors, male and fcmale, separated by hundreds or cveii thousands of generations from the present time, aud these characters, like those written on paper with invisible ink, all lie ready t o be cvolrcd under certain known or unknown conditions. ’’ I shall discuss the phenomena of reversion somemhat at length i n another place, mid wish to simply call attention a t present to the fact that here, as in the case of secondary sexual characters, me have a much simpler explmntion in tlie liypotliesis of arrc-t, and tlierefore do not necd to call in an iiiiknown factor, such as the multiple 1~ersonalityof each individual. I tliitik that the plienomcna of alternation of generations favor this latter supposition even more than the facts of reversion. The egg embryo of a hydro-medusa may give rise by budding to an indefinite number of hydroids like itself, arid each of these may give rise to other hydroids, and so on indefinitely. Em11 one of thcse may also, under certain conditions, give rise to medusa quite different from the hydroids and like tlic original medusa. As the medusa which are thus produced inherit through a long series of hydra ancestors all the specific characteristics of the origi-
116
Heredity.
we are forced to conclude that e:ich 117 clrciicl ~oiitaiiis,i n a latent state, the 1)ower to rcprodnc~a dcGnite specific medosn. As the hydra and its rncdusn differ from cach otlrcr rcry milch more than :Lmalc and :t female ni:iiiim:il. xntl have little i n common except the general plan of tlicir org:inization, there seems :kt first to be 110 r e c a p ft 0111 tlic coticlusion that the mcdasa structnrc exists side L? side with the hjdra striictiire, in each lijdruicl, as :i sccond personality. I hope to show, in the chapter on asexual rcprotliiction that alternation of gcncriitions is a seco1iii:ii.y condition of things, and tliat i t lias been bronght abutit Iiy a modification of ordinary nietamorpliosis. I think there is every reasoil to Lelicve t h t a t one time the hydra-larva wliich liutched from a rnetltis:i c ~ bcg came rnetamorphoscd, by a gmdual cliauge during gron tii, into a medusa. If this mere the case now, there mould be 110 more reason for believing in D hgdra pcrhoti:ility nud n rncdus:~ personality than thcre is for belie\ing tliiit a 11li111i~t1 child contains a distinct adnlt personality. Now me can understand that if snch D 1nrr-s slioirl~l give rise by budding to otlicr hydroids like itself, tlioy also wotild have the power t o grow into matnre medusae. We can also understand that circumstances might wise to cause the later stages in the development of some of these hydra-larv2e to become latent. We should tlrrn have two generations-hydroids without a nicdtisn st .get and hydroids with a medusa stage. The suppression of tho ligdra features of tlic l#ittcr would tlien gire 11s a generation of niedosre w i t h no liydra stage, giving birth t o a generation of liydruids with no medusa stage, and these iu turn producirig a
11111 medusa,
Various Opinions o n Heredity.
137
generation of mednsz with no hydra stage. We should then linvc a case of alternation like that which is presented by ordinary hydro-niednsae.
Szwnmwy of Chapter. A careful review of the reasons mhicli have induced various authors to believe that either sexnal element may transmit any cliaracteristic whatever, leads to the coiiclusion that its truth is not proven. It is impossible to prove i t by the phenomena of crossing, since the only animals mliicli can be made to cross are essentially alike, and differ only in minor points. The homology bet&en the ovum and the male ccll is no reason for supposing that their functions are similar, and the differences between them should lead us to believe that their functions are not alike. There is no reason for assuming that each sex transmits its entire organization to the offspring, in order to acconnt for the latent transmission of secondary sexnal eh:ir:~eteristics, since this triznsmission can be more simply explained by assuming that each embryo inherits but docs not necessarily develop all the characteristics of its Fl’ccies. Reversion and alternation of generations admit of a similar explan at’ion. We may therefore conclude that there is and can be no proof that each sexual element transmits d l the characteristics of the parent, and that there is no a priori absurdity in the liypothesis that the male and female reproductive elements are unlike i n function, and are specialized in different directions. We can therefore enter without prejudice into an examination of the evidence for this latter view.
.
CHAPTER VI. THE EVIDENCE FROJZ HYBBIDS. Importance of the subject-It furnishes a means of analyzing or isolating the influence of each sexual element-Hybrids very variable-Hybrids from domesticated races more variable than those from wild races-The descendants of hylirids more variable than the Iiybritls themselves-The offapring of a m:dc lixbrid and the female of a pure species arc miicli more vari'ible than those of a frmnle hybrid and tlie male of a pure spccies-Tl,ese facts inexplicable on any view, except tlie one lirre prcseirtccl -Reciprocal crosses-They differ in fertility and in strncture --The difference is exactly v h a t our theory requires-Difficulty in cxplnininq transmission of c1i:ir:ictcrs tritliout fusionReversion cilused by crossiiig-Two kiuds of reversion-Sum-
mary.
THEstndy of hybrids and crosses is of especial interest t o us, sincc it affords 11s a means, somewhat imperfecl, it is true, for recognizing, i n the offspring, the structure which it owes to each parent. I n ordinary sexual rcproduction between animals or plants of the same race, the parents are almost exactly alike, except for their sexual differences; and as nearly erery structural feature of the young is a fcatnre of rcsemblance t o each parent, there can be nothing t o show that it is inherited from the one rather than from tlie other. T h e n distinct races or species arc crossed, the case is somewhat different. I t is true that thc two parents are still very much alike, for species cannot be m:ide to brccd together at all unless they arc very closely rehted. Still they are more different from each other than indiridnals
T h e Evidence from Hybrids. -
119 ~
of the same species, and the study of crosses and hybrids is therefore a means of separating, to some extent, the iiifliience of one parent from the influence of the other. Tliis is true, however, only with reference to characteristics which are of recent acquisition, for the greater part of the history of two allied species has been the same, and they show in comnion everything except wliat lias been acquired by each one since they diverged from their common ancestor. Crossing gives no way of showing whether these comni& characteristics arc or are not transmitted by one parent or S11e other or by both, but it does give us this iiiforination regarding characteristics which appear in one species but not in the other, and it is therefore the best means at oiir disposal for studying the influence of each parent upon the offspring.
Crossing as a Came of Variation. Accordiiig'to our theory of heredity, we can easily see how the crossing of two species or varieties shonld lead to variability, for when two species or varieties are crossed certain cells of the body will be hybrids between tho gemmules of the male parent and the ovarian particles inherited through the female from the egg of the preceding generation. Now the ovarian particle transmits the properties of a cell like that of the female parent, while the gemmule transmits those of a corresponding cell in the father. I t is plain that corresponding cells of a female of one species or variety and of a male of anotlier species or variety must be more different from each other than corresponding cells in a male and female of the same species or variety. The hybrid cell formed by their union would, therefore, be expected to differ more from each of them, that, is, to vary more than it
-~ ~
~
does in the offspring of parents of the same variety. It is weli known that this is the case; that, in domesticated animals and plants a t least, crossing is a great canseaccording to some older writers the only cause-of variution. Darwin says that it is probable that tlie crossing of two forms when one or both have long been domesticated or cnltivated, adds to the variability of the offspring, independently of tlie commingling of the characters derived from the two parent forms. He believes that new cliaracters arise in this way in hybrids betmeen domesticated forms, forms which Iiave been rendered mriable tlirougll cultiration, but he doubts whether we have, at yresent, sufficient evidence to prove that the crossing of species which have n e w been cultivated lcads to the appearance of new characters. The following illnstrations of this lam are quoted from his Vkriuflo?~ (Vol. ii. 13, 319): ''Gartner declares, and his experience is of the highest d u e on such a point, that when he crossed native plants which had not been cnltiv:ttcd, hc never once saw in the offspring any new character; bnt that from the odd manner in which the characters deriwd from the parents were combined, they sometimes appeared as if new. When, on the other hand, he crossed cultivated plants, he admits that new characters occasionnlly appeared. . According to Kijlreuter, hybrids in tlic genus Mirabilis vary d m o s t infinitely, and he describes new and singular cliaracters in the form of the E C C ~ S , in the colors of the anthers, in the colyledons being of immense size, in new and liiglily peculiar odors, in the flowers expanding early in the E ~ B S O I I ,and in their closing a t night. With respect to one lot of these hybrids he remark6 that they presented characters exactly the reverse
. .
T h e Evidence from Hybrids.
121
of what might have been expected from their parent-
age.
‘‘ Professor Lecoq speaks strongly to tlie same effect in rcgard t o this same gcnus, and asserts that many of the hybrids from Mirabilis j a l a p and niultiflora might easily bc mistaken for distinct spccies, and adds thal, they differed in a greater degree tlian the other species of the gcnus from M. j a l a p . Herbert has also described tlie offspring from a hybrid Rhododendron as being as unlike all otlicrs in foliagc as if thcy had been a separate species. The common experience of floriculturists proves that t h e crossing and recrossiiig of distinct but allied plants, such as the species of Petunia, Calceolari:i, F ~ i c l i ~ i a , Verbena, etc., indnces excessire variability: hence the appearance of quite IICW cliaracters is probable. M. Carriers has latcly discussed this subject; he states that Erytlirina cristagalli 11:d been multiplied by seed for niany yczirs, b u t has not gicltled any varieties; i t was tlien crossed with the allied E. hcrbacia, and tlie resistance was now overcome, and varieties were produccd with flowers of extremely different size, form, a n d color.’’ Darwin, tlicrefore, concludes t h a t crossing, like any otlicr change in t h e conditions of lifc, seems to be a n element, probably a potent one, in causing variability. The variability of hybrids is qnite as explicable by Dnrwin’s Pangenesis hypothesis as i t is by our theory of hercdity, nltliough I do not see why, on the hypothesis of pangenesis, the hybrid offspring of domesticated forms should be any more variable than those produced between wild species.
122
Heredity.
The Ofspring qf Hybrids inore variable than the Fimt Geizemtion There is another aspect of the variability of hybrids which is very remarkable, and wliiclt is in perfect ngreement with our theory of ltcrcdity, but, so far as I am aware, absolutely inexplicable withoiit it. This is the law that although the offspring of the first generation are generally uniform when two species or races are crossed, the subsequent generations of children produced by these hybrids display an almost infinite diversity of character. (Darwin, Variation, ii. p. 321.) Darwin also refers to this curious law in the Origin of Species, p. 260, and attempts an explanation of it. He says: “ The slight variability of hybrids in the first generation, in contrast wit11 that in the succeeding generations, is a curious fact, and deser-res attention. For it bears on the view which I have taken of one of the causes of ordinary variability, namely, that the reproductive system from being eminently sensitive to changed conditions of life, fails under these circumstances toperform its proper function of producing offspring closely similar in all respects t o the parent form. Now, hybrids in the first generation are deecended from qecies (excluding those long cultivated) which liave not ltad their reproductive systems in any may affected, and they are not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable.” According to this view, the variability of the descendants of hybrids’is a sort of monstrosity, due to the failure of the reproductive organs to perform their proper functions; ordinary variability is not monstrosity, but is perfectly normal, and as the variability of hybrids
me
Evidence from Hybrids.
123
has precisely the same character, I think we cannot regard it as due to unnatural disturbance. According to our theory, nuithion is due to the action of cliatngcd or unnatural conditions upon certain cells of a preceding generation. Now, :is characteristics of both parents are mingled in a hybrid, i t must nearly always happen that certain cells with peculiarities of one parelit mill be in contact with, or will depend in some way upoti, cells with peculiarities inlierited from tlie otlier species. Tliere will therefore be a lack of the perfect adjustment between each cell and its neighbors, tvliicli has been brought about in each parent by natural selection, and this imperfect adjustment will cause the cell wliiuli is unfavorably placed to tlirom off gemmules. The cells of the body of a hybrid will therefore be unusually prolific of gemmules, and will transmit variability to later generations. According to our hypothesis, a hybrid is more likely to transmit Variability than a pure species, because more of its cells are placed under circumstances favorable to the production of gemmules. For tlie same reason a hybrid between two domcsticnted or cultivated forms must have more tendency to vary than one produced by crossing two wild species, for tlie domestic or cultivated parents live under unnatural conditions, and therefore have more tendency than wild species to transmit gemmules, and thus cause variability.
The Xex of tlbe Parent affects the Variability of Hybl.ids.
I hare shown that the body of a hybrid is peculiarly favorable for the production of gemmules, and that, for this reason, the descendants of hybrids are variable
124
Heredity.
to an unusual degree. Now, if our theory of heredity is true, if the seniirial flnid is especially adapted for the transmission of gemmules, ivlijle their tmnsmission by:)ii ovum is a niattcr of :tcidcnt, the tendency to Yary must be transmitted by the mule hybrid. Wlien children are born from two hjbrid parents i t is impossible to show that the variability which follows comes from the father rather than from the mother, but the subject can be put to a test by crossing tlie male hybrid with a female of one of the pure species, and the male of one of the pure species with the female hgbrid. Neither pure species has any especial tendency to transmit variation, mliile the niale hybrid has such a tendency. If, then, we cross tlie female hybrid with the male of one of tlie pnre forms, the offspring wonld not be expected to he ~~nus u a l T-nriable; ly but if the male hybrid is crossed with one of the pure females we should expect the offspring to be 1111 u sn:dly variable. Now it is very interesting to find that this actually is the case. T ~ L Gartner U states (Rnstni.derze.ligzi~zg,p. 452, 50‘7) that when the seeds of Dianthus barbatus were fertilized by the pollen of the hjbrid Dianthus chinensibarbatus, the seedlings were more wriable than those which were raised from the seeds of the hybrid fertilized with the pollen of Dianthus barbatus. Darwin states tliat Max Wichura obtained the same result with willows. Giirtner concludes from a number of experinleiits tliat when a hybrid is used as the father, and either one of the pure parent species or a third spccies as tlie mother, the offspring are more ~ a r i a b l etliaii when the same hybrid is used as tlie mother, and either piirc parent or tlie third species as the father. Darwin’s pangenesis hypothesis f urnishes n o explanation whatever of this curious fact. On the contrary, as
The Evidence from Hybrids.
125
i t requires t h a t each sexiial clement slionld contain gemmules from cvcry part of t l ~ ebody of tlic parent, it is directly opposed to any such result, and tlierc is 110 place for i t in any other hypothesis of hcrcdity. Oiir theory fits i t exactly, however, and a more crucial test could hardly be proposed than an experiment like those detailed by Gartner.
Rec roca 1 Bybl'ids. According to Darwin the two sexes play similar parts i n heredity, and any characteristic whatever may be transmitted by eitlier sexual element. This conclusion is based upon tlie phenomena of crossing, lint a little thought mill show t h a t it is impossible, from the nature of the case, to prove it from e d e n c e of this kind, althongh, as I hope to show, i t is capable of disproof. Only animals of t h e same species, or of closely related spceies, can breed together. Closely allied animals are alike i n aII respects, except as regards tile slight diffcrences wliicli distingiiish species, varieties and indiriclunls from each other. Sincc no animals or plants can cross except those mliich haye most of their past history in common, and whicli arc tlicrefore alike in nearly e n x y respect, it is plainly impossible t o prove, from tlie plicnornena of crossing, tllat encli p r e n t has power to trmsniit the features mliicli are shared by tlie other parent as well. The phenomena of parthenogenesis, or reproduction by virgin femnles, as in the case of bees and ~vasps,show t h a t the ovum alone may transmit all the established hereditary structure of tlie species, bnt there is and can be no evidence to sliom t h a t tlie malc element can accomplish the same thing. The facts of crossing, while they cannot prove that the
126 fnnctions of the t v o reprodnctive elements are alike, d o fornish convincing proof of the contrary, and show that they are not alikc. A reciprocal cross is a donblc cross bctweeii two spc. cies or varieties, one form being used in onc case as the father, and in the othcr case as the rnothcr. Thus A reciprocal cross between a horse and an ass is a double cross, between the male horse and the feniale ass on tho one hand, and the female horse and m& ass on the other. Now, if it is trne that the fuiietion of the oviim is like that of the male cell, the offspring of rccjpi.oca1 crosees shonld be d i k e in all respects, but this is by no means the case. I n the first place, the degree of steriIity often differs greatly in two species when reciprocally crossed ; for the male of the f i r s t will, in some cases, readily fertilize the ovum of the second, and thus give rise to dcscendants; whilc hundreds of attempts to fertilize the ovum of the first by the male of the second, result i n nniform failtire. It often h:qqxms also that even mlien both cross~sresult in the production of offspring, the lijbrid in the one case is sterilc, while in tlie other case it is perfectly fertile. Not only do the results of reciprocal crossing sl~orvt h i s difference, but they sliom what is still less reconcilable with the view that the fnnctions of the sexual elements arc alike, namcly, great differcnces of structure. In some cases where a rcciprocd cross is ~)crfectly fertile on both sides, the Iiybrids mliicli are thns produced are not a t all alike. When tlie male of species A a i d the femdc of B are crossed, the offspring is an entirely difftwnt being from thc one born from A as a mother w i t h B as a father. We know that allied species of animals are the deecend-
The Eoidence from Hybrids.
127
ants of a coninion ancestral form, from which they iiilicrit all that tlicy l i a ~ ei n coninion, while tiic distinctirc ~~cculiarities which distinguish them from each otlier arc niorc recently acquired. According to onr hypothesis tlie ovnin transmits estublislied characteristic?, while the cells which have rcccntly varied in the body of the male transtnit gemmules. If, then, me sclect two allied species or varieties and cross tlie male of one with the female of the other, and tlicn, reversing the process, crom the female of the first form with the male of the second, we should expect to find, in many cases, a difference in tlie offspring. Wliere tlie male of species or variety A is crosscd with the female of B, the offspring will inherit from its mother the common characteristics of both parents, a d i t will also receive from its father gemmules from those cells ~vliiclihave recently varied in the species A. The corresponding cells of its body will therefore be hybrids, and will bear a closer resemblance than the other parts of its body to the species A. That is, the hybrid will sliare, to some extent, the peculiarities which are distinctive of the species A as compared with B. The offspring of the opposite cross will, on the other hand, join, more or less perfectly, to the common race characteristics, some of the distinctive peculiarities of the species A r rod aced in it by tlie hybridization of the cells of its body by gemmules received from its father. Iteciprocal crosses between the horse and the ass have bccn reared for domestic purposes for ages, and Huxley gives the following iiiteresting account of the result: “ T h e offspring of the ass and the horse, or rather of tlie he-ass and the mare, is what is called 8 mule; .and, on the other hand, the offspring of the stallion and the
125
Heredity.
she-ass is what is called a hinney. It is a very rare thing i n this conntyy to see a hinney. I never saw one myself; but they have been very carefully studied. Now the curious thing is this, that altliongh you have the same elements in the experiment in each case, the offspring is entirely different in character, according as the male influence comes from the ass or the horse. When the ass is used as the male, as in the case of the mule, you find that the head is like that of the ass, that the eam are long, the tail is tufted at the end, the feet are small, and the voice is an unmistakable bray; these are all points of similarity to the ass; but, on the other hand, the barrel of the body and the cut of the neck are ninch more like those of the mare. Then if y o ~ look i at the hinney-the result of tlie nnion of the stallion and the slie-ass-then you find it is the horse mliieh has tlie predominance; that the head is more like that of the hdrse; the ears are shorter, the legs coarser, and the type is altogether altered, while the voice, instead of being a bray, is the ordinary neigh of the horse. &re, you see, is a most cnrious thing; yon take exactly the same elements, ass and liorse, bat yon combine the sexes i n a different niaiiner, and tlie result is niodified accordingly. " It would certainly be a wonderful thing if the conibination of the same elements should give such different results, and I think we must conclude that the elemcnts are not the same, but that the ovum and the male cell do not play the same parts in heredity. There are not many cases in which reciprocal crosses have been made so frequently, and single obscrvations are not of very great value. I will, however, cite a few, to show that the one given is not cxceptional. The Nnnx cat is a variety of the domestic cat 1ieculi:ir to the Isle of Man. I t differs from the ordinary cat i n Laving no tail,
The Evidence f r o m Bybrids.
129
a n d in some other slight pecn1i:trities; its hind legs are longer, and its habits peculiar. According to Mr. Orton (Physiology of Breeding, 1S55, p. 9; quoted by Darwin, T’cwintion, ii. SG), Dr. Wilson crossed :i male Miinx cat mitli common cats, and, out of twcnty three kittens, serentccn were destitute of tails; but when tlie female Manx mas crossed by conimon male cats all the kittens had t d s , though they mere gcnerally short and imperfect. Darwin gives tlic following in his Variation iuider Domestication (ii. 85): ‘‘ Godina lias giren a curious casc of a ram of a goat-like breed of slieep from tlie Cape of Good H o p , which produced offspring hardly to be distingnishcd from himself mlien crosscd with ewes of twelve other breeds. But two of these lialfbred ewes, wlicn p u t to a merino ram, produced lambs closely resembling the merino breed.” I quote tlic following from Darwin also (p. 8 7 ) : ‘‘ T h e silk fowl breeds true, and there is reason t o believe tliilt i t is a very ancient race; but when I reared a large numb ber of mongrels from a silk hen by a Spanish cock, not one cxliibited even a trace of the so-c:illect silkiness. Nr. Hewitt also asserts that in’ no instance are tlie silky featliers trarisniittcd by this breed when crossed with any otlicr variety. B u t three birds out of many raised by Mr. Orton from a cross between a silk cock and a bnnt:im hen liad silky feathers. Thei~eare some instances of reciprocal crosses which seem at first sight to give directly opposite results, arid therefore to contradict our theory. Thus Darwin says t h a t a hybrid which had for its mother a bay mare and for its fiither a hybrid between a male ass and a €emale zebra, had, when young, zebralike stripes npon its shoulders, flanks and legs. Here tlie only recent striped ancestor is t h e paternal grand-
130
Neredity.
mother. As the possession of stripes is a cliaracteristic which distinguishes the zebra from the horse and the ass, i t seems at first as if its transmission by a fcnxtle ancestor is opposed to our theory. We .know, home\er, that all the species of the horse genus are the descendants of a striped form, and the presence of stripes in the zebra is not due to recent variation, but to the fact that i t has not varied. The transmission of stripes by a female zebra is therefore nothing more than me might expect. We know, too, that both the horse and tlie ass show a tendency to revert to tlic striped ancestral form, and I s h l show in the next section that reversion is often excited by crossing. It is therefore quite probable that the stripes in this colt mere due to reversion. It is said that young animals born from a tigress by a male lion, as well as those born from a lioness by a male tiger, are striped, bnt many cat-like animals show a tendency to revert to a striped form, and in this case also we may explain the presence of stripes in the young by attributing it to reversion excited by crossing. Darwin says that a good authority assures hini that in South America, when hiata cattle are crossed with common cattle, though the niata is prepotent whether males or females are used, the prepoteiicy is strongest through the female line. The origin of the niata breed is not known, but there is no doubt that it originated in Paraguay from common cattle; and the fact that the niata pecnliarities arc not shared by any other living cattle, but are very much like those of the extiiict Sivatherium, seems to show that in this case also the peculiarity may be due to reversion to some remote ancestral form.
The Evidence from Nybrids.
131
Di$cdtjy of Zxpluiiziiag the Tyansinission of the CJLarn c f e m of Ti00 For.nzs ~ i t h o u tFusion. A mucli more serious difficulty is found in the fact that while a hybrid is usually somewhat intermediate between its parents, i t occasionally happens tliat the cliaracteristics of one or both parents refuse to blend and are transmitted in an unmodified state. Thus Darwin states that when gray and white mice are paired the young are not piebald nor of an intermediate tint, but are pure white or of tlie ordinary gray color. This particular case may perhaps be explained as follows : The bromn form is the ancestral form, and ivlien no hair gemmules are transmit ted tlie young are brown. All the hairs are homologous with each other, and are derived from the same part of tlie egg, and when gemmules are transmitted they may hybridize alike all the cells which are to form liairs, mid the hybrid animals will therefore be entirely white or entirely brown. I t is stated that when u black game fowl is crossed with a white, the young are either pure black or pure white. but this case is precisely like tliat of the mice. Darwin gives a number of interesting illustrations of this singular phenomenon, 'among which are the follow1ng:
TVlien tirrnspit dogs and aneon sheep, both of ivliich liarc dwarfed limbs, are crossed with common breeds, the offspring are not intermediate in structure, but resemble one parcn t only. W h e n tailless or Iiornless animals are crossed with perfect animals, it frequently but by no means invariably happens that the offspring are either perfectly furnished with these organs or are quite destitute of them. When Dorking fowls with five toes are crossed with
132
Heredity.
other brccds, the chickens often have five toes on one foot and four on the other. When the red flowered stock of Antirrliiniini is fertilized with tlie pollcn of tlic pwple Qneen stock, h u t half t h e seedlings resemble tlie inotlicr 11l:tnt, wliile t h e other lialf )Je:ir rich purple blossoms like tliose of the paternal plant. Darwin says t h a t lie fcrtilizcd the pnrplc sweet-pea, wliicli has a dark reddish-purple standard-petal and 1 iolet-colored wings and kcel, with pollen of the p i n t c d lady smcct-pca, mliicli lias a pale cherry-colored standnrd and almost wliite wings and kecl, and from the e:inie pod twice raiscd 1)lants resenibling both sorts, tlie greater n 11mbcr rcscni bl i ii g the fat 11cr. These cases are difficult to exl)lain, b n t tlie ldienonicna are so complicated that i t is l i a ~ d l ysafe to spccu1:ttc upon tlicrn until they are rc-csnn:incd by an observer who can devote liirnsclf to this subject cqwcidly. Some of them may be due to t h e c:iLiscs aboTe indicated, and sonie, possibly, to fertilization by two fatlicrs.
Crossing as n Cause of Recersion. According to Darwin’s view rcwrsion must in all cases be dne to the nianifcstation of a tendency which lias lain dormant in tlie cgg and lias bceu transniittcd for gcnerations in a latent condition, for tlic chances against tlic r c p t i t i o n , by an accidental variation, of a cliaractcristic of ii remote ancestor, arc inconccir:ibly great. According to our theor~l)tliisis not tlic case, for the conditions which causcd a cell in t h e anccstrd form to throw off genimnles and thns to produce a glvcn pecu1i:irity niity c;uise the corres1)onding cell of the parent to throw off gcinmulcs in the same may, and these, uniting with t h e .corresponding part of the cgg, will produce
The Evidence from Hybrids.
133
variation. As the gemmule and the ovarian element are both very similar to those wliicli produced the variation i n the ;~ncestor,the chmees arc not very great against t!lc rcl)rodiictioti of tlic same pecnliarity. In this ciisc IVC Jiould liave D new vmation with all the charactcristics of :I true reversion, but due to the transmission of a gcmmnle, rather tliaii to tlic sudden awakening of a tendency mhicli lias long Tain dormant in tlie egg. It is possible, therefore, tliat there may be two kinds of ixwxsion-truc hereditary reappear:tnce of features ~vliiclih a ~ lain c latent in the egg, and new variations ~ v l ~ i crepent li again certain old cliaracteristica of the race. Tliere arc, I tlilllli, certain reasons for believing tliat reversions of the latter kind arc the most common, the chief one beiiig tlie fact tliat most of the causes of variability are also c a p s of reversion. Tlins, crossing, wliicli is :t very efficient cause of vaiiiation, is also one of the chief causes of reversion. I);irwin gives a number of examides to show that, indcpcndently of the well-Bnown tendciicy of hybrids and mongrels to revert, after a nnmber of generations, to one of the parent forms, the act of crossing in itself gives a n inipnlse towards reversion, and often results in the reappcarance of long-lost characters. Tlic following interesting acconnt, from D;irwin’s Vnrintioir (Vol. ii. p. 57), will serve t o illustrate this law: “ In the chapter on tlic horse, reasons mere assigned for believiug that the primitive stock mas stripcd and d u n colored, and details were given showing tliat in all parts of the world stripes of a dark color freqnently allpear along the spine, across the legs and on tlie shoulders, mlierc they are occasionally double or treble, and even sometimes on the face and body of horses of all breeds and of all colors. But the stripes appear most
134
Neredity.
frequently on the various kinds of dun. They may sometimes plainly be seen on foals and subscqnently disappear. “The dun color and the stripes are strongly trnnsmitted when a horse thus c1i:iracterized is crossed with any other, but I was not able to prove that striped diins are generally prodnced from the crossing of two distinct breeds, neither of which are duns, although this does sometimes occur. “ T h e legs of the ass are often striped, and this may be considered as a reversion to the wild parent form, the Asinus f m i o p s of Abyssinia, wliicli is thus striped. In the domestic animal the stripes on the sl~oulderare occasionally double or forked at the estrenrity, as i n certain zebrine species. There is reason to believe tliat the foal is frequently more plainly strjped on the legs than tlie adult animal. As with the horse, I have not acqnired any distinct evidence that the crossing of differently colored varieties of the ass brings O U ~the stripes. “ B u t now let us turn to tlie result of crossing the horse and ass. Although mules are not nearly so nnmerous in England as asses, I have seen B much greater number with striped legs, and with the stripes far more conspicuous than in either parent form. Such mules are generally light-colored, and might be called fallow-duns. The shonlder stripe in one instance was deeply forked at the extremity, and i n another instance was double, thoLJgh united in the middle. Mi-. Martin gives a figure of a Spanish mule with strong zebra-like marks on its legs, and remarks that rnnles are particularly liable to be thus striped on the legs. I n South Amcrica, according t o Roulin, such stripes are more frequent and conspicnonsin the mule than in the ass. In the United States, Mr. Gosse, speaking of these animals, says that in a great
The Evidence from Hybrids.
135
number, perhaps in nine out of every ten, the legs are banded with transverse dark stripes.” Moles with striped legs can be seen in great numbers every d;iy in tlic strects of Baltimore, and the peculiarity is not in the least uncommon. Darwin gives a number of cases i n which the same reversion has been produced by the crossing of otherliorselike forms, and we must regard the tendency to revert to a striped form when crossed as characteristic of the horse family. Darwin says that when lie crossed diffcrent varieties of fowls he often got birds w i t h faint traces of the peculiar red p1um:ige of the wild GCLZZZLS bnizkivn, and that this pl timage mas almost perfectly reproduced in one magiiificcnt bird, the offspring of a black Spanish cock aiid a white silk hen, although either of these pure breeds may be reared by tens of thousands witliont the appearance of L: single red feather. Even long-lost instincts may be made t o reappear by crossing. The original mild ancestor of our domestic fomls must, like all wjld incubating birds, have had the incubating instinct. Now when two non-sitting breeds of fowls are crossed, the mongrcls frequentLy recover their incubating habit and sit with remarkable steadiness. It is said that hybrids between perfectly tame domestic :mirnals are often as wild as their wild ancestors. This has been noticed in cattle, pigs, fovls, ducks, and it is probnblc that the same thing frequently shows itself when widely separated human races are crossrd, as such good anthoritics as Liyingston and IIumboldt have remnrked uyon the savage character of half. caste liunian beings. Another intere,+ing resemblance between reversion and
136
Heredity.
ordinary variation is the fact that tlie descendants of hybrids are more apt to revert than the ligbrids thcmseives. Darwin says (Variation, 11. 65) that this is a general role. Now, whether rerersion be dne to the snddcn excitement of a tendency which has long been transmitted i n a dormant state by tlie ova, or whether it is due to t h e appearance of a new variation which rescmblcs a11 old one, we can readily understand how, according to our theory of heredity, crossing should call this poiver into action. During the evolution of t h e species eacli hereditary peculiarity has been cstablished i n tlie cgg by gemniules, and anything which prevents the egg from following its normal co~irseand developing the recently acquired c l i a r x teristics of the species, would sllow older characteristics to appear in their place. We know that animals which are w r y widely scparated are infertilc, and i r e can undcrstand tliat c w n when tlie difference betwcen two specics is not great enough to prcvent them from crossing, those cells of their bodies mliich have raried most may be so different from each other that gemmules from the one cannot fertilize tlie egg-particles which are to produce the other, or when tiley do fertilize them they may give rise to a variation which is so different from tlic 11orma1 cell that it cannot live. T h e cells which lmcede these in the order of growtli being less different in the two lmxwte, would be niuch more favorably situated, andwould thus give to tlie embryo a characteristic of longcr standing tlGn tlie peculiarities of eitlicr parent. On the other h:uid, if reversion is simply variation, me can sce that crossing might excite reversion just as i t cxcitcs Fariability.
The Boidence from Hybrids.
137
Sumrncmj of C l q t e r . The study of liybrids gives us a means of comparing, witliiii certain narrow limits, the parts which the t w o sexual elements play in,heredity. The influence of each sex can, in a certain sense, be studied by itself when a given species is used in the one case as the father of a hybrid, and in another case as the mother. The value of crossing as an experiment in heredity is greatly limited, hornever, by the fact that, although me can stndy the inflaeiice of one sexual element unobscured by the other element from thc Same species, it is obscured and complicated by the influence of this element from an allied species, and in all organisms which can breed together tlic rcproducti-re elements must be essentially alike. Hybrids do, however, present a number of peculiarities wliicli agree perfectly with what we should expect according to our hypothesis, and certain of these are hexpl i cabI e .ivi t 11o 11t it. I-Iybrids and mongrels are highly variable, as me should expect to bc the case, according to Darwin’s pangenesis liypotliesis. This hypothesis fails to account for tlic fact that hybrids from forms which have long been domesticated are more variable than those from wild species or varieties, or for the very remarkable fact that the children of hybrids are much more variable than the hybrids themselves. Onr theory not only explains the variability of hybrids, but it also accounts for the two latter peculiarities, for crossing will not give rise to a marked or conspicuous variation unless the hybrid inherits numbers of gemmules, and as domesticated animals and plants live under unnatural conditions they are more favorably placed than wild forms for the production of gemmules.
138
Her edity.
The body of a hybrid is in itsclf a new thing, and therefore in a certain sense nnnatural, and a male hybrid is, accordingly, more fitted for the production of gemniulcs than a male of a pure or unmixed race. When a male hybrid is crossed with the female of either pure species or with a third species, tlie children are much more variable than those born from a hybrid mother by a male of a pure species. It would be difficult to devise an experiment better fitted than this t o show that variation is caused by the influence of the male, and that the action of nnnatnrd or changed conditions upon tlie male parent rcsults in the variability of the child. The remarkable histor? of reciprocal hybrids is directly opposed to Darwin’s view that the functions of the two reproductire elements are esaentially similar, for in some cases it is impossible to breed from a female of one species by the male of a second species, wliile the male of the first species readily fertilizes lhe oviim of tlie second and gives rise to fertile offspring. Even wlicn both crosses are fertile the one is often much more so than the other. The hybrids of one cross often differ remarkably from those of the other cross in general structure, and in many cases they show, in addition to the common characteristics of both parents, a tendency, more or less pcrfectly pronounced, to develop the recently acqnircd characteristics of that species which is used as the fath er . This law is often obscnred by the appearance of rcwrsions, which are peculiarly apt to occur in Iijbrids, n n d by the presence, in certain cases, of a tendency f o r cncli pnrent to transmit its peciiliarities to the hgbrid, without fnqinn with t h n w nf t h o nthor nai-ent
Rnt a-Lon
ma
T72e Eoidence from Hybrids.
139
consiclcr tlic great obscurity and complexity of the case, and the great difficulty in conducting rigid cxperirnents, tlic balaiice of tlie evidence from hybrids seems to be greatly in favor of our view of the nature of heredity. It certainly presents features wliicli are ioexl)lie;ible in any other wtiy, :md perfectly simple and natural if our view is accepted.
CHAPTER VII. THE EVIDENCE PRO31 VARIATION.
Causes of variation-Clinnged
conditions of life induce vnriability-No particula~kind of cliange is necefsnry--vari:iLility i3 almost exclusively contiued to organisms produced from fertilized ova-Bud variation very rare-Ilktory of the Italian orange-The frequency of variation in organisms produced from scxual union, ns compared Tvitli its infrequency in those produced nscxunlly. rcreivcs n direct explannlion by our theory of Iicredily-Bud wriation more theory freqiient in cnllivated tlian in wild p1:rnts-Our would lead us to expect this-Clinnged conditions (lo not act directly, but they cause subsequeut generntions to vary -Tendency to vary is Iicreditnry-These facts perfectly expliaible by our tlieorj-Spccific clinractcrs more y:ui:ible t h n generic-Species of large genera innre v;iri;iblc tlinu those of sm;ill gcnera-A part tlcveloped in an utinsiuil way sexIiiglily variablc-Lnw of cqii:ible vnri:ition-Secondnry ual c1,:ir:icters v:iri~~)JIc--Nntur:i1 selection cannot act to produce pertii:incnt modificxtion unless Inany individuals theory i s tlic only expl;ination of tlie vary together-Our simultaneons variation of ~ n a n yindividoi~ls-This theory also simplifies the evolution of complex strnctures-Saltntory evolution-This is exphined by onr theory of lieredity --Correlated variation of liomoiogous parts-Parts confined t o males more variable tlian parts contined to feinales3Iales more variable than females-Summary of last two chapters.
TIM Cmses of Vnvintio?b. CERTAIN authors hare licld that rariitbility is a Iieccssary accompaiiiment of lqroduction; that it is determined by something mitliin rather than without the or-
The Evidence from Variation.
141
gmism, bnt Darwin, after long and careful study nf the snbjcct, rcachcs tlic conclusion that each variatiou is exci tcd by a chnngc of some kind in the environment. It is impossiblc to expose animals for any length of time to absolutely uniform conditions, and we therefore find that when careful attention is given to the subject, minute individual differences may be detected in animals which arc appwcntly most nniforni. A shepherd easily lcarns to recognize each sliccp in a large flock, and ants arc able to pcrccivc a difference between the members of their o ~ r ncomninnity ant1 those from another nest. I t is inipossiblc to show by direct proof tliat uniform conditions of life mould provent variation; but i t is qnitc possiblc to approach tlic subject from tlic other side, and to shorn t!iat slight external changes cause slight variability, and greatcr cliaiigcs greater variabil-
ity. Wild animals and plants mr y somewhat and hare indiridual peculiarities, for each one is under slightly different relations to tlic cxternd world from all the others, biit :is cotn~~~irect with domesticated species their contlitions of life arc very nniform. A wilJ iinimal has become habituated to the circumstances undcr which it lives, by exposure, for generations after gcuerations to the action of natural selection, and a host of competing animals tend to keep i t in its place, but domesticated animals arc protected from their enemics and competitors, they arc removed from their natnr:d conditions, and they are frequently carried from their native land and are exposed in other: countries t o unnatural food and climate. They are compelled to c1t:ingc their habits, and they arc never left long at rest, or exposed for any considerablc length of time to closely similar conditions, but they arc carried from dis-
142
Heredity.
trict to district, and their food and treatment varies considerably. We accordingly find that, with fcw exceptions, all our domcsticnted animals and plants rary more than their wild relations. Even tlie goose, one of the least variable of domesticated animals, varies more than almost any wild bird, and according to Darwin, hardly a single plant can be named, which lias long been prop:igated and cultivated by sced, tliat is not highly variable. These considerations force us to conclude tlia’t rariability is not a necessary contingent of reproduction, but that tlie production of tlie gemmnlcs which give risc to variation is excited by changes in external conditions, and we must agree with Darwin that “ it is probable that variability of every k i d is directly or indirectly caused by cliangcd eoiiditions of life; or to put the case under another point of view, if. it were possible to expose all the individuals of a species during many generations t o absolntely uniform conditions of life, there monltl be no variability.” TVlicn me come to examine tlie effect of different conditions of life we find thatwe cannot attribute the Tarinbility to one rather tliaii tlie other. The essential tliing is change, but not any particular kind of change. Variation is frequently caused by a change of climate, but this is by no means cssential, f o r most ciiltivnted plants yield more varieties when cultivated in their native country tlian when removed to other climates. (Darwin, Variation, ii. p. 310.) Change of food is often a cause of variation, but that this is not necessary is sliomn by the fact pointed out by Darwin, tliat fowls and pigeons arc the most variable of domesticated animals, although their foodis nearly the sitme as that of their wild allies, but is much less varied
The Evidence f
Tom
Variation.
143 -
than that which they would find for themselves in a state of nature. Excess of food often causes variation, yet the turkey and goose have been encouraged and tempted for generations to fecd to excess, and they have varied but little. These examples show that the chamcter of the change is uniinportant, and that variability cannot be attributed to the exclusil-e influence of any particular class of external conditions; that the exciting cause of variation is clinngc. but not any particular kind of change. Darwin quotes a number of cases to show how slight a chaugc may result in variability. Thus the mild horses of the pampasof South America arc of one of three colors, and the wild cattle are of one color; but when the same horses and cattle are domesticated, although they arc not confined, but aae allowed to run at large like the wild €orms, they entirely lose their similarity of color, and display the greatest diversity in tliis particular. I n India several species of fresh-water fishes arc rcarcd in great tanks as large as natural ponds, and they arc all very variable. Darwin quotesfrom Downing tlic statement that varieties of the plum and peach which breed truly by seed, lose this power, and like other worked trees give variable seedlings when grafted on another stock.
Variability almost Exclusively Conjined to Organisni Produced f r o m Fertilized Ova. The only method open to us besides the study of hyhrida for observing tlie influence of the sexes i n heredity, is by a comparison of sexual with asexual heredity. As I shall show in another place, all the various forms of asexual reproduction are so connected that we may pass from h i o n , or the formation of two new organisms
144
Heredity.
by the splitting of one old one, to parthenogenesis, or reproduction from unfertilized ova, without findiiig any important gap in the series, and wc may safely conclude that all these forms of reproduction are fundamentally alike. So far as regards the physical side of the problem of heredity, the only essential difference between asexual reproduction and sexual reproduction is the absence of fertilization or union with a male cell in the one case, and its occurrence in the other case. It is therefore extremely important to compare the two processes, in order to discover whether this physical difference is accompanied by any difference i n the result. I n the one case we have heredity with the male factor omitted, and in the other we have heredity with a male factor, and if there is any constant difference in the result, we may safely attribute i t to this factor. I n making this comparison we are almost compelled to restrict ourselves to plants, for although asexual reproduction is not a t all unusual in animals, it is restricted, with one exception, to animals which are not domcsticated or reared by man, and me thercfore know too little about the minute details of their life to make use of thcm for our purpose. The number of plants which hare been cultivated and carefully observed and studied by man is verygreat, and as most of them multiply asexnally by budding, as well as by fertilized seeds, we here have abundant material for comparative study, and it is well established by hundreds of thousands of observations that the prcsence or absence of the influence of the male element does have an influence upon the result of the reprodactire process, and that this result is exactly what our view of the nature of the process would lead us to expect. Plants produced from fertilized seeds differ from those
T 7 ~ eEoidence from Variation.
146
produced from bnds only in their greater tendency to riiry. Bud mriations do occur, but they are very uni i - i ~ i l ,while more or less variation in seedling plants is almost universal. As we suppose that any cell may, mlicii excited by unfavorable conditions, throw off genimules, the gemmnlcs may find their way, by a sort of accident, t o growing buds, and thus cause variation. We should thercfore expect bud variation to occur occasionally, but very much less freqwntly than variation in seedlings. This is so well known to be tlie case that many antliors have held tliat tliere can be no variation mithout sexusl union. Darwin has shown, however, by a long list of instances of bud variation in plants, that this is not absolntely true, and the weight of his authority has led to the almost universal acceptance of his conclusion that tliere is no essential difference between asexual and sexnal heredity. I shall discuss this conclusion a t length in another place, as I believe tliat the facts demand a n intcrpretntion which is somemhat different from the one mliicli Darwin furnishes. At present I simply wish to call attention to the fact that all authorities agree t h a t variation is almost infinitely more common i n sexual than i t is in asexual offspring. Asexual multiplication in animals is restricted to the lower forms which are of little use to man, and as tliese forms have not been domesticated and carefullyobscrvecl, our knojvledge of the rariability of organisms produced asexually is almost entirely derived from the study of pl;1nts. Thc only instance in domesticated animals of anything like asexual reproduction is the parthenogenetic reproduction of bees, and i t is therefore interesting to note
146
Heredity.
that the hive-bee is the least variable of all domesticated animals (Darwin, Variation, Vol. ii. p. 307). Darwin says (Variatioiz, Vol. i. p. 3 G O ) that lie procured a hive full of dead bees from Jamaica, mlierc they have long been naturalized, and on carefully comparing them under the niicroscope with his own bees, could not dctect a trace of difference. Wit11 plants it is well known to all cultivators that forms which are highly variable as seedlings can be kept perfectly true by asexual propagation, and we have Darwin's authority (Variatioiz, Vol. ii. I'. 307, and Vol. i. p. 429) for tlie statement that mliile hardly a single plant can be named which has long been cnltiratcd and p r o p agated by seed that is not liiglily variable, tlie total number of instances of bud variation is as notliiizg in comparison with seminal varieties. This contrast is the more remarkable when me recollect that in most of our cultivated plants the number of buds wliicli develop is thousands of times greater than the number of seeds which give rise to plants. It is clear that if the chance of variation were the same in both cases the number of bud variations would be thonsaiids of times greater than tlie number of seedling variations. If there were thousands of chances of seedling variation for one chance of bud variation, the number of bud varieties would still be equal to the number of seedling vane t ies. The fact that with all this probability in their fayor, bud rarieties are very rare as compared with seedling varieties, shows tliat the chance of bud variation is almost infinitely small as compared with the cliance of seedling variation. While we cannot deny that rariation may sometimes occur in organisms produced asexually, I think we are
The Eoidence from Variation.
147
jnstified in giving great emphasis to the lam that Tarisbility is a1most exclusively tlie cliaracteristic of organisms produced from fertilized ova. Di~rwinsays ( V w i u t i o i ~ Vol. , ii. pp. 361 and 377), “ When wc reflect on tlie millions of bods wliicli many trees have produced before sonic one bud lias varied, we arc lost in wonder m1i:tt the precise cause of each variation can be.” :‘ Habit, liowcver much prolonged, rarely prodaces any effect on a plant propigated by buds: it apparently acts only through succeEsive seminal generations.” ‘l‘lic curious history of the naturalization of the orange in Italy, quoted by Darwin on the authority of Gallesio (Theoria della RQivodwione Vegl, 1816, p. l%), is very interesting in this connection. Dnring inany centuries the sweet orange was propagated exclusively by grafts, and so often suffered from frost that it required protection. After tlie severe frost of 1709, arid more especially aftcr that of 1763, so many trees were destroyed that seedlings from tlic sweet orangc mere raised, and to tlic surprise of the inhabitants their fruit was fonnd to bc smeet. Tlie trees thus raised werc larger, more prod iictivc and hardier than the former kinds, and seedlings mcrc now constantly raised. Hence Gallesio conclitdes that much more was effected for the naturalization of tlie orange in Italy by the accidciital production of iiew kinds from seeds during a period of about sixty years than had been effected by grafting old varieties during many ages. It is liardly necessary to give other illustrations of this law, fur no one with any knowledge of the subject will be inclined to question it. It is strange that its significmce has been ovcrlooked, but this is probably due to the failure of students of the subject to perceive that it
148
Neredity.
is possiblc to bclicvc that tlic t m i s m i r s i o u uf I-ariaLili t y is the peculiar function of tlic ni:ilc ccll, mid iilso to :icknowledge that variation may occasioiiollg occur witlioat its influence. Oar theory that varintion is caused by tlic trausmission of gemmules, and tlint there is 110 espccitll xri’aiigenieiit for their transmission to buds or to unfertilized eggs, while there is a specia1 ndaptntioii which has bccii slowly evolved during the evolution of sex for trniisnritting them to fertilized eggs, gives us a simplc c.ul~l;~nation of tlie fact that while bud wriation is perfectly possible, i t is extremely rare as compared with the rariability of sexual offspring. Darwin has been lcd, through tlic study of nwi:ibility, to a conclusion ~vliicliis very pinch like the cslilalitition ivliicli is Iicrc presented. He sajs ( ITnrliltioa, Vol. ii. 13. 323) that ‘‘ we rnny infer from tlic occurmice of bud \-ariation tlint tlic affection of the fcm:ile clenient tliroiigli external conclitions m:iy iiidnce r ; n i d d i t y , for a bud seems 10 bc tlic ardogoe of 211 O Y l 7 k . But tlie itmle eleriaeiit i s nppai.eiitly nazicl~ qfteiier gffecfed by chanyed conditions, nt least iii n visible waiiiier, iliaii the ,female eleineiit 01’ ouule.” Brtd vnriatioti is mticli morc frcqiicnt jti cultirntcd plants than it is i n mild ones. Very fcw iiistanccs h i r e ever been observed in plants growing wild or iintlcr strictly natnral conditions, and Darwin states that ‘‘ bud variation is most comnioii in 17litnts nhich hnvc Lecu highly cultivated for a long time.” The adjustmcnt betmeeii a cultirntcd orginism and its artificial or unnatural enrironmcnt must, in most C ~ E C S , be less perfect than tliilt ivliicli lina LCCII sloivly cstablislied between a wild orgauism and its natural cnvironment. We should, therefore, expect domesticated and
The Evidence f r o n 17ar/aGoz.
149
cultivated forms to be more po?ifir of gcnmules than Tvilcl species. Tlic fact that Lad variation, like ordinary variation, is most common ii: cu!tivated fortiis, seems to shov t1i:tt the tcndency to vary is excited i n b~ids,as i t is in fcrtilizcd ova, by tlie inflnence of gemmules m1:ich are tlirown off by tlie cells of tlie body under new or unnutu r d conditions, and we can easily understand wliy it sIio:iId be niorc freqncnt wlicre gemmules are a’ouiiclant than in n forni with few gcmmulcs, for t h e chance in f:tvor of the accidental transmission of n geniriinlc to a growing or nascent bnd mill increase as tlie nnnilier of gcnimules increases.
Cliniiyed Conditions do mt act directly, b u t they cazis~
Subseqzierat r7eiieralion.s t o vary. This strange and, as I hope to sliow, higlily significant I:uv lias been noted by nimy obscr~.ers,and a long list of illustrations might be quoted. As Darwin points out, it is certainly a remark:tble fact that changed conditions should at first produce, so far as wc can see, absolutely no effect, bnt that they should snbsequently came tlie character of tlie species to cllallge. The late Dr. Jared P. lcirtland told me that for more than forty years he tried in vain to obtain varieties froxi the conimon red cherry, but that when a t last varieties began to appear the variability was yery great: tliat after it had once become established i t continued for many years with no diminution. It is well known that when new flowers are first introduced into gardens they do not vary, although all, with rarest exceptions, ultimately rary. Darwin, in his Variation, Vol. ii. p. 316, quotes the followiiig illustrations of this law: “Mr. Salter re-
150
Ner ed ity.
marks that every one knows that the chief dificnlty is in breaking tlirougli the original form and color of the species, and every one will be on the lookout for any natural sport, either from sccd or branch; that being once obtained, liowerer trifling the change may be, the result depends upon liimsclf. M. dc Joiiglie, with rcference to pears, s:iys tlie morc a type has entered into a state of variation, the greater is its tcnclciicy to continue doing so, and the more it IS disposed to vary still further. Vilmoriii says that when any particular variation is desired the first step is to get the plant to rary in any manner whaterer, and to go on selecting the most variable individuals, even though they w r y in the wrong dircction; for the fixcd cliaractcr of the species once broken, the desired variation will sooner or later appear. Darwin gives quite a list of authorities to ~ 1 1 that 0 ~ after English clogs have been bred for a fern generations in India they degenerate, not only in their mental filcnlties, but in form. According to Bachman, turkeys reared from the eggs of wild ones lose their metallic tints and become spotted with white in tlie third generation. - It will be seen from tlie instances which lime been given that the number of generations mhicli are csposed to the new conditions before variation is induced varies greatly. In the case given by Dr. Kirtland, fifty Tears elapsed before variations of the red cherry began to appear. I n the case last quoted, variation appearcd in the third generation, and Yarrell says that Australian diugos bred in the Zoological Gardens of EngI:ind, almost invariably produced in the first generation puppies marked with white and other colors. Sir Charles Lye11 mentions that some Englishmen en-
The Euidence from Vuriation.
151
p g c d in conducting the operations of the Real del 1\Ioiitc Company in Mexico, carried out with them some grcyliounils of the best breed to hunt t%e hares which abound in that country. It was found that the greyhonnda could not support -the fatigues of a long chase in this attenuated atmosphere, and before they could come np with their prey they lay down gasping for breath; bnt these same animals hare produced whelps, ~vhichhare grown up, and are not in the least degree incomnioded by the want of density of the air, but run down the hares with as much ease as do the fleetest of their race in this country. It is interesting to note in this connection that a tendency t o vary is strongly inherited independently of the inl%ritaiicc of any particular variation. Darwin believes that this tendency to vary may be transmitted by either parent, and he says (Vuriation, ii. 325) i t is certain that variability niay be transmitted through either sexual clement, whether or not originally excited in them, for Kijlreuter and GBrtner found that when two species mere crossed, if either one was variable the offspring were rendered variable. We have already pointed out that the crossing of species is in itself one of the most efficient causes of variation, and me can hardly base upon the observations above given the conclixsion that variability may be transmitted by either sex. The fact that changed conditions do not directly produce variation, but cause subsequent generations to vary, is precisely what we should expect, according to our theory: for a change in the environment of an animal or plant must disturb the harmonious adjustment which natural selection has brought about between the cells of its body and their conditions of life. Such a
162
Heredity.
change, if considerable, could hardly fail to affect certain cells unfavorably; and it woald tliereforc cause the production of gehimules, thus inducing variation i n later gener‘d t’1011s. We can also understand horn a tendency to vary may be lieredit:vy, for if certain cells of tlie body vary, tliey will exercise a disturbing effect upon adjacent or related cells, and these, transmitting gemmules, will hand on t h e tendency to vary to succeeding generations.
Secondary Laws of Varintioit. T h e law that variability is itself liercditary involves a number of secondary laws, all of wliicli find a ready explanation in our tlieory of heredity. Among these secondary laws is tlie law t h a t Y specllfic charaefers ore more vayinble than yeizeric clmmters.” Darwin 11asgiven the evidence of the existence of this law (“ Origin of Species,” p. 122), so it will not be necessary to discuss it, or to d o more than point out that t h e tlieory of heredity furnislics an explanation of it. Tlic cliaracters Tvliicli are common to all tlie spccies of n genus, and mliicli distinguish it from other genera, arc, as a rule, much older than those which dietingaish one species of tlic genus from t h e other spccies. T h e specific cliaracters or features which distinguisli each species of a genus from tlie others, are features mliich hare appeared as new variations since tlie time wlicn the various species diverged from tlie common ancestor from whom they inherit their conimon or generic characters. As spccific characters are of more recent acquisition than gencrio clinractcre, natural selection will hare had less time to act upon tlie former tlian upon the latter. T h e adjustment between a specific cliaracter and its environment will therefore be, as a rule, less complete and per-
The Evidence from Variation.
153
fect, and thc cells which are involved will therefore have a greater tcndciicy than those involved in generic characters to tlirom off gemniules. Tliese characters will tliercfore be more variable in the descendants than generic c1i:uxctcrs. Another lam,4tlie evidence for which is given by Darwin on page 4.1- of the “Origin of Species,” is that ‘(species qf fhe larger geiiera i 7 a each country vary more frepzrently than the species of the smaller genera.” When a country contains a great iiuniber of species of a genus i t is gciierally safe to conclude that they have recently varied and diverged from each other. As the tendciicy to vary is in itself hereditary, and as one variation is in itself a cause of other variations, our theory of heredity would lead us to expect species which have recently undergone considerable change to show a tendency to vary still further, and we should therefore expect tlie species of large geiiera to be, as a rule, more variable tlian tlie species of small genera, although there is no reason mliy this rule sliould be absolute. A still more interesting lam is that ‘ ( a part developed i n any species in a n extraordinary degree or inanner, in coinparison zuit7~the same part in allied species, tends to be highly vnrinble” ((‘ Origin of Species,” p. 119). When one species of a genus agrees with the other species in most particulars, but differs from them all in some one respect, we may conclude that the peculiar organ or feature has recently been modified. N a t u r d ‘selection has tlierefore had less time to perfect the ndjustnient bctwcen this part and the remainder of the body than it lins liad to perfect the relations between other parts, or between the same parts in the other species. This peculiar part will accordingly be in a favorable state for the production of gemmules, and it will there-
154
Heredity.
fore be more likely than a part which has not recently varied t o vary still farther. Walsh has called attention (“ Proc. Entomolog. SOC.,” Philadelpliia, October, 1863, p. 213) to what he calls the “Law of Equable Vnri:ition,” which is, “if any given chai.actcr is very variable iiz one speries of a yrozip, it will tend to 6e variable in allied q e c i e s , asid if any given character i s perfectly coilstant in one species of a group, it will tend to be constant in allied species.” This is by no means an absolute law, but simply a general rule. Darwinpoints out that something of tlic same kind occurs in domesticated races, and that in the forms which are now undergoing rapid improvement those parts or characters which are most valued vary the most. We can readily see that circumstances which cause a certain part t o throw oft getnmules, and thus induce variability, in one species, will be likely to produce the %me effect on allied species living under similar cii*cnmstances. We can also nnderstand that the divergent modification which has resulted in tile formation of several species or races from a parent form, will in itself be a cause of still further modification in the same general direct ion. Another well-known law, of which many examples will be given in Chapter IX. is that secondary sexual cliaracters are highly variable. I n the chapter on this subject I shall show that the distinctive sexiixl characters of a species are usually due to recent modificat’1011. Their g r p t variability is therefore due to tlic same cause as that which renders specific characters more variable than generic, and is exactly what our theory would lead us to expect.
The Evidence from Variation.
155
2CTrdurnl #election caniiof produce Race Motlijicaiion tinless the #ante Part tends t o vary i l l . a iVumber of Iiidiuiduals at the Sirvne Tints. Tliis argument, which seems to me to be the most importnnt one wliicli has ever beeii adduced against the theory of natural selection, was first adraiiced by a writer in the North B1itish Review in June, 1876. The anthor points out tliat since the cliance of surviral of m y lx~rticularindividual wliicli is borii is very slight indeed, the birth of an individual with any particular slight advant:ige, and its conseqnen t superiority over its fellows, would not be sufficient to over-balance the cliance of its destruction. The objection, wliicli is purely logical, and not experimental, will be stated at length in amtlier place. At present the fact that those who are best qualified to judge, Darwin among them, hare acknowledged its great weight, will suffice to shorn that it is a real and' valid objection, and that the foothold of the theory of natural selection wonlcl be greatly strengthened if we could show that the causes which produce variation act in such a way as to cause the mme part to w r y at tho same time in great numbers of individuals. According to our theory of heredity, this will generally be the case. We suppose that an unfavorable change in the environment of a particular cell causes this cell to throw 08 gemmnles. It is plain that a change i n the external world, which unfavorably affects any particular cell or group of cells in one individual, will usually affect the corresponding cells of other individuals of the species at the same time. JVlien any particular cell is prolific of gemmnles in one individual of a species, the same thing will usually be true of the same cell in other
156
Heredity.
individuals, and tlic corrcsponding cell will tliereforc bc a hybrid, and will tend to vary in many dcscendants. I n each of tliese descendants this hybrjcl will be composed of almost identical clements, aiid they will all tcncl to vary in the same or nearly the same manner; and as each variation causes other cells to throw otT gemmnles, the number of individuals which arc similarly modified will tend to increase from generation to generation, and natural selection will therefore act, not on a single exceptional individual, but upon a grcat number, all of wliicli are modified in essentially the samc way.
If Fariatioft is Purely Fortuitous, the Evolzction of a Corriplicated Organ composed of H a i q Pnrts by h-atural Selection demands a Period o j Time is almost Injinite. This obvious objection .to the law of natural selection lias been so frequently discussed that it is unnecessary to dwell upon it at present, cspecially as I shall examine it in detail in another place. At prcscnt I will onlycallattention to thefact tliatavariation inany part of a complicated organ will, in itself, disturb the harmonious adjustment of other parts, and mill thus cause them to throw off gemmules, and thus to induce variability in the next generation. The fact that change is needed in any part will be the cause of variation in this part, and the time which is needed to restore all parts of an organ to a position of equilibrium will thus be almost in6nitely redirced. The argument of those who hold that life has not existed upon the earth long enough for the evolution of :dl the adaptations of nature by the sclection of fortuitous variations will thus lose all its weight.
The Enidence f r o m Variation.
157
Saltatory Evolution. Darwin believes that the evolution of mild species is due, like the formation of many domesticated races, to very slow modifichon by the natural selection of great numbers of very slight and inconspicuous variations, b u t mAny other antliora have given reasons for believing that this is not the case. Many of our most peculiar domestic races have originated suddenly, and there are reasons for believing that the liistory of the evolution of each species is divided into periods of abrupt and extensive modification, alternatiirg with periods of comparative stahility. Tliis subject, likc those which have been biiefly noted in the last two sections, will be fully discussed in Chapter XI., :tnd I will only dwell upon it long enough at present to point out that our view of the cause of variation implies that any particular change should in itself be a fruitful source of still greater modification, so that as soon as a tcndency to rary becomes established it will continue to iiicrc;ise until an equilibrium is again established by the natural selection of tliose modifications which are adapted t o the environment.
Correlated Variation. Tliis snbjcct will be fully discussed in the chapter on homology, but a few words upon it will not be out of place 11cre. Damin, who freqnently nses the term, inclades under i t facts ivliicli belong t o two somew~11;ttdifferent c hsses. . Whcn any part varies, the oi.gnus with which it is most directly associated also tend to vary in such a w:~y:is to restore the harmonious adjustment bctwecn the various parts: and a variation in one part is often accompanied by mriation in homologous parts.
168
€&edit y.
These two cases shade into each other somewhat, but
i t mill be convenient to treat them separatcly. The first hits just hccn briefly cxamincd, 11. 156, and what follows rcldcs only to tlic second class of cases-the variation of honi 0 1 ogous p i r ts. The most fiimiliar illustration of this law is the fact t h z t in most bilxteral organisms homologous parts on both sidcs of tlic body tend to vary togctlier. The law holds in radially pymnictrical organisms also. All the petals of a regnlar flower generally vary in the same manner, but tlicre are many exceptions. Tlie front and hind linibs of vertebrates tend to ~ a r y iu tlic same mannci., as wc we in long and short legged or in thick and thin legged races of horses and dogs. It is stated tliat wlien the muscles of t h e arm depart i n number or arrangement from t h e proper type they almost almajs imitate those nf the leg, :tnd so converscly t h e varying mnsclcs of the leg imitate the normal muscles of the arm. There are many cases where a parent with extra fingers has produced a child with extra toes, or tlic reverse, and in otlier cases a parent with only one extra digit on one hand has liad children with supernumerary digits on both hands and both fcet. In certain pigeonsand for~ls,especiidly in the trumpeter pigeon, long feathers, like t h e primary wing feathcrs, grow on tlic outside of the leg and on the two outer tncs, and in pigeons with the feet thus feathcrcd the two outer toes arc partially connected by skin, thus slioming a marked :matomica1 resemblance to a wing. Tlie wrious appendages which are foymed from tlic skin, such as Iioofs, horns, hair, fcathers, teeth, etc., are homologous organs, and i t is interesting to notice liow frequently a pecnli:irity in one of these structures is associated with similar peculiarities in others.
T 7 ~ eE o ideizce f r o m Vuriation,.
150
Tropical sheep with long C O ~ I ' P hair ~ usually 11:~ve go;tt-lilcc horns. Iiilicritecl baldness irl nian is: often acconil)anied 1 y dcGcicnt teeth, iLritl tlic rctietval of the hair in old t i p by :t i~nev;nIof tlie tcctli. The famous liairg Burniehe 1i;tcl dcficient teeth, and Loth peculiarities mcrc liercdis,iry. A Spanisli dancer, Julia Pastrana, liad a f d l b ~ a r nand a double set of teeth, and the daily papers Iiavc rcce~tlycontained ail account of a man, Iivingnear Lebanon, Pennsjlvania, witli no hair, teeth, or sweat gla MI s. The homologons p u t s of plants often vary i n the same way, ns is well shown by certain comljound flowers, ill which tlic stamcns and pistils closely resemble petals. According to our view of the cause of viriation we can easily see how gemmules from a cell in one hand might lijlritlize, and thus c;tuse variation in tlie correspoiicling cclk of a11 four extremities, or perliaps iu the embryoiiic ccll from which all these cells arc derived, f m in tlie Snmc way that an animal can u i t e sexualiy either with mother of its own race or with one wliicli is somcwliat less closely related to it, so I assume that a gem111ule may unite with tlie particlc of the ovnm wliicli COP rcspoiids to it, or with sonie other closely related particle. For example, agemmule wliicli is thrown off from L: partku1;rr epit lielial cell may simply cause modification i n the corresponding cell of tlic offspring, or it may c m s c modification in a cell which is to produce this particular celI and a number of others. If each variation is purcly fortuitous the number of generations which mould be necessary in order to convert a species with bIacli hair into a spwies with every hair brown or with every hair red IS almost inconceivablc, bot this difficulty entirely disappears as soon as we recogtiizc tliat gemmules from oue part of the parent may
160
Heredity.
affect all the homologous parts of tile offspring in the same way and a t the same time.
Mules more Variable thaiz E’emnles. One of the most remarhble and suggestive of the laws of variation is that in all the higher anini;ils a part nhich is confined to males, or is more developed or of more functional importance in males than i t is in females, is Tery much more variable than a part vhich is confined to feniales or is more important in females 1h:m it is in males. The evidence for this remtirkahle law mill be presented at length in Chapters VIII. and IX. The cxlsteuce of such alaw is absolutely inexplicable willlout the theory of heredity, but it is exactly mliat lhis theory woulcl lead us t o expect, for an organ which is most iuiportant in one sex is most likely to be influenced in this sex by changed conditions, and is thercforo more likely to form gemmoles in the body of the sex wlierc it is most important than in the body of the opposite sex. An organ mlmh is most important in males will thercfore be most prolific of gemmulcs in males, while an orgnn mliicli is most important iii females will be most prolific of gemmules in females. Gemmules which are formed in the niale body are mstly more likely to be transmitted t o dcscciidauts than those which are formed in the female body. It fol1on.s that an organ which is most developed or most important in males must be vastly more liboly to transmit gemmules to descendants, and therefore t o \nrg in successive generations than an organ which is most developed or most important in females. Another law which follows from the one which has just been stated is that males are as a rule more rariable than females. This Inw has been noticed by Darwin and others, but no exidanation has ever becn aclranced. ,
The Eridence from Variation.
161
Summary of Last Two Chapters. The study of hybridx and of variation has led to the discovery of a great number of general laws, all of which are perfectly explicable by the theory of heredity, and are precisely what it would lead us to look for, although most of them are absolutely inexplicable without it, and have no place in any other hypvtliesis which has ever been proposed to account for the phenomena of heredity. The study of hybrids gives 11s a means of analyzing to a certain extent the influence of each sex in heredity, but our experiments in this direction are limited b j the fact that organisnis must be very closely related in order to breed together, and parents which are very closely related must be essentially alike in everything except the most recently acquired modifications. So far as tliey enable us to analyze the influences of the sexes, the results furnished by hybrids agree with the demands of our theory. This fnruislies an explanation of the great variability of hybrids, as compared with the p w e parents, and it also enables us to understand why hybrids from domestic races shoulc! be more variable than those from mild races. The remarkable fact that the descendants of hybrids arc more variable hnn the hybrids themselves receives a simple explanatio by our assumption that exposure of the various cells of the body to unnatural conditions is the prime cause of variability, and that it acts indirectly by causing the prod action of gemmule? Some of the recorded facts regarding hybrids are so very pecnliar that it mould be difficult to devise better tests tlian they furnish of the truth of our theory. What could be more curious or more opposed to the
d
162
Neredity.
view that tlie sexes play similar parts in heredity than the fact that the o3spring of a male hybrid and tlic female of a purc species is much more variable than the offspring of a female hybrid by a father of pure blood ? D:irwin’s pangcncsis hypothesis f iirnishes no explanation of this most remarkable fact, and none of the hgpotheses of heredity which haye been proposed from time to time are sufficiently dcEnite to have any bearing upon a concrete case like this, but our theory that changed conditions of life cause a production of gemmules, and that these are stored up in and transmitted by the male element, fits tliis case exactly. The curious plienomena of reciprocal crosses, a g i n , are just what our theory would lead us to expect, and it also farnishes 11s with an explanation of the fact that crossing so frequently causes reversion. A comparison of sexual with asexual reproduction also gives us a means of analyzing the inflnences of the two sexual elements, for asexual reproduction is essentially reproduction with the male element left out, and the result of this omission is, as we should expect, tlie reduction of the tendency to vary to a minimum. At the same time that OLU theory explains the great rarity of bud variations, it admits of their occasional appearance, and i t gives an explanation of the singular fact that bud variation is much less rare in plants which have long been cultivated than it is in mild forms. The most remarkable of the laws of variation is the well-known law that changed conditions do not directly produce variatibh, but cause subsequent generations to vary. As changed conditions do not in tlicnisel.F-escause hereditary modification, but simply lead to the production of gemmules, we see why their effect should be manifested in succeeding generations, and we also see
The Evidence from Variation.
163
why variation is itself hereditary, for the variation of any p:irticular cell will cause adjacent or related cells to tlirom off gemmules, and thus to produce variation in successive gcnerstions. We can also nnderstand why specific characters should be more variable than generic characters; why the species of large genera should vary more than the species of small genera; wliy a part developed in an unusual way or to an unnsual degree should show a marked tendency t o ~ a r y and , why secondary sexual characters should exhibit a similar tcndency. Uiiless our theory is true, what possible reason can tlicre be wliy a part whicli is excessively developed in males should vary more tlian a part whicli is similarly dereloped in females alone, or why the males of our Iiiglier domesticated animals should be more variable than the females? I t s power to deal with and interpret special cases of this kind scparates our theory from all other attempts to exldnin the phenomena, and seems to show that there can be but one choice between it and any otlier explanation wliicli has ever been proposed. If we accept Darwin’s view that variations are purely fortuitous, tlicre are certain grave difficulties whicli must prcvent 11s from giving the theory of natural selection unqualified acceptance as an adequate and complete explanation of the origin of species. Natural selection can rarely lead to permanent modification unless many individuals tend to vayy in nearly the same may at about the same time, and if variation is fortuitous the cliancc agxinst this is very great indeed. While tliere is no reasoii to doubt that natural selection might briiig about $1 the clianges which have led to the formation of a complicated organ, by the preservation of fortuitous variations, if time enough were given, there is
Heredity.
164 ~~~
reason to doubt wlietlier life has existed long enough to permit tlie evolution of existing forms in this way, and natural selection gives no account of the sudden appearante of considerable modifications, although the history of domestic animals shows us that sncli saltations do sometimes occur. On the one hand we find that Darwin’s assumption that variations are fortuitous involves us in grave dificnlties, but on the other hand we find scarcely any evidence to show that permanent hereditary race modifications are eves directly produced by the action of external conditions, while we do find evidence for the opinion t h a t race modifications are, as a rule, not due to this direct action, but to congenital variation. Our theory furnishes an explanation wliicli lies midway between Darwin’s view of the origin of variation and tlie Lamaskian view, and thus enables us to escape both these dificulties, for it shows us how the influence of changed conditions upon an organism may give rise to congenital variation in 1atei. generations, and it also shows‘us why variations tend to appear a t the time and place where they are needed. I t also shows how a considerable modification may appear suddenly and become hereditary. The correlated variation of homologous orgacs and the correlated modification of the various parts of a complicated organ are accepted by Darwin without explanation, but the theory of heredity shows us tliiit tlicse phenomena, the chance against the fortuitous occurrcnce of which is almost infinite, are due to the working of a very simple law. When we reriem the ground and sce how all the phenoiiiena of hybridization and variation fall into their proper places; how the same simple explanation fits the
The Evidence from Variation.
165
niost anomalous and exceptional phenomena as well as tlic more ordinary and simple cases, I tliiiik we must acknowledge that our theory is a t least an approximation to tllc t r d r . If it leads us to tho discovery of truth, and thue ultimately contributes to the establisliment of an explanation of tlie phenomena of heredity, its final acceptance in its present form is R matter of little moment. That it is R great advance beyond all the attempts which have been recorded s e e m obvious, and an examination of the ground whicli it covers ccrtainly seems to show that it is a step in the right direction.
CHAPTER VIII. T H E EVIDENCE FROIT SECONDARY SEXUAL CHARACTERS.
T h e N a t u r e of this Sort of Evidence.
~
I HAVE already given many reasons for believing that the male reproductive organ is especially adapted for gathering up the gemmules which are thrown off by the cells of the body; and for transmitting them to tlie next generation by impregnation, thus giving rise to variation; while the transmission of the gemmules which are formed in the body of the female is not thus provided for. If this supposition is correct, w e should expect to find that a variation wliicli first appears in a male should have more tendency to become hereditary than one which first appears in a female. Any slight change in either the male or the female body will, as we have already seen, cause all the cells which are either directly or indirectly influenced by the change to throw off gemmules. This will happen in a female body as well as in a male body, but the gemmules are, in tlie latter case, much more likely to be transmitted to descendants, and thus to give rise to more extended modification. We should also expect to find that an organ which is confined to males is much more likely than one which is confined to females to undergo hereditary changes, for even if the parts of the female body give rise to gemmules as frequently as the parts of the male body, tho chance of transmission is much less. We should also expect to 6nd that parts which are
The Evidence from SexuaZ Characters. 167 confined to males are more variable than parts confined to females; for variation in any part is due to inheritance of a gemniule from the corresponding part of one parent or the other, bat when the part is found in only one parent the gemmule must come from that parent. As transmission of gemmules by the mother is more rare than transmission by the father, i t is plain that parts which are confined to the male should be expected to vary more than parts found in the female alone. Finally we should expect thc male body as a whole to be more variable than the female body, for the same reason. In most cases it is impossible t o trace any particular variation back to its first appearance. This is almost out of the question with wild animals, and most doGesticated races have been formed so slowly that it is impossible to say whether the successive steps appeared in males or in females, nor can we be sure that a variation is new when it first attracts attention. Stilkit is interesting to note that the sudden variation which resnlted in the ancon breed of sheep was first noticed in a male, although it is, of course, impossible to say whether it was due to inheritance of gemmules from the father ratlier than from the mother. Certain hereditary diseases and montrosities, such as albinism or polydactylism, are fully as often traceable to a male origin as they are to a female origin, but as me know that peculiarities of this kind frequently skip a generation or two, we can xievcr be snre that we have traced them to their origin. In the secondary sexual cliaracters of animals we have a class of phenomena which are not rare and exceptional, for they are numbered by hundreds of thousands, and they can bc observed and studied by every one.
168
Heredity.
A secondary sexual character is a peculiarity which is not directly concerned in the reproductive process, although it is normally either confined to one sex, or else is much more developed in one sex than it is in the other. The presence of a beard is a secondary sexual character of man; the comb, wattles, spurs and brilliant plumage of the domestic cock, the horns of a stag, the tusks of an elephant, the mane of a lion, or the brilliant plumage of the peacock or of the drake, are all of them examples of this sort of organs, for they are either confined to one sex, or else tiley are macli more conspicuoukand important in one SCX than they are in thc other. They furnish, like hybrids, a means of disentangling or analyzing to sonic extent the influence of the two sexes in heredity, and I hope to show in this and the following chapters that they fizrnish evidence to prove1. That in most animals vith separate sexes the males of allied species differ more than the females from the ancektral type. 2. That organs which are confined to males or are of more importance or are more perfectly developed in them than in the females, are much more likely to give rise to hereditary modifications than parts which are confined t o or are most developed in females. 3. That a part which is confined to or is most developed in males is more likely than a similar female part to vary. 4. That males are, as a rule, more variable than females. 5. That the male leads and the female follows in the evolution of new races. There are two criteria which are of great use in the attempt to trace the path which a species has followed in its e-iolution. One of these is by comparison of a
The Evidence from Sexual Characters. 169 species with its nearest allies. The other is by comparison of the young with tfie adult. If most of the species of a genus resemble each other i n certain chamcters, while one species presents a marked deviation, we may in mosL cases safely conclude that the latter species has undergone recent modification in this respcct. Of course this rule does not hold good where the peculiarities of the exceptional species are featnrcs of resemblance to other genera of the family, for in this casc we must conclude that it has remained comparatively stationary, while all the otlier species of the genus have been modified. If in the second place we find that the adults of several related species differ greatly, while the young are much alike, we must attribute the difference in the adults to the fact that they have recently diverged from a common stock. Now I hope to shorn that throughout the animal kingdom, wherever the sexes differ from each other, the gencral law holds good that the males of allied species differ from each otlier more than the females do, and that the adult male differs more than tlie adult female from the young. There are many marked exceptions to this law, but the existence of the law has long been recognized by all naturalists. Every one who has worked at the systematic zoology of insects or vertebrates knows how difficult it often is to decide upon the specific identity of an immature or a female specimen, emn in cases where the mature males can be recognized and identified without difficulty. Darwin’s interesting essay on “ Sexual Selection” is well known. It is almost entirely devoted to the study of secondary sexual characters, and to a masterly discussion of tlie subject in all its aspects and relations.
170
.Beredity.
Darwin has gone over the whole field so thoroughly and exhaustively that little remains t o be said upon the subject, and the reader who is familiar with the essay will discover that almost all the facts in this chapter are borrowed from this source. Darwin’s aim, however, is simply to show the potency of sexual selection, while our present object ia to show the frequency of hereditary male modifico‘1t’ion as compared with female modifications, and I have therefore rearranged the facts, so as t o gire especial prominence t o this aspect of the subject. The critical reader will discover that in many cases I have borrowed the descriptive portion of one of Darwin’s paragraphs, but have said nothing about the theoretical portion. As Darwin’s conclusions are in many cases opposed to my own, this may convey to some the impression that I have made an unfair me of the weight of his anthority, and have quoted him in support of conclnsions which h e in reality opposes. I will refer such readers t o the chapter which follows this, where I have devoted a section to a statement of Darwin’s view of the origin of secondary sexual characters, and have given my reasons for beliering that it is only a partial explanation of the phenomena in question. Examples from Various Groups of the Animal hringdom to show that in ull Groups zultere the Sexes are Separate the Mule is, as a Rule, more Afodijed fhan the Female, and that the Adult Nales of Allied Species differ more, as a Rule, fhan B e Females or Eomg. R O T I F E R A. - h 1849, Dalrymple (Description of an Infusory Animnlcnle allied to the Genus Notommata, Pl~il.Trans. 1849) made the interesting and remarkable discovery that, in one species of the Itotifera, Notona-
The Evidence from Hexual Characters. 171 niatn A~zyZica,the animals are not hermaphrodites, as earlicr writers liad supposed, but that the mnles, which are rarely met wilh, arc very much smaller than tlie females. The latter sex is furnislied with a digestive tract which is quite complicnted in structure, and is armed at the mouth wjtli a lijglily specialized masticating apparatus. Tlie digestive organs of tlie male, 011 the other hand, are almost absent. Thc jaws, the msophagns and the mouth are wanting, and thc stomach and intestine are reduced to a functionless rudiment. The males reccive no'nonrishment after they lcavc the egg, and they live only a short time. Tile prcseiice of a digcstive tract is characteristic of all groups of animals above the protozoa, so we are compelled to believe that the ancestral form from which the Rotifera are descended had, likc the ordinary metazoa, a mouth, a stomach, and an intestine; and no one who is a t all familiar with comparative anatoniy can doubt that the male, in which it is absent, rather than the female, in which it is present, is the sex which has been modified. Tlie digestive tract is usually one of the first parts to be developed in tlie embryo, and its disappcarancc or absence in the adult male rotifer is therefore very different from the absencc of the wings in certain fcmale insects. Wings appear very late in life, and the failure of the female to acquirc them is simply at1 arrest short of perfect development, while the absence of digestivc organs shows active degeneration. I n 1855 Leyclig verified Dalrymplc's observation (Zeit. f. Wiss. 2001. vi. p. 96) in the samc s1xcies, and also in a second species of the same genus; and as Iic mas able to distingnisli tlie outline of tlic malc inside the egg, mliile this was still contained witliin the body of tlic fenialc, he remored all reason for doubting that the two sexes belong to one
172
Eeredity.
species. In these two species the feniales were much alike, while thc males mere not only very different from the females, but also froni each other.
FIQ. 1. Young female Rotifer. Hydutina senta. a. cloaca. b. contractile vesicle c. water tubes. d. stomach. e. ovary. f.ganglia. g. stomach. i . mouthparts.
FIQ.2. Male of the same species. a. orifice of penis. b. contractile vesicle. c. testes. d. ganglion. e. setigerous apparatus.
Sitice the year 1S55 the subject has been studied by many naturalists, and tlie ninles have been found in such a number of species that it is probable that the
The Evidence from Xexua.l C7~uructers.173 scxcs arc scptratc in a11 tlic Rotifern. In somc forms tlie niillcs itrc cven niorc siniplilicil than in Xotownzatn, wliilc i n otlicrs t h y arc lcss so, and in a few tlicy :ire like tlic fcni 11csi n size wid sirticturc, aiid liarc tlic digestive urgttiis perfectly dcvcloped. ANsmms.--Among tlic mnrinc p o l j c l ~ ~ t o uannelids s tlierc IS uf ten cwsidcr:tblc diffcrciicc between the sexes, and tlic points in wliicli tlie male diffcrs from tlie female are :dso points in ivliicli the males of various spccics differ from cacli other. ARTHROP0DA.-Among the Artlir0110d~,the Insects, Crnstacea, ete., the female is often very grcatly modified, and in somc case3 the females of allied spccics differ from each other inuch more than the males, and in oilier cases i t is linrdly iossible to say whether the males or tlie females of allied species differ most, but, taking the group as a whole, the Antliropods seem to follow the law which prevails in other groups of animals, and male modifications are more numerous than female modifications. I n the Branchiopod Crustaces the males are smaller than tlie females, and are mnch less abundant. The male differs from the female in the possession of a numbcr of sccond:wy sexual characters. Tlie second antennae of tlie malc are more richly supplied with sensory hairs than those of tlie female, and various appendages of tlie male may be so modified as to form clasping organs for holding the female. In Branchippus tlie second antcnnae of the male arc greatly modified for this 1mrl)osc. Fignre 3 shows the head of a female spccinicn of Brnizc?i@ps Grzibei, 'figure 4 tho liead of tlic m:dc of the same species, aiid fignres 5 and 6 tlie heads of thc males in two closely allied species. These figures show horn much the males of the various species
Heredity.
174
difler from each otlier iu this respect. Thc shape a d structure of tlie first antennae a i d of the abdomen may
FZQ.3. Eead of female specimen of Branchippus Crubei, greatly enlarged. a . Arst antennm. b. second antenna
FIQ.6. Head of male Branchippw stagnalia
Fro. 4. Head of the male of the Fame species.
FIG.6.
Head of male drtemia salina.
also show considerable modification in tho males of sarious species of Branchiopods.
The Evidence from Sexual Characters. 175 Among the Cladocera, of which the common waterpea of our fresh-water ponds and lakes is an example, the female is provided with a brood poiich, within which the eggs are carried and the young developed. I n the male these structures: are absent, and the second antenns are especially modified as organs for discovering and holding the female. They are richly supplied with sensory hairs, and they are often armed a t their tips with grappling hooks, which differ in the males of closely allied species.
FIQ.7. Antenna of male Cyclops aerrulatun.
FTQ.
8. Antenna of male Cyelopr canthocarpoiden.
The Ostracoda present sexual differences like those in the Cladocera, and in many of them it is certain that the male part deviates, more than the female part, from the typical form. I n the non-parasitic Copepods, of which the freshwater CycZops (Fig. 9) is an example, there is not very much difference between the sexes, although certain appendages, which are unmodified in the female and retain their typical form, sometimes differ greatly in the males of allied species, and may be specially mod-
176
Heredity.
ified for discovering or holding the female. The modification of the first antenna of the male for this purpose is quite general, and a comparison of this part in the males of various species of Cyclops (Figs. 7 and 8) with the same part in a female (Fig. 9), sliows how much the males of allied species differ in this respect. The second antenna, the maxillary feet, and the last pair of
FIQ.9. Female specimen of Cyclops canthoeawoides.
FIG 10. Female specimen of Notodelphys Allmani.
swimming feet, are sometimes modified in the same way i n the male. I n the male Saphirrina the wonderful display of brilliant colors is due t o the presence of peculiar color-producing organs, wliicli are absent in the female. Among the parasitic Copepods we find a departure from the ordinary typical structure, which is so remark-
The Evidence from Sexual C‘haructers. 17’7 able that no one, on first examining one of the more modified parasitic forms, such as tlic oiic sliown i i i Fig. 15, would detect any resenlblancc to tlie frce or nonparasitic members of tlie groul), or mould even suspect that the animals are crastaccans. The femalcs, which are known as “$sh-Zice,” are parasites upon fishes and other aquatic animals, while the males are parasites npon the bodies of the females, and are usually of minutc size as compared with the females. The adaptation to a parasitic life has not only produced the most profound changes in the general structiire, but it has also brought about an almost unparalleled difference between the sexes. It is true that, this is not due to the modification of males alone, for the females as well as tlie males exhibit the moPt extreme departures from the organization which is characteristic of typical or non-parasitic crustacea, and it is difficult to decide from structure alone whether the male or the female is most modified. The fact that the male has been adapted to a life as a parasite npon the body of the female, while the female has simply become adalitcd t o a parasitic life on other animals, seem to show that the male organism is somewhat more plastic than the female. Simple parasitism may be brought about by indefinite variability, but parasitism npon a parasite demands definite variation to meet the definite changes which hare taken place in the host. The liiglily specialized parasitic Copepods arc joined t o the non-parasitic forms by a long series of intermediate species, in which the parasitic habit is only slightly developed, and I give aL’femfigiires to illustrate some of the steps in this most interesting series. The female Notodelphys (Fig. lo), which lives in the body cavities
1'7s
Her edity.
of marine invertebrates, and has very limited powers of locomotion, liardly differs from the non-parasitic Cyclops (Fig. 9), except that two of the body segments are modified to form a chamber in which the eggs undergo their development. The male (Fig. 11) is somewhat
FIQ. 11. Male specimen of the same species.
Fro. 12.
Female specimen of Lernentoma corunta.
smaller than the female, but bears a close resembance to her, and to ordinary copepods. The female Lementornn (Fig. 12) is very different from the nialc (Fig. 13), and both dcpart very greatly from the typical copepod struoture, although a slight resemblance can be traced between the female and cyclops. The female is very much larger than the ordi-
The Evidence from SexuuZ C7mracters. 179 nary non-parasitic forms; the segmentation of the body is hardly visible, the power of locomotion is entirely lost, and the appendages are either rudimentary or are changed into hooks for clinging to the animal infested by the parasite. The male, like the female, has no power of locomotion, and is very much smaller than the female, the difference in size being much greater than the two figures would indicate. It is found nowhere except upon t,he body of the female, to which it clings by its rudimentary feet. The fernale of another form, Anchorella, is show1 in Fig. 15, and the male in Fig.
FIG.13. Male specimen of Lernent o m eorunta.
FIQ.14. Male specimen of dnchb rella uncinata.
14. I n tliis species the males are very small as comlmrcd with the females, to whose bodies they are firmly fastened by their rudimentary hooked limbs. We can liardly state with confidence that either sex is more modified than the other in these parasitic copepods, for both have undergone such great changes that they have lost all traces of their crustacean affinity, but in the very similar case of the barnacles, we have sufficient evidence that the males do depart further than the females from the ancestral type. The barnacles, or acorn-shells (Fig. IS), are crustacea
Heredity.
3 80
which arc prctty closcly related to tltc copcpods, n hich t h y rcecniblc somewhat, during tlic carly stilgcs of their devclolmcnt. Tlic j o n n g swim freely i n tlic water for a time, but fiii:tlly :itt:ich tliemeclvcs to foreign bodics, hc:d downrviirds, by tlicir aiitcnna, and arc scdcntiiry for tlic rest of their 1 3 . Ti1 the stitlkcd or p~di~iicuulated barnacles, the antcnns of the free 1arv:a bccortie replaced in the adult by a long peduncle, at the top of
Fro. 15. Female specimen of Anchorella unciiiata.
16. An hermaphrodite stalked barnacle. Pollicipes corn7wopia. c. csrina. t . tergum. 8 . scutum. r. rostrum. p peduncle.
FIG.
which there is an irregularly triangular box, the cnpitulum, made up of a number of calcareous plates. Inside this box tlic animal is pliiced, liead domnnmds, :ind altlioiigh it is grciitly niodifcd t o fit it for t h i s protccted scdcntary life, i t still prcsei:ts t1nniiStakiible cridcnccu of its crustnccaii affinity, such ns the mouth-pirts, tlie segmented body and limbs.
The Evidence from Sexual Characters. 181 Onc of the most rcmarIialsIc characteristics of the Bmiacles is that, with a few execptions, they arc herni:q)hroditc. Tlic Artliropoda include a vcry considcrable proportiou of a11 tlie animals mliicli arc known to US, and as all of them, except the Barnacles and a few closely rclated parasitic forms, have the sexes separated, the f x t that these few sedentary forms arc hermaphrodite is certainly very remarkable, and we must believe that they are tlie descendants of crustacea with separate sexes. The stalked barnacles resemble typical crustacea much more closely than do the sessile ones, and we must regard' the fornicr as more closely related than the latter to the ancestral form with separated sexes. It is, therefore, interesting to find that a few species of stalked barnacles are male and fcmale, and also that in a few others the ordinary hermaphrodite form is accompanied by a parasitic male, which has been called by its discoverer, Darwin, a complementary male. The study of tlie few species with separate sexes and of those with complemental males has brought to light some of the most remarkable phenomena of natural science, and the subject is well worthy of extkndcd notice. Figure 1 6 is an ordinary hcrrnaphrodite stalked barnaclc, Pollicipes. I t belongs to a genus in which no triie males or true females are ever found. Figure 17' is a species belonging to a closely related genus, Scalpellunz, and it will be seen a t once that it closely resembles Pollicipes, even in the arrangement of the plates o f d h e capitulum. I t is an hermaphroditelike Pollicipes, but with a difference, for it carries inside its shcll a small parasitic complemental male, which is shown in Fig. 18. This male is very much smaller than the hermaphrodite, and Fig. 18 is considerably magni-
fied, while Fig. 17 is of nearly the natural size; b a t with this exception the complementary male is essentially like the hermaphrodite, and i t has the structure of an ordinary stalked bai*nacle. There is a distinct pednncle, which carries a triangular capitulum, and although tile plates arc somewhat reduced in number they agree in form and position with the chief plates of such a species as PoZZicipes. The animal inside the capitulum is milch like an ordinary barnacle, the essential diffei~encebeing the total absence of female reproductive organs. I t is a male and nothing more.
FIG.17. An hermaphroditebarna-
Fro. 18. Complemental male of the
cle. Scalpellurn villosum.
same speciea
Figure 19 shows the female of another species, IbJa.
Cumnzingi, which does not differ essentially from the forms shown in Figs. 16 and 17, but the female of lbla Cunzmingi is a true female instead of an liermaphrodite, and tlierc are no traces of male reproductive organs, but inside her shell, and planted by a long rootlike process, there is a minute parasitic male, shown] in ~ i g 20, . magnified thirty-two times, wliiB t ~ i efigure of the female is magnified only five times. In Fig. 20, b is part of the -wall of the body of the female, and a is the long root by wliicli the parasitic male is planted.
The Evidence from Sexual Characters. 153 The male has a capitulum, but no calcareoiis plates, and its antenns, an., are not completcly merged in tlie peduncle. It also differs from tlie Eem:ile in the possession of an ocellus, or eye-spot It lias mouth-parts and limbs, and, except for the fact that all its parts arc somewhat rudimentary, it does not differ very greatly from other barnacles, except as regards its reproductive organs. I n other species of Scalpellum, however, as in 6cal-
Fig. 19. Female specimen of Ibla Cumingii.
F I ~20. . Parasitic male of the saw species.
pellum Regium, the male is still more rudimentary, and has no mouth or digestive organs. In two other genera, Alcippe (Fig. 21) and Cryptopliyalns, the femalee, which are true females, with no trace of male reproductive organs, differ very essentially from ordinary barnacles, and they have fastened to the outside of their bodies a number of very small males. I n the males of these two species, which are shown greatly niagnified a t Fig. 22, there are a few faint traces of muscular fibres, but the organs of digestion
Heredity.
1S4 -
are entirely gone, and tlie inside of the body is entirely filed witli a great testis, while t!ic posterior end is prolonged into a11 enormow penis; and the aninial hardly deserves to be called an animal at all, as it is scarcely more than an independent male reproductive organ attached to tlie body of tlic female. This is certainly on0 of the most remarkable cases of djfPereiiee bctmecn tlic sexes, and no one who compares Figs. 1s and 22 witli Figs. 16, 17 and 19, can doubt that
Fig.21. Female of AZcippe Zumpus.
Fig. 22. Male of the same species.
among these bariiacles the males differ from each other much more than the females. Amoiig the higher cmstacea we find great numbers of cases where tlie young male is like tlie adult female, or tlic young of both &exes, but at mntnrity acquires distinctive sexual characters. Any one who is familiar with the crnstacca will acknowledge tlie existcnce of tliis plienomcnon, and i t mill only be 1iecess:iry to give a few illustrations. The adult male Lucifer is distinguished from the adult female by tlie posses-
The Evidence from Sexual Characters. 185 sion of a very peculiar clasping organ, figure 23, upon the first swimming appendage of the abdomen. The corresponding appendage, fignre 24, of the adult female is like the other abdoniinal appendagcs of both sexes, and me must believe that die peculiar form, in the male, is due to recent n1odific:ition. I t is therefore interesting to note that, before the male reaches maturity, the limb in question is exactly like the other abdominal appcnddagcs of the adult male or adult female. The male Lucifer differs from the fernale in the sliape of the last segment of the abdomen, and the outline of the
BIG.23. First abdominal appendage of a mule Lucifw.
FIG.24. The corresponding appendage of the female.
exopedite of the tail-fin is peculiar. These differences are very slight, as mill be seen by comparing the terminal segments of the male, figure 25, with those of the female, figare 26, and it is hardly possible that they are of any direct service in reproduction. Tlie fact that, in the young of both sexes, these parts are like those of the adult female, and that their peculiarities in the adult male arc due to a final change which does not occur in the female, indicates that race-modification has gone n little further in the male Lucifer than it has in the female.
186
Heredity.
Darwin says that it seems to be a general rule among the crnstncea, that the remarkable differences of structnre Tvhich distingaish the male from the female, do not make tlieir appearance until the male is nearly mature. In proof of this he refers t o the fact that the male sandhopper, does not acquire his large claspers, which are very differently constructed from those of the female, until nearly full grown, while the claspers of the young male resemble those of the female. The history of the abdomen in crabs seeins to show
FIQ.25. Tip of abdomen of male Lucifer.
FIG.26.
Tip of abdomen of female.
clearly that this difference is due to the fact that the male has deviated further than the female from the ahcestral type. The long-tailed cmstacea, like the crayfish, have a long free movable abdomen, ending in an enlarged tail-fin, and composed of a number of segments, each of which carries ;Ipair of ;ippendages, In the female cray-fish the first of these appendages are like those behind, but in the male, the first ones are peculiarly modified and form copiilatory organs. We have ample evidence that the true crabs are the modi-
The Evidence f r o m XexuaZ Characters. 187 fied descendants of an ancestral form which had, like the cray-Esh, R long free tail, -which Tvas used in swyiniming. T h e fact tliet the young crab does have such an abdomen, is one of the proofs of the correctness of this view; but as tlie crab grows up, the abdomen becomes curled forwurds under the body; it ceases to be used as a swimming organ; its separate rings become fused together, and its appendages become rudimentary or disappear. This very instructive change goes further in the male than it does in the female, for in the latter, more of the riiigs remain distinct; a greater number of appendages persist in the adult, and these are much more like those of the young, or of the crayfish, than are those of the male. The great modification of the male as compared with the female is well shown, among the crustacea, by the fact that there may be in the same species two different m d e forms. This sexual dimorphisnz, as it is called, is well shown in a Brazilian amphipod, Ordesiia Darzoinii, in wliich species there are two male forms which differ -from each other in the structure of their large cla~vs. These claws are used for holding the female, but as both forms are now used for this purpose, either shape would certainly have sufficcd as well as the other, and this case therefore differs greatly from that of the social insects, where one form performs a certain duty in the community, while another form is adapted to fill a different place and perform a different duty. The two male forms in Orchestia seem to be due simply to the tendency of the male o i p n i s m to become modified more rapidly than the female, and not to any great advantage which has resulted from the divergent modification. In discussing this case Darwin says that the two male forms have originated by some having varied
188
Heredity.
in one manner, and some in another: both forms having derived certain specid but nearly equal advantages from their differently sliapcd organs. Dr. Hagcii lias called attcntion to tlie fact that in certain of our Anicricm species of cray-fislies, there are two slightly different male forms, and Fritz ?duller, who pointed out the existence of tlie two male forms of Orchestia, has also described a remarkable dimorphic species of Tanais, in which tlie male is represented by two distinct forms, n c v x gradnatiiig into each other. I n the one form the male is furnislicd with more nnmerous smelling threads, and in the other form with more powerfnl and more elongated claws to hold the female. Fritz bInllcr suggests that tliese differences betmeen the two male forms of the same species must have originated i n certain individunls having varied in tlie number of tlicir smelling threads, while other indi~-iclualsvaried in tlic shape and size of their claws, so that of tlic forber thosc mliicli mere best able to find tlie female, and of tlie latter those which mere best able to hold her when fonnd, have left the grcatest number of progeny t o inherit-their respcctim advantages. Wlicncvcr a number of species of a genus have any part more developed in tlie male tlian it is in the female, tliis part, as a rule, varies in the males of tlie different species, and is tIierefore of great systematic importance, since ib furnishes diagnostic cliaracters f o r distinguishing tlie species from e:di other. This rule is of gciieral application, in a11 groups of animals with separate sexes, and cvcry one who is a t all fumiliar with the systematic zoology of 0111' Iiiglier animals ILI~OIVS how difficult i t is to idciitify species without mature male specimens. Tlie crustacea furnish an abundant supply of illnstrations of tliis law, but we have space for only one.
The Evidence from SexuaZ Characters. 189 In t h e fiddler crabs, one of tlic clams of tlie malc is enomionsly dcvelopd, so t h a t i t compares with thc otlier about as a base-Tiol does w i t h its bow. In t l ~ female both clams are alike, and both small. Tlicrc arc a iinnibcr of species of fiddler crabs, forming together tlie genns Gclassimus, and the big claw of tlic male, in each species, lias certain points of differciicc from all tlie other species. Tlie fact that tlie fcatnres mliicli characterize males as distingnislied from females, arc alsn the featiircs mliicli distingnisli sl>ccies from encli other, certainly indicates t b a t the origin of specific differcncc is to be songht in some peculiarity of tlic niulc org:tnism. IKSECTS.-&ny insects haw stridiilxting organs, by ~vliicli,:is in tlic house-cricket, tlicy proclucc their sharp music. Iii many cases tliesc organs arc csclnsiwly coiifined to tlic m:iIcs; in otliers t h y are present h i t rutlimentary in the fenxrle, mliilc they are pcrfectly dewloped in both sexes of ccrt:iin others. I n :ill cases we find tliat tlie organs for this purpose differ greatly in closely rclated forms, and thus show that they arc of comparatively recent acquisition. I n the Cicadas the females are mute, and the sound is prodneed in the male, by tlic vibration of tlie lips of tlic spiracles, which arc set into motion by a ciirrcnt of air discliarged from the tracliez. It is increased by a wonderfully complex resonating app:ir>itus, consisting of two cavities covered by scales. Tliis appamtiis is present, very much less developed, in tlie female, but i t is never nscd for producing sound. The males of tlic crickets, grasshoppers, and Locnst i h e , are a11 i*emarkable for their niusical powers, wliicli are absent i n tlie females. Altliongh tliesc three g r o u p of insects are pretty closely related to each other,
190
Heredity.
and dthoiigh tlic general clinracter of tlic sonnd, and its mcc1i:micnl c:insc, arc cssentinlly nlilic in all of them, the posiilion and clinrnctcr of the sonnd-prodncing mechanism wiies greatly. In tlic nialc cricket the under surface' of each wingcover h:u a row of sharp transverse ridges or tecth, which is rapidly scraped across a projecting ridge on the outer snrface of the opposite wing, thus prodncing the music. First one wing is rnbbed over tlic other, and then the movement is reversed. Both wings are raised a little a t the same time, so as to increase tlie resonance. I n the Locustidze the oppoeitc wing-covers differ in structure, and their action cannot be reversed, as it is in the crickets. The left wing acts as the bow, and is scrapcd over the riglit, wliich serves as the fiddle. I n some forms tlie posterior part of the pro-thorax is elevated into a sort of resonating dome over the wing-covcrs. I n tlie grasshoppers tlie sound is produced in a very differcut mmner. There is usually a long row of nearly a hundred minute teeth on the inner surface of the femur, and t h i s IS scraped across the sharp projecting nermrcs on the wing-covers. I n one South African form the femur is rubbed, not against the wing-cover, .but against a notched ridge on the side of theabdomen, and the whole abdomen of tbe male is distended with air, like a great bladder, to increase the resonance. The female grasshopper has the stridnlating apparatus in a rudimentary condition, and it is interesting to note that the yonng male is like the adult female in this respect, for I m d o i s states that the teeth on the femora of the female remain throughout life in the condition in which they appear in both sexes dnring the larval state, but in the male they become fully developed and *
The Evidence from Sexual Characters. 19 L acquire their perfect structure at tlie last moult, when tlie insect is mature and rcady to breed. Many bcetles have rasp-like ridges with fine teeth on certain parts of their bodies, fur producing a stridolatiiig noise, by scraping against hard ridges or angles on the adjoining parts. Iu most stridulilting beetles they are equally developed in both sexes: in some they are rudimcntary or entirely absent in the female. These organs arc situatcd on widely different parts of the body in different beetles, even when they are very nearly related. I n the carrion beetles there are two parallel rasps with fine transverse ribs on the fifth abdominal segment, and they are rnbbed against the posterior edge of the wing-cover. I n otlier beetles the rasp is on the dorsal apex of the abdomen. I n others it is on the side of the first abdominal segment, and is scraped by ridges on the femur. I n others the rasps are on the lower surfaces of the wing-covers, and the edges of tlie abdominal segments serve as scrapers. In others the horny tip of t h e abdomen is scraped against a rasp on the wing-covers. 111a great number of species of long-horned beetles. tlie rasp is on the meso-thorax, and is rubbed against tlic pro-thorax. In still otlier beetles there is a ribbed rasp ruiiiiing obliquely across the coxa of each hind leg, and this is scraped across a specially projecting ridge on one of the abdomiiisl segments. In still others the rasp is on the pro-sternum, and tlie scraper on the meta-sternum. I n the cases where the stridulating organs are confined to the male, or wliere they are rudimentary and functionless in the female, we have every reason to believe that the successive variations which have led to their production have originated in males. I n the cases wliere each ;ex has inherited them in full perfcction, there is, of course, no direct cvidcnce to show
192
Beredity.
tliat they have originated in one sex rallier tliaii in the other. The organs are essentially alike in structure, whether they are confiued to the male or are present in both sexes; and as we liave good reason for believing, in the first case, tliat they have originated i n males, and no reason for doubting that they liave so originated in the second case, the conclusion that they all have liad a male origin certainly has a great probability in its favor. The great diversity of the males of allied species, as compared wit11 the females, is well slio~vnin those beetles where tlie males, and not tlie females, liave great horns rising from various parts of the body, as from the h a d , thorax, clypeus, or the under surface of t l ~ ebody. Darwin gives the following account of tlicse structures: '' These horns, in the great family of Lanielli.cor~is,rcsemble those of various qnadrupeds, siich as stags, rhinoceroses, etc., and are wonderful both from their size and diversified shapes. The females geac~allyerliibi t rudiments of tlic horns, iii tlic form of small knobs or ridges, but some arc destitute of even arudimcnt, wliiie i n a few others they are almost as well developed in tlie female as they are in the male. In abnost a2Z cases the horns are remarkable from tJJeir excessive cwiubiliiy; so that a graduated series can be formed, from the most highly developed males to otliers so degenerate that they can hardly be distingaished from tlie femalcs. The extraordinary size of the horns, and tlieir widely different structure in closely allied forms, indicate tliat t l ~ c yhave becn formed for some important pnrpose, bnt their cxcesaivc variability in the males of tlie same species leads t o tlie infercnce tliat this purpose cannot be of a definite nature. T h y do not sliow evidence of friction as tliey would if used for o r d i h r y work. They are not usually sharp, and do not seem well adapted for defence,
The Bnidexce f r o m EexuaZ Characters. 193 and they are not known to be used by the males in fighting %witheach other. The conclnsion mliicli best agrees with the fact of the horns having been so i n i m e n s e l y ~ c ~ t n o t fixedly dcvelopcd, as showii by tlicir txtrcme xiriability in tlic s;inic species, and 1)yllicir extreme diiersity i n closely allied spccies, is tbat t h y liave been acquired as orii a m e n t s.” One fact conncctccl with these horn-like projections gives as clear evidence as coula be desired, t h t tlie male is more liable to modification in this respect tlian tlie femde. I t sometimes happens tlixt t h e liorns are absent in the malcs of a species, h i t present i n a number of closely related spccics, and in such cases we must believe tliat t h e departure from tlie gener:il rule is due to tlic fact that tlic species in which they are absent lias becn recently modified. NOW,in such forms tlie fcmale shows her close relatioiisliip to the typical, nnnioctificd, or ancestral form by the possession of rtir1imciit:~ry horns. DtLrwin says tliat it is n higlily remnrltnble f
294
Heredity.
the lover surface of the thorax, and tliesc differ greatly in structure and development in the males of tlic several species of the genus. Darwin giyes the following illnstration to show the yemarliable nature of this case: " In most i.uniinnnts tlie males have tlie horns more developed than the females, and they may be quite small o r even absent in tlic latter sex. Now if a new sl)ecics of deer or sheep were discorered with th; h o ~ n sentirely absent i n the male, but represented by rudinieuts in tlic female. we wc should lisrc a. casc like that of 01iiti.s. Darwiii's 11lastration wonld be still more arpropriate if we suppose that tile male in tliis newly-discovered deer not onIy lacks all traces of horns on tlic head, but has a pair of very peculiar ones on his breast. In this case me should conclude that the new species is the descendant of a form with horns on the head; that the male sex lisd beconic modified, and liad lost tlie liornson the liead, and had acquired new ones on the breast, wlrilc tlie female liad renisincd without modification: and liad adhered to the ancestral type. I n tlie Stapliglinidao tlicrc are liorns 011 the head and tliorax, and the males of the same species are extrnordinarily variable in this respect. I n two genera there arc species with polymorpliic males, wliich differ greatly in tlie development of their Iioriis. In n species of Bledius it is said that, in the same locality, males can be found with the central horn of the thorax very large, but the horns on tlie liead quite rudimcntary, wliile i n other males tlie horns on the hca8 are long, and tliat on the thorax short. Darwin dcrotes more than thirty pages to R discussion of the sexual coloration of buttcrflics and moths, and the twoextracts given below willserve to show that hisgeneral
The Euidence from Sexual Characters. 195 concliision is in accordance with the demands of OLW hypothesis, :~lthoiiglihe liimself has given a different expI’lun;ition, wliicli will be discussed in the next chapter. He says : ‘‘ No language suffices to describe the splcndor of tlie males of sonic tropical butterflies. Even within the same gcnus me often find species presenting an extraordinary differencebetween the sexes, wliilc others have their sexes closely alike. Thus in the South American genus Epicalia, Mr. Bates, to whom I am mucli indebted for most of the following facts, and for looking over this mholc disciisSion, informs me that lie knows twelve species, the two sexes of wliich hannt the same stations, and tlierefore cannot have been .differently affected by external conditions. “ I n nine of these species the males r3nk among tho most brilliant of all butterflies, and differ so greatly from the comparatively plain females that they were formerly placed in distinct genera. The females of these nine species rcsemble each other in their general type of color:ttion, and likewise rcsemble both sexes in several alltcd genera, found i n various parts of the world. Hence, in accordance with the descent theory, me may infer that these nine spccies, and probably all the others of the genus, are descended from an ancestral form which was colored in ncarly the same manner; I n the tenth species the female still retains the same general coloring, h u t the male resembles her, so that IIC is colored in a much less gaudy and contrasted manner than tlie iiiales of tlie previous species. I n the eleventh and twelfth species, the fcmales depart from the type of coloring whicli is usual with their sex in this genus, for they are gayly decorated in nearly the same manner as the males, but in a somewhat less degree.”
196
Heredity.
This series of forms sccnis to show t1i:it d l twelve specirs :ire descended from a form mitli 1)l:iin niales slid 1)I:iiii females ; t h a t this character 11:~sbceu r c t a i i i 4 in botli sexes by o t c specics, bot that tlic males 1i:irc bcen greatly modified in the other elcvcn, wliile in two of tlicm tlic fem:tIcs have inherited, to an imperfect degrce, t h e modification of the iiialea of their own species, a i d iii tlie otlicr nine tlie females linvc remaiiicd stationary and 113ve sliomti no tendency to inherit the modific:ition of their male parents. I n an allied genns, Eubxgis, the males of most, of the spcciCs are decorated mitli beautiful metallic tints, in a diwrsificd mauncr, and differ niticli from the females. ‘l‘lic fcninlcs tlirouglioiit tlie genus, on tlie other h i d , rctain a uniform style of coloring, so that hlicy commonly resemble each other much more closely tlian t h y resemble tlicir own proper males. Darwin concliidcs ( Vnriution, VoI. I., 1’. 37’s) thizt ‘‘ wlicn the ECSCS of bnttcrflics differ. tlic m;ilc, ;is ;L gcnem1 rnle, is the most bcantiful, niitl del):irts moat from tlie nsaal t j p c of coloring of thc gronp to wliicli the species beloiig~. Hence in most groups tlic fcrnalcs of tlie sevrr:il species resemble each other much more closcly than do the males,” . . “niid this iiidrcrrtes f k r i ! the ninles Anve mzclerpne n greater nmouiat qf nzoclijkaiioit tl~art,iliefei?zalcs.” Tliere arc m:iny striking cxccptions to this law, mliicli is general but not unircrs:tl. Certain of tlic niost rcm:Lrk:ible cxccptions, such as the occurrence of 1mI~morphicfcmdc butterflies, and of tlie mrions fcmale forms among the socid iiisects, mill be djmisscrl at tlie end of the 11Cxt CIiiIptcr. PrsHEs.-Dar\viii gives m:lny instances of difference bctwecn t h c scxcs i n fishes, aiid liis list might be very grcatly increased, but one or two examples will be suf-
.
The Evidence from SexuaZ Characters. 197 ficic>iitto sliow that tlicsc aiiiniiils folIow tlie rulc wliich prc\arls i i i so miiiiy other g~wiipsof tlic : i i i i i i i a l kiiigdom; tliitt, tlic nialcs arc more modified tliiin tlic females; that tlic miles of allied species diffcr more t1i:tu tlic females, and that tlie mature male differs morc tliiin the mature female from the young. In many species tlic male alone is ornamented with bright colors, and h e is somctimcs provided with cnrions append:tges which d o not :il)pear to be of any iisc whatever for the ort11n:iry piirposes of life. When tlic mnlc CtdZionymz~sZyra is Ereshly captnrcd tlic body is colored witli various s1i:gies of yellow, with stripes and spots of vivid blue on the hcad; the dorsal fins are pale brown, with dark longitndinal bands, while t h e other fins arc blnish black; the fenialo fish is of a dingy reddish brown, with the dorsal fin brown and tlic others white. T h e seses diffcr iii maiig other respects, and tlic domil fin of the male is remarkably and exccssivcly elongated. T h e sexes are so different from e:tcli other that tlicy were for a long time regarded as distinct species, :ind the male is known as tlic gorgeous dragonet, l h femiale as tlic sordid dragoiaet. The males of t h e vsrions spccics of this gcnns differ from each other in these s e x i d ch:iracters, and tlic young males resemble the adult females i n structure and color. The following extract from Darwin slioms how greatly the males of closely alliccl spccics differ from each other: "In the niale of the illollieizesia p ~ f e a e m z ' tlie s dorsal fin is grelttly devclopcd and is marked with n r o w of largc, round, ocellated bright-colored spots, ~vliiletbc same fin in tlic femalc i s sniullcr, of a different sliape and mnrkcd only mitli irregularly curved brown spots. In tile m:ile t h e basal margin of the anal fin is also a Iittlc produced and dark colored. In the male of an allied form, the
198
Heredity.
~i?,hOp71O~t~Y Hellerini, the inferior margin of the anal fin is developccl into a long filament which is stripcd with briglit colors. This filament docs not contain any muscles, and apparently cannot bc of any direct use to tlie fish. As in the case of Callionymzss the males while yo~iiigresemble in color and strncturc tlie adult females.” Darwin discusses the question -whether, when thc male differs i u a marked manner from the female in color or in other ornaments, he alone has been niodiiicd witli the variations inherited only by his male offspring, or wlicther the female has been specially modified gnd rendered illconspicuous for tlie sake of protection, and lie conclu lcs tliat with most jishes i i z which t 7 ~ esexcs differ in color or in otlier ornamental c7~aracters,tlie males originally varied. LIZARDS. -Among lizards the sexes often differ greatly in various external clinracters, and the male sex is in almost every case the one wliich is peculiar. Among the many examples given by Darwin I quote the following: “ I n Anolis cristatellus tlie m d c is furnished with ;I crest which runs along the back and tail and can be erected at pleasure, but of this crest the female docs not exhibit a trace, although in other species tlie female does have an imperfect crest, whicli is much less dcvcloped th:m it is in the male. I n the genus Sitann tlie males alone are furnished mitli a large throat pouch, which can be foldcd L I like ~ a fan, and is colored blue, black and red d u r i n g the pairing season. T‘lie female docs not possess cren a rndinient of this appendage. Tlic male of Ccratophorn aspern lias a long appendage half as long :is his head on tlic tip of his snout. I n a second species of the same genus a terminal scale forms ;t minute horn on
The Eoidence From Xexual Characters. 109 the summit of this appendage, and in :I tliird species the whole appendage is coiiverted iiito a horn. I n the females of d l tlicse specics and i n tlie 3’01111g males the nppciidage is very minute. Tlic male CILameZeon Br;f‘zirczcs lias two great solid bony projections, 6overecI with scales, in tlie upper part of tlie slinll. The male C‘J~aineleon Owelzii lias three great bony lioriis on his head. These bony liorns are covered with a smooth sheath of integument, so that they are strikingly like those of a bull or a goat. I n the females and young of both species tlicse appendages are rudimentary.” BIRDS.--‘I’Le sexual characteristics of birds are most diversified and conspicuous, and most persons, even those wlio are not naturalists, know enough of tliis subject t o agree that the males are as a rule much more niodified than the females, and i t will not be necessary t o devote very much space to tliis group. Darwin lias devoted more thart two hundred p a p s to the discussion of the differences between male and female birds, :tnd lie lias brought together an array of facts a11 tending to sliow tliLit male modification is the rule, while female modification is compar~~tively rare, and although it is true that he gives another explanation of the phenomena, an exp1:ination which will be discussed in the next chapter, I e t every reader of his essay must be convinced of the correctness of liis conclusion, p. 227, “that weapons for battle, organs for prod wing sound, orna~neutsof many kinds, bright and conspicuous colors, Iinve g e n e m l l y bee?& a c p u v e d by the males, . the females and the young being left comparatively but little modified.” This conclusion mill be accepted without question by all who arc familiar with the subject, and i t is haidly necessary to dwell upon it, but tlie great diversity of tlie sexual differences in birds demands that in a general
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200
Heredity.
review of the subject they sliould receive some little notice. Darwin says: '' Male birds sometimes, though rarely, posscss special weapons for figlitiilg with each other. T h y ch:irm tlie females bg vocal and instrnmciital music of the most various kinds. They are ornamented by all sorts of combs, wattles, protubcrances, horns, airdistended sacs, topknots, naked shafts, plumes aiid lengthened fcathers, gracefully springing from all parts of the body. The beak and naked skin aboiit the head and the feathers are often gorgeously colorcd. T h e males sometimes pay their conrt by dancing, OT by fantastic antics, performed either 0x1 tlie groniid or in tlie air. I11 one instance, a t least, the male emits a musky odor, which we may suppose servcs to charm or excitc the female. Tlic ornamcnts arc wondcrfully diversified. T h e plumcs 011 tlie front or back of the head coiisist of varionsly shalml fcathers, sometimes capable of erection or expansion, by wliicli tlicir beaiitiftil colors are fully displayed. E1egr;iiit ear-tofts arc occasionally prcseu t. The liead is sonictimcs covered with velvcty dvivii like that of t h e pheasant, or is naked and ~iv-icilycolored, or suppxts flcsliy appcndagcs, filaments and solid protuberanccs. T h e throat also is sometimes ornamented with a beard, or with wattles or carunclcs. Such appenduges are gcnerallv brightly colored, aiid 110 doubt ~ c r v cas ornaments, tliougli not always ornamental i n our eyes: for while the malc is in the act of conrting thc fem:tle, tlicy often swell and assume inore yivid tints, as in the case of t h e malc ttirkey. A t such times the flesliy appendages about the Iicad of tlie male Trngopan pheasant s\vcll into a 1:irgc 1;tppct on the throat ;tiid into two horns, oiic on each side of the splendid top-knot, and tliesc are then colored of tlie most intense blue wliich I have c w r beheld.
T 7 ~ eB’oicZence .,#?om 1SezuaZ Cfinl-acters. 201 T he African liornbill inflates the scarlet bladder-like wattlcen its neck, aiid with its \ring drooping arid tail cxpaiided nmhes quitc a grand ;ipl~eanince. E\ cu tlic iris of tlie eje is sonictimes more briglitly colored in the male than in tlie fcm:ile, aiid this is frcqncntlg tlic caee with the beak, for iustancc in our comnioil blackbirds. In Buceros cumigafzis, thc whole beak and immense cnsquc are colorcd mure conspicnously in tlic m d c flian in tlic female, and the obliqnc grooves upon tlie sides of the lower mandible arc peculiar to tlic inalc sex. The males are often ornamentcd with elongated feathers or plumes, springing from almost every part of the body. The feathers on the throat and breast are somctimes dereloped into beautiful ruffs and collars. The tail featliers arc frequcntlg increased in Icngth, as me see in tlie tail of tlic Argiis pheasant. The body of this latter bird not larger tlian that of a fowl, get tlic lengtll from the end of thc beak to tlic cxtrcmity of tlie tail is 110 less than five feet tlirce inchcs. . Nor nccd much bc said on the mondcrfnl differences of color between the Eexes, or on tlic extreme beauty of tlie males of many biids. Tlic common peacock offers a, striking instance. Fcmale birds of Paradisc are obscwely colored and destitute of all ornaments, wliilc tlic males are probably the most highly decorated o f :dl birds, and in so many ways that thcy must be seen to be appreciatcd. Tlic elongated and golden orange plnmes wliicli spring from beiieatli tlie wings of the Paradisen apodn, wlieii vertically erected and made to vibrate, are described as forming a sort of halo, in the centre of which the ’head looks like a little emerald sun, with its rays formed by the two plumes. I n aootlier most bcaiitiful species t l r e h e d is bald and of a rich cobalt blue crossed by several lincs of black velvety feathcrs. Male humming birds almost vie with birds
. .
Neredity.
2 0 '
of Paradise in their beauty, as every onc will admit ~ 1 1 0 ]ins seen Mr. Gould's spleiidid volumes in liis ricli eol-
lcction. 1.t is very remarkiible in how many different ways these birds are ornamented. Almost every part of t h e plumage lias been taben advantage of and modified. . When the sexes of birds differ i n beauty, i n the power of singing, or in producing instrnmental music, i t is almost invariably the male wliich excels tlie female." This extract is enough t o show the wondcrfnl diversity of the chnracteristics of male birds, and the following examples bring out w r y prominently t h e fact t h a t male birds of allicd species often differ greatly i n their sexual cli;iracters, while t1i.e females are Terg much alike. In thi? South American bcll-birds tlie females of t h e four species resemble each other very closely, and are of a diisky green color, while t h e male of one species is ptire white; in a second species white with the cxception of a large space of naked skin on the tliroilt and round the eyes, which dnring the breeding season isof a fine green color, while in a third species only the head and neck of the male are white and t h e rest of the body chestnut-brown. I n one species t h e male alone is proGded with three filamentous projections half as long as the body, one rising from the base of the beak and t h e others from the corners of the rnuiith, while in anotlier species the male has a spiral tube nearly three inches in length wliicli rises from the base of the beak and is j e t black dotted over with minute downy feathers. I n the Indian chats, honeysnckers, shrikes, kingfishers, Knllij pheasants, and t r r e partridges, tile males of allied species from distinct countries are quite different from each other, while t h e females a i d the yoniig of both sexes are iiidistiiiguisliable.
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The E’uidence from Sexual Characters. 203 111the cases where the females of allied slwcies d o dif,fer the difference is rarely so great as between t h e m;ilcs. I).irwin says : ‘cWc see this clearly in the whole family of tlic Gallinace= : tlie females for instance of the common and J:ip:in pheasant, and especially of the gold and Amherst plie:isant, of the silycr l)hcnsant and thc wild fl,wl, resemble each other very closely in color, mliilc the nxiles differ to an cxtr:iordinary degree. So it is w i t h the Cotingidae Priiigitlzdne and many other f:imilics. 7‘Jicre cau indeed be 110 doubt tJiat as a general m b the f m a l e s J L ~ V C bee72. inodiJetl t o a less exteiit tliata die inales.” ( Variation, Vol. 11. p. 184.) As regards tlie relation bet\reen the jonng and the adult, tlie general ~ w l eis t l ~ nlicu t the Ecxes differ the young of both sexes in their first plomnge rescmble tlie adult female as they d o in the common fowl or the peacock, or else they rcsenible licr more closely thsn they do the adult male. Darwin sags that innumerable instances of this law could be given in all orders, but that it will suffice to call to mind the common pheasant, duck, and house 6pal’rOV.
There are a fcw cases in which the yonng male is like the adult male?and the young female like the adult female, and there are also a few cases where the young of both sexes resemble tlie adult male, but the difference between the sexes is never, in this case, very great, and in stances are so rare that Darwin, who says that he has recorded all lie could find, gives only nine. In his summary he says : “ We thus see tlmt the cases in mliich female birds are more conspicuously colored t11:in the males, with the young in their immature lilnmngc reeenibling the adult males instead of tlic adult females, are not mmerOGS, though they are distributed in various orders. The
204
Heredity.
amoiint of diffcrencc between the sexes is also imam :Irably less than t h a t wliich freqnently occurs i n thc last class ; so t7iat t 7 c~a m e of the dffference, zahateaer ir rnuy have b m a , ?ins acted i y o n t?ie.fe?-Iales i i i the present class either less energetically or less persisteiitly than o n the males i i z the lasf class. (Descewt of J f a i i , 11. p. 198.) MAnmALs.-Aiiiong the mammalia the Sexes often differ in their weapons of offence and defence, as we see i n the deer, when the horiis are usually absent in the female ; i n their voices, as in the case with t h e cow and bull; in odor, as goats for ex:iniplc, and in the musk deer, where both the mnsli-producing oigan and other organs of a sinii1:ir c1iar:ictcr are confined to the male; i n color, as i n many antelopes, and in the c1i:tractcr and distribution of the hair, as we niay see by comparing the lion with the lioness, or tlie liunian male with t h e hnm:in fcni:tlc. A little thought d l sliom that among the mammals, as in otlicr groups of the anini:tl kingdom, tlic males are more modified t1i:tn the fenxiles. T h u s nian differs from woman by t h e possession of a beard, b a t the boy resembles t h e girl or tlic mature female, thus showing that the linman race is iiiflaenced by the gcnerai law of which we have seen the cyidciice in so m:my gronps of animals, and that the a d u l t female is more like tlie young of both sexes than the adult male. So, too, tlic young stag, or the young male goat, resembles the adult female in tlie absence of horns. The fact that different human races are c1iar:ictcrizcd by the presence or absence of a be:~rd in the ni:tlcs, anti that tile horns of differcnt F I ~ C C ~ C Yof deer ditt'er w r y gre:ltlv, shows that the m:tles of allicd slxzics of nianimals differ more than the femalcs. Among the mammalin we sometinies find that t h e
Tlze Bvidence from XemaZ C7~aracters.205 -
ninle l i m been modified by tlic acquisition of new structiircs, wliile i n otlier cases organs common to both scxcs :iiicl t o g i u t groups liavc become cliangcd in tlic male, bii t liitvc rcmaiiicd comparatively unmodified in tlie fcnl:llc. l'lic spiirs on thc leg of t h c nialc Ornithorinclius may, pcrhap, bc rrgardcd as n cnsc of tlic first kind, as niny also tlic Iiorns of tlic rhinoceros, vliicli arc longer and more important i n tlic malc tlian they arc in Ihc fcmale, whilc tlic great tiislis of tlic boar arc org:ins wliicli must linrc been present in both seses of tlic rcmotc ancestors, al tlioiigli t h y have rcccntly undergone great cliange i n tlie male. No onc who mjll compare t h e liead of tlic common boar with tliat of tlic male lkrbyrnsa, tlic m d c wart-hog, and t h e male river ling, can doubt that tlic males of tlicsc allied species differ much more tlian tlic females. I n some cases certain tccth of t h c male are so greatly modified that t h y illlist be regarded as ncw organs. This is trnc of the narwlial, in which one of the teeth is greatly elongated, and forms II long, spirally-twisted spciir, nine or ten feet long, mliile tlic corrcspoiiding tooth in the male, aiid both teeth in tlic fcmale, are rudimeri tary. T h e tusks of tlic male walrus, and those of t h e inale elepli;tnt, arc grcatly modified teeth, but they differ so greatly from ordinary teeth that tlicy are almost as truly new o i g ~ n sas the Iiorns of ruminants. It is interesting to note how greatly tlie various races of e1epli:tnts differ in the dcveloprnent of the tusks. I n Ceylon they arc ncrcr found in the females, and they occlir i n only abont one per ccnt. of the males. I n india tlicy occur in :dl or nearly all the males, but i n tlie males alone, while in Africa the female usually has small tusks.
206
Heredity.
Tlie same thing is true of the horns of ruminants. I n tlic hollow-horned species, as in cattle, they arc not :it all uncommon in the females, altliougli they arc L1Suillly m i d i less important than they are in the males. Among the antelopes the females of somc species have horns like the males; in other species they arc someyliat smaller in thc female than they are in the male; in others they are large in the male, but rudimentary in the female, while in others they are entirely absent in the female. I n female deer they are usually absent entirely, but in some they arc rndimentary, and in tlic female reindeer t h y are fully developed. I t is interesting to note that in femalcs which nornially lack them, they may be developed as the result of injnry or disease of the reproductive orgms, and that their development in the male may be arrested by castratioa
CHAPTER IX. THE
EVlDENCE FROM
SECONDARY SEXUAL CHARAC-
TERS CONTINUED. - T H E
CAUSE O F T H E EXCESSIVE
MODIFICATION O F NALE CHARACTERS.
Thc Explanation of Daines Barrington and Wdlace-Reasons for considering it inadequate-Dalwin'swiu's explanation-History of domesticated races shows that this does not go to the root of the matter-The view that the male is more exposed than tlie female to the action of selection-A more fuudamental exis furnished by our theory of plauatiou is needed-This heredity-Special difficulties-Summary .
THEsexual characteristics of airimals have been made the subject of considerable discussion by various naturalists, and among birds especially there have been many attempts to explain why the female has not acquired the same ornaments as the male. The Explanations of Daines Barrington and Wallace. Wallace points out that conspicuous ornaments and brilliant plumage would render the female bird prominent while incubating, and would thus enable enemies to detect the presence of the nest. He believes that since incubating females are exposed to this danger, natural selection has acted, by the destruction of the most conspicuous females, to gradually produce races in which the females hare nothing to render them conspicuous. I n 1773 the Hon. Daines Barrington called attention (Phil. Trans. 177.3, p. 164) to tlie fact that singing birds are all smalll, and he believes that this arises froni
208
Heredity.
the difficulty larger birds would haye in concealing themselvcs I f they called the attention of their eneiiiies by loud notes. He also says that lie conccires it is for the same reason that no lien bird sings, because this talcnt mould be still more dangerous during incubation, and he suggests that the inferiority of tlie female bird in point of plumage may be due to the same cause. This argument, that the dull color and lack of ornament in female birds is a direct adaptation to their peculiar life, has been elaborated by Wallace. ( O u 1\Taturnb Selectzoia, p. 231.) He says that in the struggle for existence incesssiitly going on, protection or concealment is one of the most general and most effcctual means of maintaining life, and i t is by modifications of color that this protection can be most readily obtained, since no other character is subjcct to such niimcxous and rapid variations. He says that, as a general rule, the female butterfly is of dull and inconspicaons colors, even when the nialc is most gorgeously arrajed, and that in all these caws the differcnce can be traced to tlie greater need of protection for tlie female, on whose continued existonce, while depositing lier eggs, the safety of the race dcpends. Since a male insect is, by its structure and habits, less exposed t o danger, i t docs not need any special means of protection, as the female does, t o balance the greater danger to which she is exposed, and TV:illacc believes that on account of this danger, and because of her greater importance to the existence of the species, tho female insect always acquires this protection in one may or another throiigh the action of natnral sckction. H e also says that “ t h e female bird, while sitting 01 her eggs in an uncovered nest, is mnch exposed to thc attacks of enemies, and any modification of color whicl
The Evidence from XexuaZ Characters. 209 rendered licr more conspicnons mould often lead to her tle~truction,and that of lier offspring. All variations i n this direction i n the female would therefore, sooner or later, be eliniinttted, mliile such mod~fications :IS rendered lier inconspicuous by assinii1:iting licr to surrounding objects, :IS tlie earth or the foliitgcl, would, on tlie whole, snrvive the longest, and tlins lead to the attainmen t of those brown or green and inconspic~ious tints whicli form tlic coloring (of the upper surface at least) of the vast majority of female birds which sit upon open nests.” As a proof that this is the trne explanation of tlie dull plumage and lack of ornaments i n so many feniale birds, lie states tliat mhcrever tlii? iiest is domed or covered, or so placed as toconceal tlic sitting bird, tlie plnmagc is strikingly gay and conspicuously c o l o i d i n both sexes; but tliat in those species where tliere is a strong contrast, in colors, and tlic male is gay and conspicuous, mliile t h e fernale is dull and obscure, tlie nest is open, and the sitting bird is exposed to vicw.
Reaso-m for Holding that this ErplaizatioiL is 171a d q u ate. Tlie argument of Wallace, which is fully stated in the cssiiy ;ibovc quoted, is bricflg, that tlie dull plumage of so marig female birds, as contrasted mitli the gay colors of tlic miiles, has been directly acquired in the females by the destrnction of tlie most conspicuous ones, and tlic natnral selection of tlie inconspicnous varieties. Darwin lias discnssed i t a t length in liis essay on sesn:tl sc.lcction, and Iias given many reasons for refnsing 10give i t unymlificd acceptance, but I will give here a fcw additional reasons for beliering that tlie plienomciia in qnestion depend upon some more fundamental law. In the first place, we must bear in mind that,
210
Heredity.
,
e l m among birds, the male diffcrs from the fem;ile by the possession of numerous secondary sexual characters besides brilliant plumage, and that many of t h e , like the spurs of male gall in ace^, are not a t all Conspicuous. Bechstein (i\Tatz6~gesci~Deutschland) says that a breed of fowls formerly existed in Germany in which tlie hens mere furnished with spurs, but that they could not be allowed to sit on their own eggs, as, although they were good layers, the spurs disturbed the nest and broke the eggs ; and it might perhaps be urged that the absence of spurs in the females of wild species of Gallus may be due to the selection, €or this reason, of females without spurs; but we must recollect that natuyal selcction acts upon every part of the organism, and would, if the female were as liable as tlie male to give rise to heredihry variations, have acted, during the evolution of spurs, to bring the structure and habits of the female into harmony with these new weapons, so that she could enjoy their protection without injury to her eggs. Darwin s n ~ sthat when we think of the multitude of birds which with impunity gladden the country with their songs during the spring, it does not seem probxble that the females have been saved from acquiring this power on account of the danger to which they would have been exposed by attracting the attention of birds and beasts of prey. If female birds have had the power of song, it would certainly seem simpler for them to have acquired the habit of restraining tlicir voices in dangerous places than to suppose that the power has been renioved by natural selection. Wallace's view fails to account for the fact that the plumage of allied species of females is, as a rule, much more alike than that of the males ; and this fact is quite
The Evidence from Sexual W r a c t e r s . 211 incxplicable if the dull colors of the females are due to direct modificntion by natural selection. Again, we must recollect that among the lizards,where tlie females do not incubate, the males are often mucli more conspicuously colored than tlie feniales, and tlie fcmales of allied species are more alike than the males. Here the dull colors of the females as compared witli those of the males cannot be accounted for by the niLtLlra1 selection of those females which are least exposed to danger during incubation. Among fishes tlie same rule is adhered to, and tlie males arc usually more conspicuous than the females, and licre the female is certainly no more exposed to dnngcr tlian tlie malc. '' & far as there is any difference, tlie males, from being generally of smaller size, and from wandering about more, are exposed to greater danger than the femalcs; and yet when the sexes differ, t'lie males are almost always the most conspicuously colored. Tlie ova are fertilized immediately after being deposited, and wlien this process lasts for several days, as in the case of thc salmon, the female during tlie whole time is attended by the male. After the ova are fertilized they are, in most cases, left unprotected by both parents, so that the males and females, as far as oviposi tion is concerned, are equally exposed to danger, and both are equally important for the production of fertile ova; consequently the more or less brightly colored individuals of either sex would be equally liable to be destroyed or preserved, and both mould have an equal influence on the colors qf their offspring or the race." (Darwin, Sexual Sdection, Vol. 11, p. 19.) The male stickleback does all the work of building the nest, and after the eggs are laid and fertilized he drives the females away, and performs for a long time
212
Heredity.
tlic duties of ;t iiiirsc with cscmp1:iry c:irc and \igil;ince, gciitly 1c:iding h c k tlic yoiiiig to tlic nest wlrcn tlrcy stray too far. Yct tlic nrdc is niorc biil1i;iiitly coloi.cil tlian tlie fcninlc, and liis colors iirc e s i m i d l y brilli~int a!iil conspicuous during tlic brcediiig sc;ison. I slrall show fiirtlicr on t1i:it tlic m;tlcs of domcst icatcd l)rcctls of fowls and pigcons are niorc co~ispiciioi~s and dircrzificd than tlic femdcs, biit as f m c y 1)igeoiis :ire re:trccl in confiiicnicnt, and arc protected from e w r y dangcr, this cil~inotbc dnc to tlic iiatural sckctiun of the bcst-protcctccl fenialcs. Wc must co~icltidc,tlicn, that t h e brilliant plumage of m:ilc birds is dnc to some niors gciier;il and fiiiidamcntd cause than tlic one Iyo1)oscd by IVdlaec, siiice female reptiles which d o not iircnbatc, and funialc JLlics which arc e v q i less C X ~ O S Cto~ daiigcr tliaii the males, and fcrndc donicstic:tted birds wliicli are thorouglily protcctcd from enemies, all follow tlic mmc law. Tlic fact that many structnres wliicli arc not a t ,211 cotispiciions arc confined, ljlie gay pliim;ige, to. male birds, also indicatcs t h e cristcncc of an cxlhnation more fuiid;tmental than the one proposed by Wdlncc, m c l this latter explanation gives no reason wliy tlic fcmalcs of allied specics slionlcl so often be almost cxactly alike when the males are very different.
Darzoiis Expbaization. Darwin has givcn a different cxplanation, and he believes that the greater modification of rnalcs tlrrongli. out t h e animal kingdom is chiefly clue to scxniil sclcction. He lias devoted more than firc hundrcd p ; i g p to the development of this idea i n his casny on scsua.1 selection (Descent of Man, Part II.), and he lias marshalled an overwhelming array of facts with mas-
The Boidence from LYexua2 Characters. 213 teily skill. The nttcnipt t o point out within the *limits of a single cliaptcr tlic crrors of his conclnsion is beret with many difficulties, and I sliall be compelled to treat the subject with brevity, and to leave unsaid much wliicli might be urged did spax pernii t. As an introdnctioii t o the discussion of the subject, I sliall quote D:tryin’s statement of the m e m i n g of the tcrm “sexual selection.” R e says : “This depends on tlie advantage which ccitain i n d i ~ i d n a l shave over other individuals of tlie same sex and species in exclusive relatioii to reproduction. TVlieii tlie two sexes differ in structure in rc1;ttion todiffercnt habits of life, the?. have, no donbt, k e n modified tlirotigli natural selection, accompanied by inheritance limited to one and the snnie sex. So, again, tlie primary sexual organs, and those for n our is11i 11g and protecting the you 11 g, come u 11de r tlie same head; for tliosc i n d i ~ i d n a l smliicli generated or noui islied tlicir offspring best, mould Icave, cceieris 33arib74 the greatest number to inherit their soperiorit!;’ ivliile those wliich gcnclnted or noui~islicd their offspring badly, wonld leave b u t few to inherit their ~veakcrpoivcrs. As the male has to scarcl: for the female, lie requires for this purpose organs of sense and locomotion, b u t if tliese organs arc necessary for the other purposes of life, as is gencrally tlic case, they will hare been developed through natural selection. When t h e male has found the female he sometimes absolutely requires prehensile organs $0 hold her; tlins Dr. Wallace infornis me that t h e males of certain moths cannot unite with the fcmales if their tarsi or feet nre broken. .. TTilien the two sexes follow cxnctly the same liabits of life, and the male has more liiglily dcveloped sense organs or locomotivi? organs t h a n tlie female, i t may be that these in their perfect state are indispen-
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214
Heredity.
sable t o the male for finding the female; but in tlie vast majority of cases they s e n e only to give one malean advan tage over another, for the less well-endowed males, if time were allowed them, would succeed in pairing with the females; and they would in all other respects, jiidging from the strncture of the female, be eqiially well adapted for their ordinary habits of life. In such cases sexual selection must have come into action, for the males liave ncqnired their present structure, not f r o m being better fitted to survive in the struggle for existence, but from having gained an advantage over other males, and from having transmitted this advantage to their male offspring alone. It was the importance of this distinction which led me to designate this form of sclcction as sexual selection. So, again, if the chief service rendered to the male by his prehensile organs is to prevent the escape of the female before the arrival of other males, or when assaulted by them, these organs will have been fortified through sexo:il selection, that is, by the advantage acquired by certain males over their rivals. Biit in most cases it is scarcely possible to distinguish between the effects of naturaland sexual selection. . There are many structures and instincts which must have been developed through sexual selection, sue11 as the weapons of offence and the means of defence possessed by the males for fighting with and driving anay their rivals-their courage and pugnacity-their ornaments of many kinds-their organs for prodiicing vocal or instrumental music, and their glands for emitting odors; most of these latter structures scrviug only to ailure or excite the females. Tliat these ch:iracters arc the result of sexual and not of ordinary selection is clear, as unarmed, unornamented, or unattractive males would succeed equally well in the battle for life, and in leav-
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The Evidence from Sexual C7~aracters.215 ing a numerous yogcny, if better endowed males mere not present. We may infer tliat this would be tlie case, for the fcmales, wliicli are unarmed and ~~nornamciitcd, are able to survive and procreate tlieir kind. Secondary sexual cliaracters of tlie kind just referred to mill be fully discussed i n the following chapters, as tliey are, in many respects, interesting, but more especially as tliey depend on the will, choice, and rivalry of the individuals mf either sex. " When me bcliold two males fighting for the possession of the female, or several male birds displaying tlieir gorgeous plumage and performing tlie strangest antics before an assembled body of females, we caiiiiot doubt that, tlioligli led by instinct, tliey h o w wliat tliey are about, and coneciously exert their niental and bodily powers. I n tlie same manner as man can improve tlie breed of his game-cocks by the ?election of those birds wliicli are victorious in tlie cock-pit, so it appears that the strongest and most vigorous males, or those provided with the best weapons, have prevailed under nature, and have led to the improvement of the natural breed or species. Through repeated deadly contests, a slight degree of variability, if i t led to some advantage, however slight, mould suffice for the work of sexual selcction; and it is certain that secondary sexual characters are eminently variable. '' I n tlie same manner as man can give beauty, according to his standard of taste, to his male poultry-can give to the Sebright bantam a new and elegant plumage, an erect and peculiar carriage-so i t appears that in a state of nature female birds, by having long selected the more attractive males, have added to tlieir beauty. . It is certain that with almost all animals there is 8 struggle between the males for the possession of the fe-
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Heredity.
216
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males. . Of the males the strongest, and, with some species, t h e best armed, drive away the wealier m:iles; and tlie former moulcl tlien unite with tlic more vigorous :tiid best, nourished females, as tlicse arc the first to breed. Such vigorous pairs mould surely rcar a larger nnmbcr of offspring tlian tlie retarded females, mliicli wonld be compelled to u n i t e with tlie conquered and 1css powerfnl males; and this is all that is wanted to add, in the course of successive generations, to the sizc, strength, and conrage of the mdes, or t o improire tlieir w q o n s . B u t i n a multitude of cases the males which conrliicr other males do not obtain possession of tlie females indepcndently of choice on the part of the latter. The conrtsliip of animals is by no means so simple and short a n affair as might be tliouglit. Tlie fcm:ilcs are most cxcitcd by, or prefer pairing with, the more ornamented males, or t h c x which are the best SongJters, or p h y tlie best antics; but i t is obviouslg probable, as h:is bcen actually observed i n some cascs, tliat tlicy mould at tlie sanie time prefer the most vigorous and lively males. . And this apparently liris sufficed during a long course of generations to add not only to tliestrengtli and fighting power of the males, but likewise to tlieir varions ornarnents or other attractions. . . . To sum u p on the means through which, so far as we can judge, sexual selection has led to the development of sccondary sexual cliaraeters : I t has been shown tliat tlie 1;irgcst nnmbcr of vigorons offspring will be reared from the pairing of the strongest and bed, armed males, wliicli hare conq~ieredother males, with tlie most vigorous and best-nonrishtd females, wliicli arc tlie first to breed i n the spring. Snch females, if they select tlie most att r x t i r e and, at the same time, vigorous males, mill rear a larger number of offspring than t h e retarded fema!es,
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The Evidence frona Sexual Characters. 217 mliicli mnst pair with tlie less vigorous and less attractirc males. So i t will be if the morc vigorous males selcct tlie more attractive and, a t tho same time, liealthy and vigorous females; mid tliis will especially hold good if the male defends tlie fcmde nnd aids in providing food for tlic j o m g . The a d ~ a n t a g ethus gained by tliemore vigorous pairs in rearing a larger number of offspring has apparently sufficed to rcnder sexual selection efficient.”
The Xtudy of Donaesficnted h’nces slrozus tJint this ExpIatiutiai~does not go to the Root of tlie Ndter. This long extract mill, I hppe, fully explain to those readers wlio arc not familiar mitli Darwin’s essay, the naturc of s e x u d selection. I t will be sern that he attributes the grcatcr modification of the males as conipared with the females, i n most of the groups of:inimals where tlic sexes differ, to tlic fact that the males have struggled wvitli each other for the possession of t h e females, or have been chosen by the females. This process, long continued, is believed to hav’e resulted in the perpetuation of the strongest, best armed, or most attractive males. I fully acknowledge the great potency of sexual selection, and believc with Darwin that it must act in esscntially the manner described by him, b u t I do not believe that i t goes to tlie root of the matter. Portiinntely there is simple experimental test wliicli is easily tried and giyes a satisfactory solution of tlie question mlietlier tlie phenoniena do or do not depend upon sometliing more fundamental tlian the exposure of the n d e to the action of seiectioo. If we take animals in mliich the sexes differ but little, and prevent them from following their own inclinations,
218
Heredity.
and pair them without any reference to their own preferences, and continue this for a number of genera‘1 t.ions, until we have produced a number of divergent races or breeds ; if we then find that tlie males of these breeds differ more fr6m each other than the females, we must conclude that there is, behind the action of selection, some more deep-seated law, wliicli determines that males shall, as a rule, be more modified than females.
Domesticated P i g e m s . The study of domesticated pigeons is extremely interesting in this connection, for it sliows, conclusively that the tendency which we have shown t o exist in nearly all groiips of biscxual animals, the tendelicy of the male t o deviate more than tlie fenidc from the typical structure of allied forms, caiinot be attribu ted exclusively to the fact that the male is more exposed than the female to tlie action of either sexual or ordinary selection. There are more than two h~iiidredwild spccics of the pigeon family, &id throughout tlic whole group tlicre is an almost total absence of exteriial difference between the sexes. In a few species the plumage is somewhat more brilliantly colored in the male tIian i t is in the fenialc, and it is stated that in one species, Caq~opl~nga oeennzca, the excrcscence at the base of tlic beak is a sexual character, but these differences between the sexes are sIigIit and exceptional. In domesticated pigeons, on the contrary, the sexes often differ considerably, and i t is a remarkable fact that here, as i n so many other gronps of tlie animal kingdom, the characteristics of the different breeds me oftcn most strongly displaycd in tlie male bird.” (Darwin, Vayiation, Vol. I. p. 199.) In many cases tlie sexes are
The Euidence from 8exuaZ C7~aracters.219 d i k e ; thus tlie female trumpeter has a t u f t like that of tlic male, and tile liood of the Jacobin and tlie frill of tlic turbit are alike in both sexes ; b u t wlicrever tile sexes do diffrr the males are, as it rule, more modified tlian tlic fen1ales. I n all ordinary domesticated brecds as ~vvellas in most wild species, tlie number of tail-featliers is twelre, bnt in the fan-tail h e e d tlicre are from thirty to forty, and they are permancutly exp:inded like a fan. We must believe tliat this deviation from tlie typical number of tail-feathers i n the pigeon f:imily is duc to recent moclification, and we find that tlic iiumber is often ninch greater in the male fan-tail than i t is in tlic fcmale. “lie pouter pigeon is a w r y remarkable domestic breed. All doniestic pigeons h a r e some slight power of inflating the crop, but this power is so greatly developed in the pouter breed that the bird is xble to blow liimsclf up like a b:illoon, and Darwin says that after one of liis pouters had smallomcd a good meal of peas, lie could hear tlie peas rattle as if in a bladder as the bird flew through tlie air with its crop inflated. Darwin says that the males pout more than the females, and glory i n this po~vcr,and strut about puffed lip with wind and pride. H e also sajs tliat i t is a very nnnsnal thing for tlie female to excel in pouting. We must therefore believe t h a t tlie male pouter lias departed further than the female from ordinary pigeons. T h e tumbling habit of tumbler pigeons is p e h p s the most remarkable of all the hereditary modifications of domestic animals mliicli man lias produced. Tlie following account of tlie English tumbler is quoted by Darwin from Brent: ‘‘ Every few seconds over they go, one, two, or three somersaults a t a time. Here and there a bird gives a very quick and rapid spin, re-
220
H2redity.
volving like a mlicel, though they sometimes lose their balance and make a rather ungraceful full, in wliich they occasionally hurt themselves by striking some object. They begin to tumble almost as soon as they can fly; :It three months old tlicy tumble well bat still fly strong; at five or six months they tiimblc excessively, and in the second gear they mostly give n p flying on account of their tumbling so much and so close to the ground. Some fly round with the flock, throwing a clean somersault every few yards, till they are obliged to settle from giddiiicss and exhaustion. Thcse are cdled air-tumblers, and they commonly throw from twenty to thirty somersaults in a minute, each clear and clean. I have one red cock tlitlt I liwe on two or tlirce occaSions timed by my watch, and counted forty somersaults in tlie minute. Others tumble differently. At first they throw a single somersault, then it is douLled till it becomes a coiitinnons roll, NlliCh puts an end to flying, for if t h y fly a few yards, over thcy go, and roll till they reach the ground. Thus I had one kill lierself, and mother broke his leg. Many of them turn over Only a few inches from the gronnd, and will tumble two or three times in flying across their loft. These are called house-tumblers, from tumbling in tlie house.” The tumbling habit is shared by both sexep, tmt as in the case of the ponter, it is the mnle wliicli excels. The carrier and barb races hf domestic pigeons are characterized by the prcsence of naked carnnculated skin owr the beak and around the eyes, and in both of these races this featurc is most pronoiinced in the males. These illustrations are scfficient to show that tlie distinctive characteristics of cadi breed of domesticated pigeons are either alike in both sexes, o_r clse most de-
The Euidence f r o m Sexual ChaTacters. 221 wloped in t h e males, a i d t h a t the males of allicd breeds differ. from each otlicr more than the females. The indiridnals of choice breeds of donicstic pigeons arc not allowed to follow their own inclinations and t o pair a t will, but thcy are very carefully watched by the breeder, for reasons mliicli h a r e no reference to tlie inclinations of tlie birds, so that there is no chancc for sexual selection, nor does tlie breeder confine his attcntion to the male sex especially, bnt seeks to impox-e the fcmale as well as t h e male ; and R l r . Eaton asscits in his '' Trcatiw on thc Almond T~iniblcr'~ tliat a hen tumbler would bc worth twice :is much nioncy as a cock if slic 1i:id tlic characteristics of the breed eqnnlly m l l de~cloped. We find, then, that among the two linndrcd or more mild s p i e s of the pigeon family, \\here sexual selcction has every chance to act, there is no great diffcrcncc between tlic sexes ; but tliat in llic Inore ~a1n:iblcdoniesticatcil breeds, wlierc all choice is lmxlnded, aud ~exo:il selection oiit of the question, tlic m:ilts arc, as a r111c, niorc modified t l i m the females nlienever tlic sexes differ. We must tlicrcforc c o ~ c l n d etlint tlic greater moc1ific:ition of the m:tles, in pigeons at least, is not dne to the fact that the male is more exposed t l i i ~ nthe female to tlic action of selection, bnt that tlie male lias morc tcndcncy than the femnle t o dcpart k o m the ancestral type. In pigeons, a t least, we mnst believe tliat something witlrin tlic animal determines that tlie male slioald lead and the female follow, in the evolntion of new breeds.
Doniesticated Animals iit General. When TVC stntly otlicr domesticated mimnls in tlie same way, we find that in some ease^. as in liorscs, thcrc is little difference between the sexes, and in other cases
222
Heredity.
tlie efforts of t h e breeder are directed towards n p e e d iwity of one or the other sex, as mlicn cattle are rwi.ed for the s;tlie of tlieir milk, or wlien fowls are kept fur figlrtiug, or for tlieir eggs; but whenever the sexes do d ffer ive find that the same law exists, and tlitit tlie males of allied races differ from each otlier more than the females. Rcgarding sheep, Darwin says t h a t there is a strong tendency for cliaracters wliicli have been acqnired under domestication to become attached exclnsively to tlie male sex, or to be much more higlily develolwd in the male th:in in tlie female. As illnstrative of tliis law he refers, among otlier jnsta~iccs,to tlie fact that the accumulation of fat in tlie fat-tailed sheep of the plains of India is greater in the male tlian in the fcrnnlc, and the maiie of the African maned race is far more d e w l o y d in the ram tlian in the ewe. Anio:ig fowls, erery one is familiar with tlie fact tliat tlie mnlea of different breeds are, as a rnle, mncli more different tlian the females, and that most of tlie breeds arc distinguisl~edfrom each otlier by pecnliaritics i n organs ~vliicli,like the comb, spurs, and long tail-featliers, are confi:iecl to the male. As a rule tlicre is considerable difference between tlic sexes of fowls, but exccptions are not at all nnnsiial, and j n many lJreeds the 6excs c:m hardly be tlistingnislied. 'rlic males and fcmules of tlic gold and silver laccd Sebriglit bantam can be barely distingaislied from each other, cscept by the comb, wattles, and slmrs, for they are colorcd alilx, :ind t h e rnnles have not Iiacltles, nor the flowing, sicl
tlic featlicrs of tlie hen, lie retains all liis couragc and high b1)irit. 111 a few cases the fcmalcs of allied breeds differ ~ n o r c tliw tlic males, atid Darwin refcrs to two strams of black-breasted red games, iii wliich tlic cocks were SO mnch alike that tlicy could not be distinguished, nliile tlic licns \vcrc partridge-brown in the oiie cnsc and fan 11brown in tlic other. The pencilling mliich is cliaractcristic of tlie Earnburg lien is almost absent in the male, but as a rulc tlic various breeds of fowls are distingaislied by peculiarities of organs which arc almost or entirely confined to the males. Of tlic comb Darwin says that it diffcrs mucli in tlie various breeds, and its form is eminently cliuractcristic of ench kind with the exception of tlic Dorkings. A siiiglc deeply scrratccl comb is tlie typicti1 and most coninion form. I t diffcrs much in size, being immensely dcvclcpcd in Spanish fowls; and in a local brccd called Itcdc:ips, it, is sonictimcs n p ~ ~ a r dofs threc inches in breud~lia t tlic front, ancl moic t ~ i a nfour inches in Iengtli, nicasnrcd to tlic ciid of the peak behind. In sonic brccds the comb is donblc, and wl,en tlic two cnds arc cemented togctlier i t f o r m a “ cap comb;” in the “rose comb” i t is dcprcsscd, co-icrcd with small projcctions, and produccd backwards; in the liorncd and CrBvc-Ceur fowl i t is proclnccd into two horns; it is triple in tlic pca-combed Brahmas, short and truncated in tlic Malays, and absent in tlic Gncldcrlands. In tlic tmclled game a fcm long fcntlicrs arisc froin tlic back of tlic comb, and in many breeds a crcst of fcatlicrs replaces tlic comb. Tlic crcst, wlicn little dcrclopcil, ariscs from a ficsliy mass, but mlicn mncli dcvclopd, form a licmisphcric31 protuberaucc of tlic sliull. In tlic bcst Polish fowls it is so largely devclopccl that tiic birds can liardlj picli up their
224
Heredity.
food, and they are said to be particularly liable to be strncli by hawks. With reference to variation in tlic plumage of t h e malc fowl Darwin says ( Variafioiz,1’. 507): “ A s in some orders of birds t h e males display extraordinarily shapcd featlicrs, sucli as naked shafts mitli disks a t tlic end, ctc., the following case may be worth giving. In tlic wild Gnllus baizkivn, and i n our domestic fowls, tlic barbs wliich arise from cacli sidc of the extremities of t h e liackels are naked or not clothed with barbulce, so that they resemble bristles; b u t Xr. Brcnt sent me some scapular haclicls from a young Birchen Dnclin-ing gamccock, in which the riakcd barbs becamc densely rcclotlicd with barbulcs towards their tips, so that tlicse tips, which mcrc dark colorcd with a metallic lnstre, r e r e scparatcd from tlie lower parts by a symmetrically-sliapcd transparent zone formcd of the naked portions of the barbs. IIcnce thc colorcd tips appeared lilic littlc scparate metallic didis. Tlic sickle featlicrs in tlic tail, of which there are tlirec pair, and lvliicli are eminently characteristic of tlie malc sex, differ mnch in tlic n r i QUS breeds. They are scymntcr-shapcd i n somc IIamburgs, instead of being long and flowing as in tlic t-i)ical breeds. They are extremely short in tlic Cocliins, and are not at all developed in IIcnnics. Tlicy arc cnrried, together with the whole tail, erect in Dorliings :ind games, but droop much in Malays and some Cocliiiis. Sultans arc cliaracterizcd by an additional number of lateral sickle featlicrs. T1:c spurs vary mucli, being placcd liiglicr or lower on the shat~li;bcing cxtrcmely long and sharp in games, and blunt and short ia Cocliins.” Tlic nnmbcr of tlie s p i m varies, somc fowls linving as niany :is fire on cncli leg; thcir position 03 the lcg also wries in different breeds.
me Evidence f r o m
Sexual Characters. 223
These extracts are sufficient to show that organs mliich arc cocfiiicd to tlic cock arc especially Tariablc, : i d that t l i c characteristics of eacli Lrccd arc cliicfly moilificaticns of tlicir male lmrts. It is tlicrcfore cvidcnt that tlic malcs of tlic vz' rlOUS ' brecds arc as a Inle inncli more diffcrent from eacli oilier than tlie fcmales, in fowls, as mcll as in sliecp, 1x w o n s mid otlicr domestic animals. Tlic rule is by no means uni~-ersal,however, and tlierc arc a few reinarkablc exccptions. I h a w already mcn5oncd two cases of blackbrcastcd red game €owls, i n wliicli t h e fcmalcs mere quite distinct, mliilc tlie malcs of the two forms could not be distinguislicd. Tlie breed of domestic ducks known as tlie Call Duck is remarliable €or its small size and from tlic extraordinary loquacity of the fcmalc, while tlic drake only liisscs likc ordinary d d i c s . Darwin sires (F-uriutioiz, Vol. I. p. 309) an interesting account of the origin of tlie crest in Polisli fowls. IIc s q s that in most fowls head ornaments of all hinds are morc fclly developed in tlic male tlian in t k c fcm:dc; but i n Polisli fowls tlic crcst or top-lcnot, wliicli in tlic rnalc rcplaccs tlie comb, is equally dcvclopcd in both sexes. '' In certain snb-breeds, which from llic licii having a small crest arc called larli-crcsted, a single upright comb sometimes almost entirely takes tlic l h c c of tlie crest in tlic male. From this latter case, and from Gomc facts presently to be given mitli rcspcct t o tlic protnberaiicc of tlic skull i n Polisli fowls, tlic crcst in this breed ought pcrliaps to bc viewed as a fctniiiinc cliaractcr which lias bccn transferred to tlic male. . . At llic prcwiit day a11 tlic breeds of Polisli fowls h a w tlic grcat bony protabcrancc on tlicir sliulls, mliicli inclndes part of the brain and supports tlic crest, equally dcvclopcd i n both sexes. But formerly in Germany tlie skull of
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296
Neredity.
tlic lien alone was protuberant. Blumcnbach, who particularly attended to abnormal peculiarities in domestic : i t ~ ~ n i dstates, s, in 1813, that this was tlie case; a i d Beellstein had previously, i n 1793, observed tlic same fact. This latter author . expressly states t1i:it Iic never observed this protuberance in male fowls. IIciIcc tliere c;m be 110 doubt t h a t this remai kable cliaracter in t h e sliulls of Polisli fowls was formei.ly in Germany confined to tlic female sex, but has now been transferrcd to the mclcs, and has thus become common to both sexcs.” Tlicse few cases are clearly escei)tional, and tlic study of domesticated animals shoms us that, as a rule, t h e males of allied breeds, like the males of wild species, are more different from e:di other than the fcrnalcs. TVe cannot :Lttribnte this difference to sexual selection, for most of our domesticated animals, especially those of pure blood, are prevented by man from €ollowiiig tlicir own inclination in tlie salect;on of mates. h’cjther can me assert that man has devoted especid attention to the selection and modification of males, atid Iias aimed a t cliangcs i n those oi‘gans wliicli ore most dcvcloped in ni:tIes, for, among pigeons a t least, the opposite of this is the case, and a female bird of equal exccllence js more valued thnn a male. We are thus forced to conclitde not only that “among domesticated animals tlic m:ilc is morc vari:ible than the female” (Darwin, Scxzicd SeZcctiou, Vol. I. p. 26G), b u t also t h a t organs wliicli arc confinctl to males, or iiiiusually dcrelopcd in them, arc more a p t tlian organs wliicli are confined to females, to transmit their T-ariations, and thus to g i m rise to licrcditary mcc modifications. As onr domeeticatcd r:iccs E ~ O W ,by tlicir close similarity to ii:itnra1 specics, tliat tlic causes wliicli hare produced them are very simjlar to tlmsc wliicli have acted upon wild organisms, we are justified
.. .
The Eoidence f r o m XexuuZ Clum-ncters. 227 in doubting, from the analogy of domesticated animals, wIietIier the cxcessive modification of the males of mild animals is due cntirely to the €act tliat males arc more exposed than females to tlic action of selection. As the study of domesticated races leads us to tlic conclusion that sometliing wit1ii:i tlic animal compels tlic male to lead and the female to follow in the evolution of new breeds, we must believe that a similar lam regulates in the same way tlic evolntion of wild organisms. The study of domesticated races, like tlie stndy of wild species, also compels 11s to believe tliat this law is not immutable, but that variations which originate in a femiile may beconic hereditary, :iltboiigli this is someivliat rare, as compared Fit11 tlie liercditary cstablislimen t of male modifications.
The TTiew that the illale i s i ~ i o r eErposed than the Female t o the AcfioiL qf Selectioir. According to Darwin the csccmive C ~ ~ O S I W of C tlio malc to tlie action of selection, nntural and sesu:tI, is the cansc of liis great modification. IIc points ont that the distinctive cliaractcrs of tlic male arc, in mang cases a t least, of especiul use to him, as a male, and he shows that tlic individuals mliicli possess these peculiarities are benefited by tlicm, and liavc thercfom been ~~reservcc?, vliilc tlic €males, deriving no adwntnge from tlicm, liaw not bccn thus sclectcd. No onc c:tli doubt tlic truth of this statement, bnt it docs not go to tlic root of tlic matter. The qucstion is not how pecnliaritics useful to tlie rnalc tllonc have been restricted t o that scs, bnt why the female Iias n o t ticqiiired anothcr sct of cliaracteristics to fit licr for her peculiar nccds. No one can doubt that a lien niiglit have special organs, as useful to her for the care and pro-
228
Her ediity.
tcction of licr brood, and for her oltin defence while incubating, ;is tlic cock’s spurs and ornaments are in another w:iy t o him: nor can TVC doubt iliat such organs would Lc prcscrrcd and pcrfcctcd by natnml selection if proper variations sliould appear and should become lie rcd i tary. Among the mammalia the peculiar organs of t h e male, liis so-called secondary sexual characters, are often of great use t o him in ways which are not connected with reproduction. This is espccially true of his weapons of offence, for tlie bull not only nses liis horns i n fighting with otlier males for the fcmalcs, b u t also i n protecting himself and tlic rest of the lierd from cncmics. T h e elephant uses his tusks in many ways. IIc tc:irs’doivn trees with tlicm for the sake of the foliage, and hc rips open lmlm trees in order t o obtain the nntritious farinaceous core. IIc uses tlicm to prod tlic ground to disco1 cr mlictlicr i t is firm enough to bear his meiglit, and with them Iic iittaclis and kills liis encmics. M x i ~ moniitnin goxts, wlicn tlicy accidcntally fall from great heights, strilic npoii tlicir strong and elastic liorns, nncl tiins break tlic forcc of tlic blow. In fact, most of tlic w.xipo m nliicli occur in malc animals’ arc nscd for delcncc or protection, as well as in tlicir conflicts with otlicr males. Tlic prcsencc of tlicsc organs often ares tlic life of tlicir possessor, and i t monlcl tlicreforc semi as if tlicy would bc more niodiGcd by natural selection t1i;in by sexual sclcction, for n a t u r d selection nsnally means death to tlic unarmed male, wliilc tlic result of sexnal selection is simply a dccreascd niinibcr of c1ceccntl:int~. Bnt 11ati1r:il sclcction acts upon tlic fcni:ilc as wl! :is the malc, and :is tlic care and protection of tlic ~ o u i i gnsiidly falls to tlic female mammal, i t mould seen1 as if die as well as the male ought to have special n-capons of de-
The Evidence f r o m XezuaZ Characters. 229 fcncc. Tltc n-clfare of tltc rncc docs not dcpcnd upon tlie niimbcr of p i n g wliicli arc born, bnt 111ion tlie nnmber wliicli grow 1117; aud if wc take two cascs, onc mrietg i n IF liicii tlic mnlc 11::s spccial ~~cnpoiis n liicli cnable liirii to drire my liis rirnls and thus to lirodocc n p a t numbcr of children, and nnothcr wricty iii nliich tlie femalc has special wcnpons wliicli cnable her to lirotcct her yoiingfroni encniics, and thus rear tlicni a11 in safety, it ccrtninly scenis as if the modification would be most sure of pcrpctnation in the sccond case, and tliat tlie sccoiid rnriotp should, in time, cxtcrminatc tlic first. As ;L matter of fact IVC do find tlint tlic ncnpons of manininls exist in many cases in tlie fcmnlc, but tlicy :ire most dcwlolicd a d most modified in tlic ninlc, nnd it is hard to nndcrstand why nritltions of this bind should not more frcqiicntly arise and bccomc licrcditnry in the female, unlcss sonietliing besides sexual selection determines that malcs should be more plnstic tlinn fcninlcs. Thc modification of the female is certainly quite possible, for thcre are nunibers of cascs in all groiips of tlic animnl kingdom where the females alonc ham some IICculiar clinractcristic wliicli is not directly concerned in reproduction. Thus Darwin says ( Va~iatioiz,Vol. I. 13, 333): ‘‘ The tarsi of the front legs arc dilatcd in many mnlc bectles o r are furnished with broad cushions of hairs; and in many genera of water-beetles they arc armed with a round flat anchor, so that the male may adhere to the slippcrj- body of the female. It is n much more tctzuszial circzmsfame that tlie females of some matcr beetles (Dytiscus) hare their elztrs dccply y o o ~ c d and , in Acilius sulccitzis thickly sct with liairs, as an aid to the male.” We have seen that tlie males of many spccics of crustacee liaye various parts of their bodice especially modi-
230
Heredity.
fied for clinging to the female, and we can understand that natural selection will perpetuate modifications of this kind, for the mdeswhich adhere most firinly to the femalcs will leave the greatest number of descendants, who will inherit their peculiarity; but the same rule would hold good if certain females were so modified as to afford a good surface for the male to cling to, as we may see from the fact that in a few forms the females are thus modified. Fritz Miiller lias described certain species of nmpliipod crnstacea, of tlie genus Melita, in wliicli the fernale does have special hook-like processes for tlie male to cling to, and cases of tliis kind are sufficiently numerous to show that wlicii a useful female modification docs appear i t becomes hereditary. I n all cases wlicre the sexes are separated and different from each other, the female nndonbtedly miglit be benefitcd by peculiar organs as frequently as the male. How tlieii are we to account for the remarkable fact tliat the cases of m d e modification of this kind are so very much more numerous then the instances of fernale modification? Darwin concludes that we must believe that the male is more variable than the female, and we shall subsequently see that this is so, and the reason for it. Still the female does vary, and vary greatly, and unless there is some reason wliy female rariations should be less apt than male variations to become hereditary, tlie great preponderance of special male modifications is incomprehensible.
The Male ?nore Eager tliait tlie Fentale. Dnr\yin attribntcs this to the greater eagerness of the male. He says (Sexual Selection, Vol. I. p. 263): “Throughout the animal kingdom, when ’the sexes
The Xoidence from Xexuul Churacters. 231 differ from each other in external appearance, it is the male which, with rare exceptions, has been chiefly modified: for the female still remailis more like the young of her own species, and more like the other members of the same group. The cause of this scems to lie in the males of almost all animals having stronger passions than the females.’’ He points ont that i t is the males that fight together and display their charms before the females ; that among mamm:ils, birds, fishes, reptiles, and batrachians, the male is known to be niuch more eager than the female ; that among insects i t is a law that the male seeks the female ; that among spiders and crnstacea the males are more active arid erotic than the females, and that in these latter groups the orgins of sense and of locomotion are often more liiglily developed in the male than in the female. The female, on the other hand, is, with the ritrest exceptions, less eager than the male : she is coy, requires to be courted, and may often be seen for a long time endeavoring to escape from the male. He gives the following cxp1an:ition of the manner in which the male has becn rendered more eager than the female, so that he searches for her and plays the more active part in courtship in so many widely distinct classes of animals : “It would bc no advantage and some loss of power if both sexes were mutually to search for each other; but why should the male almost always be the seeker ? With plants, the ovules after fertilization have to be nourished for a time; hence the pollen is necessarily brought to the female organs-being placed on the stigma, tlirotigh the agency of insects or of thc wind, or by the spoutaneoua movements of the stamens, and with the algae, etc., by the locomotive power of the antherozooids. With lowly
232
Her edity .
organized animals permanently affxcd to the same spot, and hn-iing tlieir sexes sel?aratccl, the male element is invariably brought to tlie female; and we ciin see the reason, for tlic o m , even if detached before being fertilizcd and not requiring snbsecinent nourishment and proteetion, would be, from their larger relative size, less easily transported than the male element. Hence, plants and many of tlic lower animals are in this respect analogons. In the case of animals not affixed to the same spot, but enclosed mitliin a shell, with no power of protruding any part of their bodies, and in the case of animals Having little p o ~ e rof locomotion, the males must trust the fertilizing element to the risk of a t least ashort transit through the waters of the sea. It would, thercfore, be a great advantage to sncli animals, as tlieir organization became perfected, if the males, mlicii ready to emit the fertilizing clement, wcrc to acquire the habit of approaching the female as closely as possible. The males of various lowly org:inizecl animals lime thus aboriginally acqnircd the same habit w1iich would naturally Lc transmitted to thcir more highly developed male descendants; and in order that they should become efficient seekers, they would have to be endo-wd with strong passions. The acquirement of such passions mould naturally follow from tlie more eager males leaving a larger number of offspring than the less eager.”
This is all nntlorilhdly true, as far as it goes, but it docs not cover tlie c ‘,i)leground. The sexual passion of the male is undunbtcdly stronger, as a rule, than tlint of tlie female, and as tile existence of the species depends upon the strength of this passion, there will un. doubtedlp be a selection of the most. eagpr males,
The Ec'idence f om EexuaZ Characters. 233 We must recollect, however, that the sexual passion is not tlic only one upon which the perpetuation of the species depends. The parental feeling or passion is fully as importnnt, and as a rule this is most developed in the female. I n the same way tlint the males which are best fitted for pleasing and commanding the females are nattially selected, those females which are best adapted for protecting, feeding, and educating the young would be picked out from generation to generation. If any hereditary variation should appear wliicli con tributed in any way t o this end, i t would be a t least as raluable to the species as an extra ornament or a new color in the male; and there are certainly as many possible ways to improve a female animal as thcre are to improye a male. If these variations of parts which are confined to the female, or wliicli arc of use only or chiefly in this sex, are as apt as the similar parts of a male to give rise to licrcditary modifications, we should cxpect the evolution of iicm improvements in tlie female body to keep pace with tlic improvcrnent of the male body. We should expect, when allied species are comparcd, to find that the females differ from each other as much as the males; and that while the males are gradnally becoming more and more specialized for conflict and riralry with other males, and for miiining the favor of the fcmales, the females are Ixcoming specialized along another path, for the better care and protection of their yonng. The fact that we find nofliing of the kind; that evolution shows itself especially in tlie males, while the f e n d c s remain comparatively stationary, shows that we must search for some other explanation than the one given by Darwin. We are, therefore, compelled to recognizc, in the general rule that the male is more modified than tlic female, the evidence of .some cause
234
Her edity.
more fundamental and general than the great exposure of the male, through the intensity of tlic sexual passion, to the influence of selection; for tlie parental instinct is fully as important for the welfare of tlie race as tlie sexual instinct, and the former is, as a rule, rnost de,veloped in the female, just as the latter is greatest in the male, and it might be expected t o lead to the selection ant1 modification of feiiiales, as the latter passion does to the modification of males.
The Theory of Heredity FurnisJhes the Oidy Adequate ExplaIiatioi8. We must acknowlcdge tliat the great body of facts detuiled in the bcginning of tliis cliapter have no adequate explanation, except on the liypothesis that a part which is present, or functional, or most impurtuit in tlic male alone, is very much more likely tliaii a part wliicli is limited to females in tlie stlnie way, to give rise to Iiereditary variations. The facts receive a ready explanation 011 tile hypothesis tliat tlicre is an especial adaptation for the transmission to the egg of gemmules tliromn off by the cells of tlic malc body, while their trausmission in tlie female is not thus provided for, but is due to accident. According to this view we must, in animals where the sexes have long been sepnritcd, look to the cells of tlie male body for the origin of a large proportion of the rariations which lime gradually beeii accnmnlated in the past to give specics their present character; and we must regard secondary sexual characters as differing from ordinary specific character~stics,simply i n being especially useful to one sex, nsndly tlie malc, or i n being disadvantageous to the other sex, so that natural selection has developed them to a greater degree in one sex than in the other.
The Evidence from EextcaZ Charactcrs. 235 It will be seen tliat tlie eyidcncc from this cotircc is, as far as i t goes, very similar to the esidcnce from Iiybricls. A reciprocal cross between two species furnidies a means of analjzing the iuflaence of tlie two sexes, :ind of distingnisliiiig, to some slight degree, the effect of each sexual element in heredity. The stricly of sexn:il cliaracter gives us another means of doing tlie same tliing on a more limited scale. As each cell of tlie body niny tlirow off gemniules, there is no may of showing tliat a variation in a part nliich is alike in both sexes, is due t o the tr:iusmisaion of grmmules from tlie cells of one parent rather than from those of tlie other, but the case is different with a part uliich is more developed in one sex tli:in i t is in tile other. In tliis case we sliould, according to our tlieorj of liei,cdity, expect i t to tlirow off gcmmulca most frequently i n tlie sex in which it is of most functional importance, and as we suppose tliat tlicre is nu especial arrangcment for the traiisrnission to the egg of those gcmmules wliicli originate in tlie male body, we can see that an organ which is most iniportant in the body of the male is much more likely to give rise to hereditary modific:ition than one which is most important, aud therefore most prolific of gemniules, in tlie female body. The Iijstory of secondary sexual ~11aracte1'sis, thmefore, what our theory of heredity wonlcl lead us t o expect, and no other explanation which has c ~ e rbeen proposed fully accounts for all the phenomena. Instnnces qf Female 2lfodi'cafioia. We should not expect, liomercr, to find sccoudary sexnal cliaracters esclnsiwly confined to ni:tles, but simply more general tlinn tlicy arc in fcnialcs, :ind as a niiittcr of fact we'do meet with many cases wlierc the female has been more modified than the male.
236
Her edit g.
I will now give a fern of those wliicli seem to me t o be most opposed to my general coiiclnsion. Jentab Modtjkation.
I n certain species of the amphi pod crust ace:^^^ genus Melita, the females differ from all other amphipods by having the sexnal lame1lze of the imiii1tiiii:~tcpair of fect produced into hook-like processes, of mliicli the males lay liolcl with tlie hands of tlic first p i r . In anotlicr amphi pod Bradl yscelus, the niale l?(JSSeSSCS, like all other mnpliipod3, ;I pair of posterior m t c i i i i ~ ,hit tliey are absent in the fcinale, so that tlie latter differs more tli:un the male from allied fortiis. 1)arwin states that the fenides of c c r t i n water-beetles, :is D y f i s m s Acilizis a d N y d r o p o m s , lia~retlieir wing-corers grooved or thickly set with Ixiirs or p~inctnrccl,in order to ennblc the mnlc to cling to the slippcry snrfucc of tlicir liard and polislictl bodies. The cnll duck is a domcsticnted breed wliich receives its name from its extmordiu:~ryand ex~eption;il1oqu:ucity, and as this loquacity is confined to tlic fem:ilc, while t h e male ltisscs like other ducks, we must regard this as a case of fcmnle modification. We know from the statements of Blumcnbach and Beclistein that, previonsly to the year 1513, tlie great bony protuberances on the skull which characterize the Polish breed of fomIs, iverc confined to the females, altliough they are now equally developed in both sexes. Tliere c:m be no doubt that this pccnlinrity originated in tlie females, and mas snbseqnently inlicrited by t h e males. Among the P 1 1 : ~ m i dor~ spectre insects the females alone, in snme speries, sliow a most strihing resemblance to leaves, while tlic males show only a r d e approximation, and Darwinhas pointed o u t that, as me can
The Eoidence from Sexual Characters. 237 hardly belicvc that such a rcscmbiancc is disadrantagrons t o the males, we must conclndc that the fenxiles nlonc have viiricd, :uid t h a t tlicsc variations have becii preserved and angmeiitcd by natiii-a1 selection for tlie w h e of protection, aiid have bccn transmitted to tlie female offspring alone. In two species of Birds of Paradise, Pnrndisin npoda and Parndisia Papziana, tlie females differ from c a d i other 1noi.e tlian do their respective males; the female of the Iat ter spccics having the nnder surface piire white, tvliiic tlie female of 1'. a p d a is decp brown beneath. T h e males of two species of shrikes (Oiywotzis) in the islands of Mauritius and Bourbon, differ but little in color, mliile the females differ much, so t h a t the fcmnie of the Boiirboii species might a t first siglit be mistaken for the young of t h e M;iuritiiis species. In this case there seems to bc every reason for belicring that t h e female of the Mauritius spccics lias varied, while tlie male has remaincd unmodified. Seinper stales (BnimaZ L i f e ) on the authority of Dr. H:igcn that the fenides of many species of cave-bcctlcs arc blind, while the males have perfect eyes. As we may fcel confident that these bcetles are descended from ordinary forms, we must regard this as a n instance of female modification. The remarkable shell which is secreted by the large fan-like arms of the paper nautilus (Argonau ta) occurs in the femnles alo:ie, and i t probably owes its origin to fcni,ile modification, although it i t not impossiblc t h a t our i w e n t spccies may be descendcd from a form in wliicli the male had a shell. The most remark:ible cases of female modification are those mliich arc presented by polymorphic insects. Papilio turnus is one of our common yellow butter-
238
Heredity.
flies, and it is found over almost the whole of temperate h-ortli America. I n New Englund and Ncw York tlie sexes are alike, but south of h t . 42' some of tlie fenialcs are black, and tlicy arc so different frcm tlie jcllow male and tlie northern yellow female, tl!at they were for a long time regarded as a distinct species, and hire rcceivcd a specific name, Papilio y l m c z m Betrvccii ]at. 42" and 1;zt. 37" both forms are found, and Prof. Uliler of Baltimore, has rcared tlic jellow female Pupilio tzcmzcs, and the black one, P. ylaiiczis, from tlie same lot of eggs, but further south only tlic black female is found, althongli tlie male is exactly like that wliicli in Ncw Eiigland is associsted with tlie ycllow female dlonc. Wallace lias recorded a number of similiLr cases nnicing t lie Ma lays11 Pap ilion id@, of mli icli Papilio Xemnon is one of tlie most strilkig. In tliis spccies there are two kinds of females, one closely resembling the male, and tlic other differently colored, and ftirnisheci rvitli long spatulate tail-like clongations of tlie hinder wings. These tails are not prcsent on the wings of the male nor on those of the second female, altliongli they arc found in both sexcs of other species of Pitpilio, and in some otlier less slwcialized genera of the P q i l i o family. Tlie males, tlie tailed and the tailless females have all bcen reared from a singlc group of eggs, so tliere is no doubt that they all belong to tlie same species. Wallace lias given otlier cases in which the same male form is found associated, in cliffercnt countries, with their three different female forms. I t is possible, and indeed probable, that in some of tliese cases certain females have resembled the male, wliilc others liave either remained unmodified or else hare reverted back to an ancestral form. Darwin refers to a case of sexual dimorphism which
The Evidence from SexuaZ Characters. 239 occurs in several species of tlie dragon flies of the genus Agrion, in which a certain number of females are of an orange color, and tlids differ from the males and ordiliary females. He suggests that this is probably a case of reversion, for in tlie true Libellulae, whenever the sexes differ in color, tlie females are always orange or jellom, so that, supposing Agrion to be descended from sonic primordial form liaving the characteristic sexual colors of t!ie tjpical Libellulae, it would not be surprising that a tendency to vary in this manner sliould occur in the females alone. This explanation seems to apply to sereral of the recorded cases of female polymorphism, but not to :ill, and we must acknowledge tliat in these cases the female shows, in a far greater degree than the male, a tendency t o deviate from the primith-e form of the species, and to giye rise to new race modifications. We have already called attention to tlic fact that among the Crnstacea there are many cases of male polymorpliism, and many cases of tlie same kind are known among =ale insccts; as well as many cases, besides those I have mentioned, of female polymorphism. I n many of tlie social insrcts we have most pofound structural mddifications, and most complex instincts, which can only have arisen in females; and as allied species of social insccts differ from c:xcli otlier in characters wliicli are confined to the females, ivc must acknowledge that in these forms there is no lack on the part of thls sex of a power to give rise to hereditary race modifications. That facts of this kind present a serious difficulty 1 cannot deny, but we must recollect tk.t our hypothesis does not demand tliat the power to transmit variations should be confined exclusively to males, but simply that
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Heredity.
il should be much more active in thcm than it is in the females, and we certainly find that this is the case. I believe that we may, in justice, conclude that, with greater knowledge of the few cases where females give evidence that they have tliis power to an exceptional degree, the difficulty will disappear, for they are certainly deviations from a general rule, and they must therefore be regarded as special cases, to be studied by themselves. It is interesting to notice that both parthenogenesis and female race-modification are inore frequent among the Anthropods than in most other groups of animals, and that parthenogeneais is known to occur in the Lepidoptera and in the social insects, two of the groups where great modifications can be most clearly traced to a female origin. I t is not improbable that the power of the egg to develop without fertilization, and its power to store u p and transmit geemmules, maybe related in some way, so that when the one power is acquired the other is also. Every one is aware that we meet, in the most diverse groups of animals, with structures and instincts which are confined to the females; such as the brood-chambers of Daphnia, thc ovipositor of the ichneumon fly, the sting of the honey bee, the marsupial pouch of the oppossum, the nest-building and incubating instincts of birds, or the nursing habit of female mammals. We must bear in mind, however, that in many of these cases a male origin for the successive variations is not ont of the question. The fact that the male Hippocampus and not the female has an incubatory poiich, and that mammZe are present in most male mammals, certainly shows the possibility of a male origin for these structures, and m many male birds either share i n the work of nest-building and incubating or aid the female in this duty, there
The Euidence from Sexual Characters. 241 is certainly no difficulty in believing that these instincts hare liad a male origin. r 1 l l i e reniarkable instinct which 1e:ids some species of cuckoo and crow blackbirds to lay their eggs in the nests of other species, must have originated in females, and a collection of all the cases mliich must be explained in tlic same way wonld make a formidable list, b u t the fact mould still remain true, that among animals with separate sexes, male modifications are T-ery mnch more freqiient than female modifications, and this is all that our theory requires.
CHAPTER X. THE EVIDENCE FROM THE IXTELLECTUA L DIFPERENCES BETWEEN MEN A R D W O M E N .
(This clinpter, wliich was published in the Pupulai. Science dIonl?@/ for Jane aud July, 1879, under the title, (‘The Condition of Women froin a Zoological Point of View,” is repriuted here. almost without change.)
ZOOLOGY is the scientific study of tlie past history of animaI Iifc, for the purpose of understanding its future history. Since mail has, in part at least, conscious control of his own destiny, i t is of vital importance to human welfare in the future that we should le;irn, by this comparative study of the past, wliat are the lines along which progress is to be expected, and what the conditions favorable to this progress, in order that we may use our exceptional powers in harmony with the order of nature. The study of the growth of civilization shows that human advanccment has been accompanied Ly slow but constant improvemen t in tlie condition of women, as compred with men, and that i t may be very accunttely measured by this staiidard. Judging from tlie past, we may be sure that one of tlic paths for the future progress of tlic race lies i i i this improvement, stid tlic position of momcii must therefore be reg:u.ded :AS a most important social problem. If there is, as I shall try t o show, a fundamental and coiistant,ly increasing clifference between the sexes; if tlicir needs arc different, and
The Ecidence f r o m Intellectual Diferences. 243 if tlieir parts in tlie intellectual, moral, and socid evoliitioii of tlie race are, like tlieir parts in the rclwoductire process, complemental, the c l e y recognition of this dlflerencc must form botli tlie foundation and superstrncturc of all plans for tlie improvement of momcn. If there is this fundamental difference in the sociological influence of the sexes, its origin must be sought in the phpsiological differelices bet~vecnthem, nl tliovgh tlie subject is now very far removed from the province of ordinwy pliysiology. While we fully recognize the insignificance of tile merely mima1 differences bctween the sexes, as corcparcd with tlieir intellectual and'moral influence, i t is none the less true that the origin of the latter is to be found in thc.former; in the same manner -to use a honible illustration-that the origin of the self-denyiiig, disinterested devotion of a dog t o his master is to be found in that self-negation which is necessary in ordrr that a herd of rvolres niay act in concert under a leiidcr, for the general good. 10 order to trace tlie origin and significance cf the differences which attain to snch complexity and importance in the human race, TVC must carry our retrospect back far beyond tlie beginning of civilization, and trace tlie growth and meaning of sex in the lower forms of life. 111 so doing I sliall ask attention to sevcral propositions which m:Ly not a t first appear to h a r e any bearing upon o ~ i subject, r or any very close relation to each other. I shall then try to show what this relation is, and point out its bearing upon tlie education of women. Every organism which is born from an egg or seed is a resultant of the two systems of laws or conditions which nxty be spoken of abstractly as tlie law of heredity and t h e law of variation, or, to use the old teleological terms, each organism is a mean %etmeen tlie principle of adhe-
244
Her edit y.
rencc to t j p e and the principle of adaptation to condi tions. T1i;it like prodncp like is nnivcrsal!y but never :ib-olntely true. Tlic offspring resembles its pirents i n d l fuiidament;il cliaracteristics. T h e human child, for i n stance, rcsembles its parents in the possession of :ill tlie cliaracteristics which distinguisli living things from those mhicli are not alive, as well as those wliicli distinguish animals from plants. The chemical, plijsic;iI, and physiological changes which take p l x e in its body and the histological structnre of its tissues are like those of its parents, and its rarions organs are tlie same in form and function. All tlic cliaractcristics mliicli unite it with tho otlier vertebrates, as a member of the subkingdom Vertebrata, are like tliosc of its parents, and also those wliich place i t i n tlic class Mamnialia, nnd i n its proper order, family, genus, and specics. I t also shares with its parents tlic featnres or race cliarncteristics of tlic particular tribe or race to which they belong. If they arc Chinese, Indians, or negroes, tlic child belongs t o tlio s m i e race, and manifests all tlie slight, superficial peculiarities of form, constitution, and c1i:iracter by wliicli tliat race is distingnislied. Even tlie i n dividual peculiarities of the parents, intellectual and moral as well as physical, are now known to bc licreditary. Since this holds true of any other animal or plant, we must recognize the universality of the lam of heredity, but we must not oTerlook the eqndly well-est:tblishcd fact h , t each organism is tlic resiiltant of this law and another, the law of variation. Tlic child is likc its parcnts, but not exactly like tlicm. It is not cvcn a compound of characteristics found i n one or tlic otlier of tliem, b u t has individual peculiarities of its own; slight variations which may riot have existed in either
T h e ELtidence from Tntellectual Diferences. 245 ~
parent, or in any more rcmote ancestor. Tlic diglit iiiclivldu,il diffcrenccs are so ovcrsliadomcd by the mncli more conspicoons rcscmblmces due to licicdit~--witli which they conipaie about as the grceii buds nt tlic tills of (lie twigs of it large tree compare with tlic 1i:ird mood of tlie trunk and bmnclics, tlie growth of preiious jciirs -and they are so flnctunting a n d inconstant, tlint their iniportance may easily cscapc atten tion. Ctircf ul observation sliows, Iiomcvcr, tlint every clinrnctcristic may vary: tliosc distinctivc of tlie class or order as n.ell as those which maik tlic specics or variety. 'l'lie \ariatioxis may nianifcst tliemselres iii tlic adult, or at aiiy.otlier period in tlie life of tlic individual. E r c n tlic eggs Iiave intlii idualities of tlicir own, and among manv gionlis of aiiimitls tlic eggs of tlic same parent, wlicn ~rlaccdnilder ~)rcciseIysiwil'ir conditions, may differ i i i tlie rate and manner of dcvelopmcnt. Although moot of thcsc individual diffcrcnccs are transient, iind disnppc:ir witliin a few generations, there can now be n o doubt that tliuse which tend to bring t h e organism into more perfect liurmony witli its environmeiit, and arc tliereforc advant:igeous, may be established as hereditary features, t h o u g h the action of tlie law of the surviwl of the fittest; and it is hardly possible to over-estimate the value of tlic evidence which paleontology and embryology now furnish to prove that all hereditary characteristics, even tlie most fundamental, were originally individual variations. T h e series of hereditary structures and functions which makes up the life of an organism is constantly being extended by the addition of new features, which, nt first mere individual variations, are gradually built into tlic hereditary life history. I n this way newly acquired peculiarities are gradudly puslied furtlicr and fnrtlicr from what may be called the growing end of tlic series,
246
Bered ity.
by the addition of nemer lariations above tlicni. It can also be sliowii that from time to time tlie lxcnliaritics at the other end of the series, tlie oldest hereditary fcatures, are crowded out of the life of the organism, and droppcd, so that an animal ivliicl1 is high i n the scale of evolution does not repeat, in its own deielopment, all of the early steps through which its most remote ancestors have passed. The series of hereditary characteristics, thus growing at one end and fading away at the other, gradually raises the organism to new and higher stages of specialization, and its evolution by variation and heredity may be compared with the growth of a glacier. The slight individual differences are represented by the new layers of snow added by the storms to tlie deposit which fills tlie valley in- which tlie glacier arises. The snows wliicli are soon blown away are tlioso wrjations mhich, being of no use, soon disit1)par; wl~ilcthe snow which remains in the valley, and IS gridually conT-cited iiito ice, represents those individual diffwcnces which are seized u p m by natural selection, aiid gradually rcndered hereditary and constant. The long stream of ice stretching down to lower regions, and made np of tlie siiows of thousands of winters, receiring new additions at its upper end, and at the enme time melting 9way at its lower, is n o bad represcntation of tlie long series of hereditary features, once variations, which form so l u g e a part of every organism. If the glacier were not i n motion, but stationary, so that the melting of the oldest portion and the additions to its iipper elid ~ l i o u l dgradually carry the body of ice 1113 to Iiiglicr mid Iiiglier levels, we should have a cry perfect 1~:iiallclto the evolution of an organism by variation and heredity. The steps in this progress-arc embodied in a long series of individuals, each of which is, either immediately
T h e EEidence fi-om Intellectual Diferences. 247 or indirectly, the p i d u c t of a fertilized egg or seed, through which tlie laws of lieredity and lariation act, to bind the sepl.ute indi\.iduzlls into a progressive whole. The seeds and eggs w i t h wliich we are most familiar are liiglily contl)lic:ated, and consist of the protoplasmic germ, wliicli is intimately uiiited to a mass of food destined to be convcrted into protoplasm during development. The germ with its food forms the yolk of such an egg as t h t of the bird, and is surronnded by layers of albnmen, whicli are also used as food, and by a complicated series of investing membranes. It originates in a special organ, tlie ovary, and is incapable of perfect dexlopment until it lias been fertilized by the male reproduetive element. I n its earliest stage of growth i t is simply one of the cells or histological elements of tlie ovary, but as i t grows i t soon becomes very much Iargcr tlian an ordinary cell, and its protoplasm becomes filled with food material, aud the outer laycrs and walls are added t o it. In many animals the external enrelolws are wantitig, and the egg is simply a w r y large ovarian cell, filled . w i t h food material, and capable of developing, under tlie itiflnciice of the male element, into a new organism. I n still otlier animals the food-yolk is wanting, and tlie egg i s small, and does not differ fJom an ovarian cell; and i n still otlier animals the ovaries are lacking, :ind cells may become specialized as ova in various parts of the body. T h e series is so complete that we may be certain that we are comp:iring strictly homologous structures, and we may therefore conclude t h a t the egg is nothing but one of the cells of the body, mliicli may, ml~en actcd upon by tlie male clement, develop into a new organism, substantially like its parents, with some of t l ~ cindibidual pecnliaritics of cacli of them, and also with new peculiarities Qf its own.
248
Heredity.
From the necessity for impregnation
In
most cases, i t
l m bceii assumed tliat the essential fuiiction of the niale clement is to quiclien tlie germ, and thus start tlie process of development. It is true tliat i t does lime this fnnction in maiiy cases; bnt comparative study sliows that the egk itsclf is alive, and does not need quiclicning, and tliat this must be regarded as a secondary and derived function of the male elcnient, not tlie essential and primitive fniiction. T h a t tliis is tlie casc is shown by the fact that, while t h e earlier stages in tlie dcvcIopment:~I 1)roccss are saPficiently alike in different aninids to admit of a coniparison betwecn them, the stagc at wliich iniprcgnation takes place is not fixed, but variable. In sonic c:iscs tlie ovarian cgg remiins without change until i t is impregnatetl; aud tlie first step in tIic dcvelopmcntal process, tlic dis;qqmiraiicc of tlic germinative vesicle, is tlie imniediatc result of tlie union of the spermatozoa witli the oviim. In otlicr eases the germinative vesicle dieap~ : L I ’ S ,and tlie egg tlicii reniniiis inactive until i t is imprcgiiatcd; and this is followed at once by segnientntion. In other cases segmentation takes place mithont impreg-’ nation. Otlicr eggs develop still further; and, finally, tliere are many animals whose unfertilized eggs not only conimence but complete tlie developmental process, aiid give risc to adults which may in turn produce yo~ungin t h e snmc nap: and this may go on indefinitely, withont tlic intervention of a male. Tlie queen bee is ablc to lay fertilizcd or unfertilized eggs, and they are eclLially d i r e and c:tpablc of development. Tliesc f;icts sliow conclusively t h a t thc essential fnnction of the male element is riot the vitalizatioil of tlie germ. Turning now to snothei. aspect of our subjcct, we find
T h e Evidence from InteZZectuaZ Diferences. 245 tlixt among pI:ints, and among all tlic lower and simplcr groups of animals, new indiyidn:tls are produced by tlie T-arious forms of asexual generaticn, as well :is sexn:tlly. In ceitain anim:tls, such as tlie tnnicates, this form of generation is liiglily specialized, and the stolon from which ncw individuals are budded off is a liiglily complex structure, which contains cells or tissues derived from all the essential organs and systems o i the parent, a n d from these the corrcsponding organs and systems of tlie new individual :ire dcrired. As a rule, however, thc procesS of budding is very simple: a mass of tinspecialized cells a t some definite point upon llie body of t h e parent animal or plant becomiirg converted i n t o a new indiridnal, instead of contributing to the fuitlier growtli of tlic old. Among the lower animals, such as the lijdroids and sponges, the process is still more simple, a i d cells may become con\-erted into a bud a t almost any point upon tlic body of tlie parent. T h a t tlie process of reproduction by budding is not in any may absolutely distiiiguished from the process of ordinary growth by cell-multiplicatioii, is shown by tlic fact that an accident may determine mliicli of tliese processes is to resnlt from tlie actiyity of a giren ccll. Comparison shows tli:bt there is, on the one hand, no essentid distjnetion between ordinary growth and reproduction by budding, and, on the other liarid, nolie except tlic necescity for impregnation to distinguish asexual from sexual reproduction. All these poccsses are fundamcntally processes of cell-multiplicatioi7. As none of the animals with which we arc thoroughly f:iniiliar reprodnce asexnally, we are unable to make any rcry exact comparison of the rcsnlts of tlie two processes of reproduction in animals; but among plants siich comparison can’ be made without difficulty, and v i l l be found t o show
250
Heredity.
t h a t variation is much more marked and common in plants raised from fertilized seed than in those raised by budding. A marked bnd-variation is a very rare occurrence, but in many cases the tendency of plants reared from seeds to differ from the parents is so great that choice varieties are Iwopagated entirely by buds. I t is almost hopeless to attempt to propag:ite u chojce variety of grape or strawberry by seeds, as the individuals reared i n this way seldom hare the valuable qualities of their parents, and, although they may haTe new ynahties of equal or greater value, the cliaiices are of course greatly against this, since the possibiIity of iindesirabIe varintion is much greater than the chimce of a desirable sport. There is no difficulty, however, in perpetuating valuable varieties of these plants by asexnd reproduction. P u t t i n g together these Tarions propositions-that t h e evolution of life has bceu bronglit about tlirongh the combined action of the law of heredity and the lam of variation; that in all except the simplest organisms the process of sexual rcprodnction by o m which liave been acted upon by tlie mule element is m e t mitli; that the ovum is alive, and capable of cleveloprneiit in itself, and that the essential fiinction of the male element is something else than tlie vitalization of the ovum; tli:it tlie process of sexual reproduction differs from the process of asexual reproduction only in the occurrence of i m pregnation, while the result of the former process differs from the r e s d t of the latter i n its greater wri:hilitjwe seem warranted in concluding that t h e ol-um is t h e material medium through which the law of Iicredity manifests itself, while t h e male element is the vehicle by which new variations are added. T h e ovum is the conservative and the male element the progressire or variable factor in the process of evolution of the race as
MALE.
FEMALE.
MALE AND FEMALE RUFFED GROUSE.
YOUNG MALE.
ADULT MALE.
ADULT WMALE.
ADULT MALE, YOITNG MALE AND ADTTLT FEMALE OF THE RED HEADED WOODPECKER.
[From photographs of stufled specimens in the collection at Druid Hz Park, Baltimore.]
T h e Bvidencefrom InteZZectuaZ Diferences. 251 well as in tlie reprodnction of tlie individual. The adcqnutc st:itcnicnt of the evidence upon mliicli tliis generalization rests, or even a full statement of the gcneralization itself, with its qnalifications, would bc out of place liere, but the facts wliicli have been given seem to be snfficient to marrant its use as onc step in our :~rpnment in reg:ird to the relations of tlic sexes. From tliis as our basis we mill now trace tlie evolntion of sex. Among tlie lumest organisms, animal and vegetable, multiplication is usually by the various forms of asexual gener;ition, buddiiig or fission, or ccll-mnltiplicationan organism wliicli has by ordinary growth increased in size bcyoncl the limit of exact liarniony mitli its environmcnt, diliding i n this way into two, like each other as well as hlie tlieir parent. In tliis way the preservation of tlie established cliaracteristics of tlie species-licredity-is proxided for, bnt in order t h a t progress diould take place, by the preservation of favorable varieties, lariation must also be provided for. This is acconiplislied by the process mhicli is known as conjog;itioir : two protoplasmic organisms approach, come into contact, and a transfnsion or mixture of tlic semi-fluid contents of tlicir bodies takes place. T h e result of this process is tlie production of new individuals which, deriving their protoplasm from two parcnts wliicli are not exactly alike, are theniselves different from either of them, and have individual pecnliarities which are, i t is true, the resultant of the peculiarities of tlie parents, b~7twliicli are nevertheless new rariations. I n the simplest forms of conjngation the functions of both parents appear to be identical, b u t in organisms wliicli are a little more ~pecializedwe find male and female reproductive bodies, and tlic offspring is tlie result of the union of the male element of one individual with
252
Heredity.
the female element of anotlier; thnt is, we hare true sexnal reproduction i n its simlilest furm. Among tlie lower aninids :und most pl:lnts botli sexes are united i n the same individual, but tlie law of plivsiological division of labor, tlie principle t h a t an oi*ganor orgimisni, like a macliine, can do some one tliing better and with less expenditure of force when i t is specially adapted to this one thing than when i t is generally adapted for several functions, mould lead to tlie preservation by natural selection of any variations in tlie direction of a separation of tlie sexes, and me should tlierefore expect to find among t h e higher animals what we actually do find-the restriction of t h e male function to certain individuals, and tlie restriction of tlie female function to others. From this time forward tlie maIe is an organism spccialized for tlic poductioii of tlie mriable element in tlie reproductive process, xiid tlie female a n organism specialized for tlie prodnction of tlie conservative element. WC soon meet wit11 structural pecnliarities a d q h x l to aid and perfect tlie performance of tliesc respective functions; :tnd tlie various org:ms. habits, and instincts by mhicli, among the higher animals, tlie rearing of youag is providecl for, form O I I C of the most interesting cliapters of natural wience. On n priori gronnds wc should expect a still greater speci:iliz:ttion to make its appearance. Since the mnlc organism has for its function the production of the rarialslc reproductive clement, and since rariations mliicli originatc in :I m:dc liare tlicir perpetuation especially provided for, i t mould cle'wly be of admntnge tliat tlie mnlc org:iiiism shonltl : q u i r e a peculiar tendency t o vary, :tnd any steps i n this direction would accordingly be seized u p o n by natural selection and perpetnnted. Tlie female organism, on the other hand, having for its function tlie
The Eoidence from Intellectual Diferences. 253 transmission of tlic cstablished hcrcditary features of the slwcies, mc sliodd expcct the fcniale to gradnally acquire n t c n d c ~ ~ ctoy dcrclop thcsc general cliaractcristics niorc perfectly t1i:in tlic male. Tile m:ilc organism mould thus gidu:iliy bccome the variable organism, :is wcll as tlie transmit tcr of variations, and the female organism mould bccome the conservative organism, as well as the originator of the conscrrat~reclcmcnt in rcprodnction. Tile study of the liiglicr forms of life shows tliat this spcci:iliz:itiun lias :ictu;illy talicn 1d:icc in ninny cascs, :tnd tliat, in itc:irly all C:IECS i n wliicli tlic scxcs diffcr in pcculi:n itics not nctunlly conccrncd in rcprodnction, tlie male lias rai icd niorc t h n the fcninle. Tlic amoniit of xiriation mliicli any organism 11as 1;itcly undcrgonc may be lcnriictl i n two wn~s-by Q comparison of :dlied specics, and by n comparison of the adult with tlic young. In a gcnns wliicli compriscs several spccies tlic cliaracteristics wliicli tlicsc species liarc in common are dnc to heredity from n common ancestor, and tire therefore oldel. than fcntnres mliicli are confincd to any one species. Kow, i t is n mcll-knov\~n ornithological law tliat tlic fem d c s cf allied species of birds arc vcry much morc alike tlian the m:tles, and tliat iii soinc cases wliere the fcm:ilcs can 1i:irdly be clistingnishcd tlic males are very conspicnondy diEcren-so much so t h t there is not the least danger of confonnding them. C)ountless cxampks will prese~ittliemselrcs to m y one rvlio is at all familiar with birds, niid tliosc who are not can a t oncc find ample proof by g1:incing tliroiigh any illustratcd work on ornit hology- Gould's " IInm ming-B ird s, " for example. The gt'cater mriability of the rn:ile is also sliown by a compni.is*,n of the adult inale and female with tlic inimature birds of both sexes Since the growing animal tends to recapitulate, during its own development, the
254
Neredity.
changes tliroiigli wliicli its ancestors 11ni;c pnsscd, snbst;iiitially in the ordcr in wliich they first :ipl?e;tred, it folloivs tliat, in c;tses mlicre the sexes arc unlikc, the oric wliicli I S most different iron1 the young is the onc wliicli Iias varied. Now, i t is only necessary to comp:irc the ncarly fnll-grown young of our doriiestic fowls with the adult cock and lien, to perceivc t h a t tlie adult hen agrees with tlic young of both sexes in lacking such male characteristics as tlic liiglily ornameiited tail-feathers, the brilliant 1damnge, tlic distended comb, t h e s p r s , and tlic c:ip:ici ty to crow. Countless similar illustrations might bc given to show tlic great tendency of the male to wry, but t h e nbovc arc snfficieiit for the purposes of OUT argument. As both sescs usually retain the more gcncral spccific and generic clinracteristics, and are alike :IS f;w as tliesc :ire coiiccrncd, i t is a little more difficnlt t o show tlic conscri utivc coiistitution of tlie female than i t is to prom the inale tendency to vary. Amorig the B;trnncles tlicrc are a fern slmics the males and fcnialrs of which differ remarkably. The fetnalc is an ordinary barnwlc, with all the pecnliaritics of the group fully dcveloped, while tlic male is a small parasite upon tlie body of the femdc, and is so differcrit from the female of its own species, and from all ordinary barnuclrs, that no one mould ever recognjze, in t h e adult mile, any affinity whnterer to its closest allies. All of tlie hereditary race cliaracteristics are wanting: tlie limbs, digestive org:ins, and most of tlic mascks arid nerves hare disappcurcd, as they arc not needxi by a parasitic anin d ; aiid the male is little more than a rcprodnctire organ attached to the body of the female. I t is only mlieii tlie dcrelopmcnt of the male i s studied tliat we obtain any proof of its specific identity with the feinale. The young of both sexes are alike, and the d e d o p i n g
17Le Eoidence from Irdellectual Diferences. 265 m:tic shares with the female tlic cliaracteristics wIiicIi uuitc them to the otlicr b:trn:te1cs, and which are due to descent from a common foimi. ‘l‘hc femalc keeps tliese licred i tary cltuructcristics tlirougli life, wliilc the male soon loses tliein entirely. These facts seem to be sufficient t o prove that the specialization wliicli we slioiild cxpcct to find among the higher animals witli separatc sexes docs exist, atid tlliit t h e male orginism is espechlly and pcciiliarly mriable, and tlic female organism especially and peculiarly conservative. Leaying this aspect of our snbjcct for the present, let us look at i t from a somewhat diffcrcnt point of view. Tlic history of the evolution of life lias not only an objectire sidc, but something whicli may witli perfect propriety be spoken of as a subjcctive aspect. Tlie progress wliich is sliown objectively as grcater and grcater specialization of strncturc, atid a closer atid closer adapt:ition of tlic orgaiiisni to the conditions of tlic external world, lias been well described by IIcrbcrt Spencer, as the increasing delicacy, cxnctness, and scope of the ndjustrnent betmeeii intertial and external relations. Sccn in its subjcctive aspect, each of tlie steps in the growth of this adjnstnicnt is a recognition of a scientific law, t h e perception of the permmeticy of a relation betwccn external p1ienomeii:i ; for science is simply the recoguition of the order of natnre. TVlten a R1iizul)od discriminates bctrveen the contact of a largc body and that of a small one, atid draws in its pseudopodia arid slirinks into as compact a shape as possible ill’ order to escape tlie danger wliicli the past experience of tlic race lias diown to be related to t h e former sensation, or when i t expands its pseudopodia in order to ingulf and digest tlie body which has caused
256
Heredity.
t h e second sensation, it fnrnislies proof that its scientific edircation has begun. Of conrse I do not illtend to say that the ordcr of nature, according to which t h e Rhizopod adjnsts its actions, is conscionsly apl~relicnded, b u t simply that i t is the expcricnce of t h e existcnce of this ordcr mliich determines the action. Throtighout t h e whole conme of tho evolution of one of t h e higher or&anisms each variation mliich served to briitg about a closer harmony bctween the organism and its environment, and was accordingly presrrvcd by natnral selection, and addcd on to the series of hercditnrg struetiires and fonctions, was in its snbjective aspect the c x p r i ence of a new cxternal conuection, D new step in the recognition of natnral lam, an advance in scientific knowledge. 13uman advaiicenient is of course miclely different from the slow progress of t h e lower forms of life, b u t i t is fui~dnmcnt,allyt h e same. Experience is continu:~llyspreading o w r ncw fields, and bringing about a more wide and exact recognition of the persistent relations of the external world. T h e scientific laws thus recognized then gradually take t h e shape of principles or laws of condnct, accordi~igt o wliicli actions are determiiicd in those cases where cxperience lias shown t h a t they apply. Those laws of conduct mliich have becii long recogiiizccl gfiidaally asslime the shape of habits or intuitions, according to wliich conduct is :i1most nnconsciously regulated, and the habit finally becomes established as one of t h e hcrcditary characteristics of the race. We are a p t to confine oiir attention to tlic snbjectire side of human adrancement, and to neglect the strnctural side, and a t the samc timc to nrglcct the subjective side of t h e evolntion of the lower forms of life, and to confine our attcntion t G t h e strnctnral side, bnt of
The Eoidence froin Intellectuul Diferences. 257 course no one can donbt tliat a new linbit is represented by a new specialization of structure, and is transmitted, lilie any other pccnliarity, by liercclity. If this is so, :ind if the female orginism is t h e conservatire org;inism, to which is intrnstcd t h e lieeping of all that lias been g:tincd during the past history of t h e race, it mnst follow tliat the female mind is a storehouse filled with t h e instincts, habits, intnitions, and laws of conduct which hare been gained by past expcriciice. The male organism, on tlic contrary, being the Tarinble organism, the originating element in the process of evclution, t h e male mind mnst liave the power of extending experience over new ficids, and, by comparison and generalization, of discovering new Ia\rs of natnre, which are i n their t n r n to become rules of action, and t o be added on to t h e series of past experiences. Our examination of tlic origin and significance of tlie physiological differences bctween tlie sexes, and of the parts which tlicy have taken in the progrcss of the past, wonld therefore lead ns to expect certain profound :ind fandamental psychological differences, having tlic siinie importance ; and i t will be interesting to cxaniine ~vliat; these intellectud and ethical diffciciiccv arc, and how far expcriencs :~ndtlic common coiiseiit of niaiikiiid accord with tlie dcniands of our hypothesis. If, as me snppose, the cspecial and peculiar fnnction of the male mind is tlie expansion of onr circle of cxperience ; the morc cxact apprehension of :dl onr re1:itious to the external world ; t h e discovery of tlie 1:iw of thought, of society, of pliysiology, and of t h e matei.iiil universe, and of the bearing of tlicsc lams upoii individu:il condnct-it mill follow that men must excel women i n their power to discorer the manner in which a new cxteriial relation shall bc met and proyidetl for by a new
235
Heredity.
internal adjustment. In a case mhere our instincts, intuitions, feelings, or past experiences furnish no guide t o conduct, tlic judgment of a man as to the proper course of action will be of more value than the judgment of a woman. On tlie other hand, only a rery small proportion of our actions are directed to new conditions ; experience lias already determined the proper conduct in all the circumstances upon wliicli our preservation and wellbeing most directly depend; and action in tliesc circnnistances docs not demand conipnrison and judgment, while it must usnally be so prompt as to forbid deliberation or thought. The 3ower of quick and 1)roperaction J. in the innumerable exigencies of ordinary life, indepcndcnt of rcflcction, is nt least equally important with tlie power to extend our field of ratioiial action. By the former power we hold on to wliut has already been gained, while the latter power enables us to increase onr advantage in the struggle for existence, and t o widen our coiitrol over the laws of nature. Psycliological variation is tlie result of tlie latter power, psychological licredity tlic rcsult of the former, and psychological evolution and huninn progress the result of their combined action. If the female mind is especially ricli in the fruit of this pist experience, we should expect women to exccI men in the promptness and acciiracy with which the conduct of ordinary life is decided, and in the range of circumstances 0ve.r which this powcr of rational action witliout reflection extends ; that is, me should expect men to excel in judgment, women iii common sense. This important and fiindanie~italdifference between the male intelIect and the female must have a very great influence in clctermining tlie occupations or professions
T h e Eaiclence from In.tellectua1 Diferences. 259 i n which each sex is most liliely to succeed mlieii brought into f;iir competition with the other sex. T h e originating or progressive power of t h e male miiid is shown in its highest forms by tlie ability to pnrsue origind trains of abstract thought, t o reach t h e great generalizations of science, and to give rise to the new creations of poetry and art. Tlie capacity for work of this character is of course rery exceptional among men; and, although history shows tliat i t is nlniost exclusively confined to men, i t must- not enter into our conception of tlie ordinary male mind. The same l1ower of origin:tting and of gencrdizing from new experiences is possessed, in a lesser degree, however, by ordinary men, and gives them :In especial fitness for and an advantage over women i n those trades, professions, and occupations where competition is closest, and where marked success depends u1)on the union of the knowledge and skill shnred'by competitors, to the inwntij-mess or originality necessary to gain tlic advantage over them. Women, on the other hand, wonld seem to be better fitted for those occnpations where ready tact and wrsatility arc of more importance than the narrow technical skill which comes from apprenticeship or training, :und where success does not involve competition with rivals. The adequate examination of this aspect of our subject mould furnish material for a treatise, and i t is out of place herc, as all that is necessary for the purposes of our argnment a t present is to point o u t tlie difference, and to shorn that i t is the nccessary conseqnence of our Fiew of the manner in which sex has been c\olvcd: that i t is not due to the subjection of one sex by the other, but is the means by wliicli the progress of the race is t o be accomplished. Turning now to aiiotlicr part of our subject, and bear-
Heredity.
260 ~
ing in mind the fact that by far the greater 11 art of tlic external relations t o which our actions are :id justed, and t o wliicli i t is necessary that tlicy should conform, i n order to scciire our preservation, safety, and welfilrc, are fixed and definite, and liave been substantidly nncliangcd for almost, if not qnite, the wliole period of hnman developmcnt, we see a t once that, if tlie femalc mind is especiilllyricli in the past experiences of the mcc, sofar as these have resulted in lams of conduct, i t f o l l o ~ sthat, sincc these cxpcriences have been the same for :dl members of the race, there must be a greater nniformity in female cliarncter than in ninle character. As this statement is very abstract, I will try t o put i t in a less gcnera1 form: E q w i c n c e of the order of events has shown that undcr certain circumstances, of €rcqucii t occurrence, certain condiict is proper and condiicive to welfare, while its opposite is hnrtful. This expcricncc bcing constantly repented, the tendency to do the proper tliing when tlic circamstilnccs occur griidoally takes the slitlpe of an instinct, intnition, lithit, or law of duty. Henceforward, all persons who have tlic i n i p l s e wliicli lias thns been formed will act in the same way wlien the circumstmces arise, but two persons who liavc not tlie impulse mill follow their in&d i i i d jnilgnients, and may or may not act alike. As tlie fernale mind is c1iar:icterixed by tlic possession of these impulses, i t is plain t h a t i t milst be mucli more easy for one average w o m m to predict y h a t another arcrage woman will do, or feel, or tliinl~,or say i n any given case, tlitin for one average man to prcdict in the Fame w;iy of :wether averngc inan. We niny carry this line of tlionglit n little further. Since niale niitids liavc the elenicn t of originality, male charactem differ among tlicmselres; bnt, since all are
The Eaidence f r o m 3nteZZectual Diferences. 261 members of the same species, fundamental similnrity must underlie this individnd diversity, and tliis fanclamcntal similarity must subsist betmeen female and male cliaixcters also. The arernge fcmale character will tliercfure liare more resemblance to two or ~iioremale cliaractcrs tlian these lattcr will have to each other, and accordingly, i n all cases where relationship or educat'ion has not led two men into the same may of looking at things, a woman will be better able tlian either of them t o forcsee the condnct of the otlier nnder given circnmstances, and of conrse the adrantage of a woman o ~ e ar man in understanding the conduct of a woman will be still greater. Since, on t h e whole, the differences between male characters are slight wlien compared with tlicir resemblances, and since the points of rcsemblance are also points of resemblance to women, me shonld expect that, although the power of women to foresee male conduct is greater than the power of men to foresee fcmale conduct, t h e superiority is not so marked as i n the otlicr tlircc cases. This superiority of women i n predicting conduct mill be shown by their possession, to a much greater degree than men, of the power to influence or persn''1d e as distingnished from the p o m r to convince or niove by arguments; for to convince is to innovate and place matters in a new light, b u t tlie secret of influence is a Tivid appreciation of t h e established'motives and incentivcs to conduct. T h e relative power of persnasion of the two sexes, then, may be tabulated as follows: The power of Women Women Women Wamen
i
To foresee the conduct of or to i i ~ ence
I
Women Women
Elel
1
f
l
Men Men
El
u
1
To foresee the con- diict ~ of t or~ to ~influence
1
Men Women MenWomen
.
262
Heredity.
It seems hardly necessary to point out the fact that in cmes where sex is a motive and inflneiices tlie conduct directly, the law stated in this table does not hold. According to our hypothesis, the first line of the table sliould give the arrangement in mliich tlie difference is greatesl. In the next line the difference is less; still less in the next; and least of all in the last case. In all cases, however, tlie superiority of women in this respect should be very marked. Since our feelings arc necessarily mnch more numerous than our judgments, we should expect to find it much more easy to persnade either a nian or a woman than to convince; but, if our theory is correct, the advantage of influence over argunient sliould be mnch greater when a woman is to be moved than when the effort is directed to a man. Another difference between the sexes will at once be seen to follow from the above pmallel. Since male character has the variable element, and may m r y toward either good or bad, it follows that the ideally perfect male character will be more hind to define and more seldom realized than the ideal female c11:iracter. I t is difficnlt to prove such a statement as this, for the sentiments upon which individual opinion of the sul#xt is based hardly admit of exact statement, but that there is an accepted standard of female excellence, and that the women w ~ i orealizeft are not rare exceptions, can, I think, be shown by the study of fcmde character as depicted by dramatists, novelists and poets. An appeal t o this test is uiifavorablc t o our hypothesis, for charnctcrs arc selected for novels or poems on account of their origiii:ility; bnt I tliink that any one wlio will review Sllilkespeare, Tliackeray or George Eliot with the subject in mind, and who will compare the more important female characters, will fiiid that they might be trans-
The Eaidence f r o m 3nteZZectuaZ Di'erences.
261
members of the same species, fnndamental siniilnrity must underlie this individnal diversity, and tliis fandamcntal similarity must subsist betmeen female and male cliai*:tcters also. The arernge fcmale character will tliercfure liare more resemblance to two or more male cliaractcrs than these latter will have to each otlier, and accordingly, i n all cases where relationship or educat'ion has not led two men into the same may of looking at things, a woman will be better able than either of them to foresee the conduct of the otlier tinder given circnmstances, and of conrse the advantage of a woman o ~ e ra man in understanding the conduct of a woman will be still greater. Since, on the whole, t h e differences betmeen male characters are slight wllen compared with tlicir resemblances, and since the points of reseniblance are also points of resemblance to women, me shonld expect that, altliougli the pomer of women to foresee male conduct is greater than tlie power of men to foresee fcmale conduct, t h e superiority is not so marked as i n the otlicr tlirce cases. This superiority of women i n predicting conduct mill be shomn by their possession, to a much greater degree than men, of the power to influence or 1mwi:tde ;is distinguished from the p o m r to convince or niove by arguments; for to convince is to innovate and place matters in a new light, b u t tlie secret of influence is a Tivid appreciation of t h e establjslied'niotives and incentives to conduct. The relative power of persnasion of the two sexes, then, may be tabulated as follows: The power of Women Women Women Women
1
1
To foresee the con- Is greater than To foresee the conduct of or to illflu- the power of diict of or to influence ence Women Women
g~g
I
Men Men
$2
1
Men Women Men-
Women
.
264
Beredlty.
differences between the'sexes whicli the study of tlie evolntion of orgrnnisnis wonld lead us to expect. I sl1:11[ now quote afew extracts from authors whose mritings npnn the position of women are accepted as val tiaLle contributions to our knowledge of tlie subject, in order t o shorn that they ham recognized the cxistence of tlie very differcnces wlijch we have been led, by theoretical reasoning, to expect. Mill's essay on " The Subjection of Woman" must be regarded as the most important contribution to the discussion of the rcliitirc positions of tlie sexes as rcl;~tiug to future progress; and it is interesting to note that, wliile he holds that the exiating differences arc not natural, but are d m to the suLjection of one FCX by tlie other, lie fully recognizes certain profonnd and c11:iracteristic differences, which are p;ccisely i n accordance with tlic present Tiem of their origin and piirpose. Mill's evidence as to important diffcrenccs between the scxcs is of the greatest value, 110th on account of the weigli t of his opinion in itself, and on ncconn t of his bcing in this case an unwilling witness. IIe says: '' Looliing at women as they are knomn in c ~ p w i e n c e ,it may be said of them, with more triith than belongs to most generalizations 011 the subject, that the generid bent of their talents is toward the practical. This statement is conformable to all the public history of wonicn in the present and in the past. It is no less borne out by common and daily experience. Let us consider tlic special nature of the mental capacities most clisrncteristic of a wom:in of talent. They are all of s kind whicli fits tlicm for practice, and makes them tend toward it. What is meant by n woman's capacity of intnitire perception? It means :Lrapid and correct insight into present facts. It has nothing to d o with pmcrnl principles.
The EEidence from Intellectual Difemices. 265 Nobody ewi* percrjwd a scicntific law of iiatnrc by intuition, or arrived at a gcncral rule of duty or prudence by it. Tliesc are results of slow and careful collection and comparison of experience; and neithcr t h men nor tlic women of intuition usudlgsliine in this dcpartmcnt, unless, indeed, the experience is such as they can acquire by themsclres. . T o discover general principles bclongs to tlic cpecnlative faculty; t o discern and discriminate the lmrticular cases in which they are o r arc not applicable constitute practical talent; and for this ~vorncn,as they now are, have a peculiar aptitnde.” It is only ncccssnry to change two or thrcc words in this last sentence in order to show its complctc ,agreement with tho demands of oar theory. Its meaning will not be altcrcd by the Eollowjng reading, which serves to bring out move clearly its implications: To discorer gericrd principles bclongs to the progressive aspect of the mind, n.liich i s most strongly developed in men; t o prcserre and apply the general principles which are already estnblkhed bclong to tlic conservahive side of tlic mind, a n d for this n-omen, as they liave bcen made by the TOlntion O€ the race, liarc and sliould have a pcculiar aptitude. Mill continues ns follows: (‘I admit t h a t there can be no good pmctice mithont principles, and that the predominan t place whicli quiclrriess of observation holds aniong a woman’s facnlties makes her particularly a p t t o build over-hasty generalizations upon hcr own observation, thongh a t the same time no less ready in rectifying these generalizations as her observation takes a wider range. B u t the corrective t o this defect is acceps t o the experience of the hnninn race; general knomledgc-cxactlg tlie thing wIiich edncntion can best supply.’’ This sentence, when viewed in connection with our present theory of tlic relations of t h c sexes, gives the Bey
. .
266
Beered ity.
to the question of female education-for that form of education mliicli supplies the general kiiomlcclge wliich is so i m p o p n t for tlie correct application of principles t o special cases is culture, as distinguished from tlie technical training wliich looks to the discovcry of new laws. The nest passage ivliicli I slinll quote is of the greatest importance, for, founded as Mill’s autobiography and nnmerons passages in his various works tell us it is, upon the personal experience of his life, it coiitains the germ of tho idea wliich, if fully investigated, might have led him to entirely remodel liis css;iy upon women; the idea that the sexes do not naturully stand in the relation of superior and inferior, nor in t h t of indcpcndent equals, but are tlic complemental parts of a compound whole. H e says: “Tliis grinitation of women’s minds to the present, to tlic red, to actual fact, while in its exclusiveness it is a source of errors, is also a most useful counteractive of the contrary error. The principal and most ell aracteris tic nberrn t i on of speculative mi nds, as such, consists precisely i n the ilcficicncy of this lively perception and ever-present scnse of objective fact. . H:~rcJly ariytliing can be of greater valuc to a man of theory and speculation, mho employs himself, not i n collecting materials of knowledge by observation, but in working them up by processes of tliouglit into conipreliensive truths of science and laws of conduct, than to carry on his speculations in the compnnionsliip and under the criticism, of a really superior womm. There is nothing comparable to i t for keeping his tlionghts witliin the limits of real things, and the actual facts of nnture. Women’s tlionghts are thus as useful in giving reality to those of thinking men as men’s thoughts in giving width :md largeness to those of women.” IIere we liave a clear recognition of the law that width and largeness, mentd
. .
The Evidence from Intellectual Diflerences. 267 growth, originate in tlie male, and are tlien prcscrvcd by women, and the context lcavcs 110 rooni to doubt that t h e '' really superior worn:iii" wliicli fillcd tlic author's memory ;it tlie tinic this pissage wiis mrittcu, was :I woman i n whom this feminine characteristic was well developed; tliiit slic mas a wornmi fillcd with tlic frnits of humin ca~~ei~iencc; and i t is a little strange tliat lie fails t o see that tlic relation with which, for :t man of spcculation, tlici*c is iiotliing cotn1xuxbIc, may Iiavc a wiclcr valuc, and be of the greatest importance to humanity as % WIlOlC.
The next pissage wliicli I sliall quote is still more to t h e point. Hie s a p : '' Lct us now consiclcr anotiier of t h e admitted su1)eriorities of clcvcr women, greater quickness of ap~)rcliensioti. Is this not ~ ~ r c - e r n j i i c ~a~ t l y quality whicli fits :I 1msoii for practicc? In action cvcrything depcncls upon deciding promptly. In speculation nothing does. A mere thinker can w i t , can take time to consider, can collect :tdtlitiond evidence; he is not obliged to complete his pliilosopliy a t once lest the 011portunity should go by. The power of d ~ n w i n gthe best conclusion possible from insufficient data is not, indeed, useless in philosophy; the construction of :I provisional hjpotlicsis consistent with a11 known facts is oftcn tlie needful basis for further inqujry. Btit this faculty is rather scrviceablc in philosopliy than tlie main qualification for it; and for tlie auxiliary ns well as for the main question the philosopher can allow liimscl€ any time hc plcases. He is in n o ncccl of doing lapidly wliat he docs; wliat lie rat,licr ncccls is p:iticncc t o work on slowly until imperfcct lights h a w bccornc pcrfcct, and a con jcctaro has ripcned i n t o a thcorcm. For tll(JSc, on tlic conti'ary, wliose busincss is with the fngitire and pcrisliable-with individual facts, not kinds of facts-rapidity of thought
268
Heredity.
is a qLtdificatioii ncxt only i n importmcc to the powcr of tlioiiglit itsclf. 1Ic wlio lias not his f;iculties uiidcr imnicdi;itc ci~nimand in the contingcncics of action might as wcll iiot 1i:tre tliem a t all. I-Ic niay bc fit to criticisc, but lie is iiot fit to act. Now i t is in this tliat women, and tlic Inen who arc most lilic woiiicn, confessedly excel. Tlic other soit of man, Iiowevcr 1)rc-eminent may be his faculties, :uriws slomly a t conildete commaiid of tlicm; rapidity of jitdgment and 13roniptitndc of judicious action, cvcn i n the things lie knows best, arc the g r d i i a l and late result of strennous cffort grown into liabit.” I 1i:we quoted tliesc I X L S S ~ ~ Cfrom S IIill at Icngtli, as tlicy give ;I very cle:ir altlioiigli soine\rliat ii:irrow statenieiit, by tlic strongest :idvocate of tlic fand:inicntd likciicss of the scxcs, of what I t:ihc to be tlic most iinport:in t pspcliological cl iffewlice b c t w c n tlieni. Accordiiig to Mill-mid I tliiiili that iiiiivcrs:il cspcriencc wi!l justify his view-tlic liiglicst t j p e of wonim is distinguished by Iicr po\vcr or iiitnition, by Iicr concrete acqn:iiiit:incc with tlic 1;i~vs:ind l)riuciples mliicli 1i:irc been cst:tbhshcd by cxpcricncc mid gciicr,ifiz:itioii, by :t constitntioiiiil 1;nowlcdge of tlicsc ILLW wliicli anlotints to habit, so that slic is :~blc:to rccognizc in actual 1mctic:il life tlic action mliich is lyoper i n :my gircn cast, mitho u t the necessity for a slow process of comparison and tliouglit; by that immediate command of tlic faculties wliicli is necessary for action. This poiver of correctly and Iiromptly applying tlie eskiblislicd scientific laws, wliicli :ire tlic rcsnlt of all tlie expcriencc of the p s t , to the actions of ordin:try p r x t i c a l life, is comnion sciisc, :is distingiiislied from originality. Tlic highest tj-pc of male intelligence, on the other hand, is distinguished by the power to abstract and com-
T h e Ecidemc fI-om Iniellectual Diferences. 269 1) ire, :tiid by a doiv process of thought to rencli new gcuer,iliz;itions and I;LWS, and to see thcse in their abstract :ind ii1e;il form, freed from all the complications of tlicir conci~ctc maniI'estations. To this power is often joined it wofnl and disastrous lnck of common sense, or power of p r om pt imd p r o l ~ c rdecision and action i n special cases. Lecky, in liis " IIistory of Enropean Morals," gives an e.iccllcnt sumnmry of tho most marked differences betwccri the m i l e mind and tlic fcmalc; find, altliongli we d o n o t agree w i t h him in tliinking that :t cleparturc from tlic mule t j p e is i n all c:i.;cs to be regardcd as inferiority, n e c;innot f a i l to note liow esactly his accouiit agrees i b i t l i the dem:unds of onr ligothcsis. He s;iFs: '' Intellcct~ially;I certain infcriority of t h e fcmulc sex can 1i;irdly bo clcnicd \\lien wc remember how almost cxclnsiicly tlic foremost 1)laces in every dcpartm c n t of scicnce, litci;iturc, aiicl :irt liave beeii occupied by mcn; horn infiiiitcsinially small is the nuniber of \YOmcii who liarc shown i n any form the veryliigliest order of gcnins; horn m:my of the greatest men liare achieved tlicir greatness in dcfiancc of the most adverse circomstances, ant1 Iiow completely women h a w failed in obt:tining tlic first position, crcn in music and painting, fur tlic cnltirntion of wvbicli their circnmstanccs would : i p p i r most propitious. It is as impossible to find a fcnidc R;iplinel or a fcmxlc Handcl xs a female Sliakes p c ~ r cor a fcnxilc Neu-ton. Woincn arc intcl1cctu:illy more desultory aiid volatile tliaii mcn; they are more occnpicd with practic;il instnnccs than with general p i n ciplcs; they judge ratlicr by intiiitive pcrccption than by clcliber,ite reasoning or past cxperieuce. They are, however, usually superior to men in iiiiiibleness and rapidity of tliouglit, and in the gift of tact, t h e power of seizing rapidly and faithfully tlic Encr impulses of feeling, and
270
Heredity.
they have tliercfore often attaincd very great eminence as conversationalists, as actresses, and as novelists. I n tlie etliics of intellect they are decidedly inferior. Women very rarely love trnth, though they love passionately wliat they call ‘ the truth,’ or opinions they have received from others. They are little capabIe of impartiality or of doubt; their thinking is chiefly a mode of feeling; though very gcnerons in tlieir acts, they are rarely generous i n tlieir opinions, and their leaning is naturally to the side of restriction. They persuade ratlier than convince, and value belief rather as a source of conso1;itioii tlian as a faithful espression of tlic reality of things. They :ire less capable than men of distiiignishing tlie person$ cliaractcr of an oppoiicnt from the opiniims be maiatni~~s.!l’J,cjraffcctioiis are concc’ntmtcd rather on lcadcrs than on causes, and if they care for a great cause it is generally bccauae it is rcpresentcd by n great nian, or coniicctcd with some one whom they love. I n politics their cntlinsinsni is more naturally loyalty tlian patriotism. In benevolence t h q excel in cliarity rather than in pliilanthropy.” Wliilc I cniinot believe tliat Leekj’s statement is entirely n ~ i ~ ~ r e j i ~ d i Ic cthink d, 110 one will deny tlint tlie views wliicli I liave qnoted ngrec in tlic main with those wliicli l i n w gained general acceptance in the past. At tlie present time, however, therc is a growing tendency to regard the relations of the seses ns due in great part to male ~clfisliness;and while tlie snbstmtinl correctness of our view of the differences bctwecn the male and the female clinracter is acknowledged, its origin is attributed to the “snbjection” of women by men. I n this paper I have attempted to prcsent reasons, which I believe are new, for regarding t h e differences as natural and of tlie greatest importance t o t h e race.
me Evidence from Irhtellectual Diferences.
271
Those who acknowledge the weight of my argument, as applied to evolution in the past, may, hobever, question its applicability to the human evolution of the futnre. It may fairly be urged that while we grant that the course of evolution from the lower forms of life up to rational man has been by the slow process of variation and heredity, we have now passed into a new order of things, and the great advances of the human race have been and now are brought aboutby the much more rapid and totally dissiniilar proccss of intelligent education. It may be urged that heredity does very little more for the civilized than for the savage child, and that thewide difference between the savage a i d the civilized adtrlt is mainly the result of the training and instruction of tlie individual; that it has not been brought abont by tlie destruction of those children whose congenital share in the results of the intellectual advancement of the race is most scanty. It may be urged that, since man has reached a point where progrcss is almost entirely intellectual, and depends upon his own efforts, he is free from the lams by which development up to that point was reached. We are not concerned at present with the question how far progrcss might be accelerated hy intelligent selection, and we may therefore conditionally accept the view that future progress, for some time to come a t any rate, must depend almost entirely upon education; but, far from holding that this conclusion will allow us to ignore or obliterate the differences between the male and the female intellect, I believe that the full significance of these differences can be appreciated only in their relation t o higher edncation. The scope of the present paper will only allow the space for an outline sketch of the reasons for this belief. As the field of human knowl-
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Heredity.
edge widens in all directions, as society becomes more complex, and as the points of contact between man i ~ n d his inorganic environment multiply, tlie arnoiiiit of p i eral education which each individual mnst receive before he is in a position to hold his own, and to guide himself rationally in all tlie emergencies of life, and to enjoy his share of the benefits wliich oiir intellectual advancemeii t has placed within his reach, increases in a geomctricd progression, and the amount of time demanded for gcneral liberal education increases in the same ratio. Meanwhile the amount of special preliminary training which must be undergone in order to fit a person for new and original work in any department of knowledge or art increases a t the same rate, and makes greater and greater inroads upon the time which is iieeded for general education. At present the most important, delicate, and complicated of educational problems, the problem ~ ~ l i i ceach li individual mnst meet and decide upon, and the problem which engrosses most of tlie thought of edncational bodies, is where to draw the line between general culture and practical or technical training. Culture in its midest sense is, I take it, thorough acquaintance with all the old and new results of intellectual activity in all departments of knowledge, so far as. they conduce to welfare, t o correct living, and to rational conduct; that is, cuItnre is to tlie iiitellectnal man what heredity has been t o the phjsicxl man. Cnlture is concerned only with results, not with demonstmtions, and it does not look to new adrances; w.bile tcchnical training is concerned with metliods and proof.; and it values the results of the methods and invcstigations of the past only as they contribute to new adnncez. Technical training looks to progress i n some one definite line, one radius of the growing circle of the domain of
The Euidence-from InteZlectuaZ Diferences. 233 linman intelligence, and ignores the rcst of tlic circunifcxrcnce. I t is to tlic intellectual man mliat variation is to tlie 1)liysical nxin. By culturc we liold our own, and by teclinical training we advance to higher I P W ~ S . Both arc equally important to 1ium:tn welfilre, and tlie grcat problem of tlic future is liow to secure eacli to the greatest degree without sacrificing tlie otlier. Tlie analogy of tlic rest of tlic organic worhl mould sceni to indicate that this is to bc accomplished by “ division of 1:ibor.” If tlie fcniale mind has gained during its evolution an especial aptness for acquiring and appljing tlic results of pnst progress, by an empirical method and without the necessity for studying proofs and reasons, i t would seem especially fitted for culture, as distinct from training, mliilc the male mind is best fitted for education by tliat process of inductive training by demonstration aud experiment mliicli lends to new advances. Tlie methods entployed in tlie general instruction of young nieii mid young women should not tlicrefore be idcntical. With t h e one the field may be very wide and tlie methods empiricnl, and with tlie otlier tlic range niorc 1i:trrom and tlie mctliods more strictly logical. I n tliis way each type of mind will be developed in tlie nianner for wliicli i t lins an especial fitiicss; atid we liave the strongest grounds for the belief that tliis method would also gradally result i n tlie extension of tliat congenital ncquaintance with nature wliicli is the common stock of tlie race, :tnd would thus lcavc more time for tlie special training of tliosc minds mliicli are by naturc best fitted to receive it. I t is unavoidable tliat a bald outline of :L view mliich lias such wide implications should afford many openings for serious criticism; b a t tlic present article does not admit of the expansion of the idea, even if its detailed examination could be fairly iiicluded in the province of
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biology. I-Iaving traced the origin and significancc of sex from its lowest manifestations to a point wlierc it beconies p r e l y intellectual, thc biologist niay fairly leave tlic subject in the hands of the psyhologist. When this chapter was printed, several scars ago, I was told by several teachers of great experience in tlie cducation of both boys and girls that tlicir observations showed no coilstant difference in thc intellectual powers of the two sexes. They therefore disputed the accuracy of my view. Taking the cliapter alone, this is, 110 donbt, a fair criticism ; but I believe that any reader who mill examine the subject in connection with tlie other chapters of this book, as a part of tlie whole, and not as an isolated essay, will 1)erceive that we should not expect tlie intellcctual differcnces bet\vccn men and women to be so well mitrkcd and conspicuous during childhood as they become after maturity is reached. Tile enbjcct is such a frnitfnl source of controwrsy that I c:iu Iiardly liope to escape adverse criticism, aiid I can only say that I have not approached it in a spirit of controwrsy, and -sliall gladly welcome any discussion which leads to the discovcry of truth. The acceptance of my view should put an end to all discussion as to the relative intellcctnal rank of meii and women; for if the two sexes contribute in different wags to the welfare, of tlic race, and fill eyiially important but dissimilar places, there can be no question as to relative superiority or inferiority.
CHAPTER XI. THE THEORY OF HEREDITY CONSIDERED AS SUPPLEMEN.
TARY TO THE THEORY OF NATURAL SELECTION.
Darwin believes that variations are purely fortuitous-Natural selection c;iniiot give rise to permanent race modifications 1111less many individuals vary in neaily the Same way, at about the snmc time-The cliarices against this are very great if variations are fortuitous-Argument from North British Review-Darwin acknowledges the great weight of this objection -It is removed by tlie theory of heredity-The co-ordinated modification of complicated organs-The time demanded by D.irwiu practically iufiuite-Murpliy’s argument froin the complexity of the eye-flerbert Spencer‘s illustratiou-Our tlicory removes this difficiiIty-Mr. Couii’s objectiou-Sjal tatory evolution-Evidence tlint it occurs-Spike horn buckAncon and Mauchamp slieep-Blacli-slioiildered peacorkThe theory of heredity accounts for saltntory evolutionPiirnllel vari;itiou-Evidence of its occmrrence-Evolution of the meclus-General and special Homologies.
According to Danvin’s view, variations, thongh determined by definite causes (for the most part nnknown), arc, so far as their usefnlness to the orgmism goes, fortnitous, aiid lie makes use of the following illnstration t o explain his conception: “ I have spoken of &electionas tlie paramount power, yet i t s action nbsolntely depends upon what wc in o m ignorance call spontaneous or accidental variability. Let an architect be compelled t o build an edifice with uncut stones, fallen from a precipice. The shape of each fragment may be called accidental, yet the shape of each has been determined by tlie force of gravity, the
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Heredity.
nntnre of tlic rock, a n d tlic slope of tlic pi~ecipicccircumstances a11 of which depend on i i i i t t i i n l l a w ; Init tlicrc is 110 rc1:ttion bctwccii tlirsc 1:i~rs: i i i c l the p-,i~yosefor wliiclr e:icli friignicnt is uscd Ly tlic builder. 111 tlie smie niaiincr the v:iriutions of r:icli cre:hire arc dctcrmiiicd by fiscd and immntiible laws; b u t tlicse bear n o relation to tlie l i v i n g structure wliicli is slowly built u p by tlie power of sclection, wlietlicr this be n;itur:ll or artificial sc1ection.” “If our nrcliitect succeeded in rcaring a noble cdifice, using the rongli mcdge-sllnped fragments for t h e arclics, t h e longer stones for thc lintcls, and so forth, we should admire his skill even in a higher dcgree tlian if lie 11acl used stones sh:ipcd for the piirpose. Soit is with selcction, mlietlier a1y)licd by man or by n:iture; for tlioiigli rai.i:ibility is indispntably ncccssary, Set when we look :it some liiglily complex and csccllcntly :id:iptcd org:iiiisni, T ari:tbility sinks to a quite subordiuutc position i n cotnlwison with selection, in tlic same m:inner as the sliape of r;rcli fragment used by giir.sn1yoseCd architect is uninipii tarit i n comparison wit11 111sshill” ( TTwintioiz, xsi. 11. 301). It, is qiiitc 13o;siblc that Darwin niny be riglit iii :ittri bn ti ng the mod I ficat ion :in d ad ;I])t at ion of o i g t i i i > m s almost cntircly to the inflncnce of n:itni.:iI wlcrtion, and, at the s:mc time, wrong i n 11;sbelicf 11~:ih the r-a& ations arc fortnitons. Scicri~lcritics 11:irc pointrd vrrt tliat if i t is true tliat variations linrc 110 reliltion w1i:ite ~ c to r tlie needs of the organism, t l i c i ~:ire grarc ilifliculties i n t h e w:iy of ii:itnr;tl selection: b u t the t1icoi.y rests upon too firm a b:isis to be e;isily w t aeitle, i t t i t 1 thesc objections hnc-c 1i;trclly received t h e nttcntion which they fkiirly ~ C E C ~ T Cfor , those :nitliors wlio 11:i~c poiiitccl tliein out haw, a t tlic same time, attacked tlic general theory in a liostile spirit without proposing anj-
ercuts aiid
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thing to take its place. This has iiot prevented Darwin hintself from pcrcciviiig the meiglit of tlic criticism, but i t lias certitinly caused tlic objcctioris to be jgnorcd or ovcrloolicd by otlier less candid writers. ATCtttral selectioz cnimot act .zsiikss m a n y indiuidua2 MIYJ toyether. Oiic of tlie most scrious objections t o Darwin’s theory is bused u p o n tlic fact tli;tt wliile natural selection requires that great numbers of iiidividoals sliall vary iii esseutially the same way at nearly tlie same time, tlic cliancc ilgtilist this, if variations arc fortuitous in Darwin’s sense, is great bcyoiicl d l conipiitatioti. 1111864 the wiitcr of wltnt Darwin terms < ’ a n able and valunblc articlc” i n the A W h BritisJ~Review, c:tllcd attention to tlic fact tlmt, according to tlic I:LW of cIianccs, slight variations, homever iiscfiil, mill tend t o be obliterated, instead of lmyetnatcd, by natural sclcctioii, iinless tiicy simnltancously appear in a g1’citt number of indi\idnals. Unless we can sliow that tlie c:iLises of variability act in siicli a way as to affect tnany individoals at the same time. and cause the same part to vary in all of them, we must regard this as a very serious objcctioii to the tlieory of 1iiLtLir;Ll selcction, and Darwin liimself acknowledgcs ( Oriyiit of Species, p. ‘72) that tlie justice of this objection cnnnot be disputed. IIe admits in the later editions of the Origin of Species, p. 71, tliat until readiug the able a i d valuable article i n the NortJh Britislh Revieza, he did n o t appreciate how rarely single variations, wlietlier slight or strongly marked, mould be perpetuated. Tlie revicwer points out that i t is difficult t o see how a species can be changed by the survival of the descendants of a few individuals yhich possess some favorable
278
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Neredity.
variation, even when tlie variation is of tlie very greatest advantage to its possessor; and tliat this difficdtg is very much greater when as must nsu:tlly be the case, the advantage p i n e d is very blight. He sags: " The advantage, ahatcrer i t may be, is ntterly out-balanced by numerical inferiority. A million creatures are born; ten thousand survive to produce offspring. One of the million has twice as good a cliaiicc as any otlier of surviving; but tlic cliances iirc fifty to one against the gifted indisidual beiiig onc of tlic ] i n n dred surriyors. No doubt tlic chiices a m tn-icc as great against any one other individual, but this does n o t prcvent their being enormously in favor of some average individiial. Hon.erer diglit the adr;uit:tgc m-y be, if i t is shared by lialf thc individu:tls produced, it will 1wub:ibly be prcsent in at least fifty-one of tlic kiir\-iyors, and in a larger proportion of tlieir offspring; btit tlie chances are against the preservation of any onc 'sport' (i.c., sudden marked rariation) in a numerous tribe. Tlic \-ague use of an imperfectly understood doctrine of cliancc lias led Darwinian supporters, Erst, to confiisc tlic two cases above distinguished; and, secondly, to imagine that a very slight balance in favor of some indiiidual sport must tend to its perpetuation. All that caii bo said is tliat in the above example the favored sport would be preserved once in fifty times. Let us consider what mill be its influence on the main stock when preserved. I t will breed and liare a progeny of say 100; now tliis pogeny will, on the wliole, be intermediate between tlic average individu:tl and the sport. The odds in fayor of one of this generation of tlic new breed will be, say, one and :i lialf to one as compared with the aver:tgc inclividuul; tlic odds in their favor will, thercfore, be less than t h t of tlieir parents; but, owing to their greater number, tlie chances
Heredity and Natural Selecticp.
279
arc that about one and a half of tlicm would survive. Unlcss these breed together, a most improbable e m i t , tlicir progeny would again approacli the average individual; tlicrc would,Le 150 of tlicm, and thcir superiority would be, say, i n the ratio of one and a quarter to one; tlie probability w o ~ i l d now hc that nearly two of them wonld survive and have 200 children with an eighth superiority. Ratlicr more than two of tlicse mould snrvive, but tlie superiority would again dmindle, until after a few gener:itions it'would no longer be obscrred, and mould count for no more in the struggle for life than any of the hundred trifling advantages whicli O C C I I ~ in tlie ordinary organs. An illustration will bring this conception home. Snppose a white miin to have been wrecked on an island inliabited by negroes, aiid to have establishcd himself in friendly relations with a poverful tribe, whose customs he has lcxrncd. Suppose him to possess tlie physical strength, energy and ability of a dominant white race, and Ict the food and climnte of tlie island suit his constitution; grant him every advantage mliicli we can conceive a white to posS ~ S Sover tlic iiatiye; concede that in the struggle for existence his clinrice of a long life will he much superior to tliat of the native chiefs; yet from all these admissions there does not follow the conclusion that, after a limited OT unlimited number of generations, the inhabitants of the island mill be white. Onr shipwrecked hero wonld p ~ o b ~ l , become lg king; he woold kill a great mauy blacks in the struggle for existence; he monld h a ~ ae great many wives :ind children. I n the first Sener;itiw there will be some dozens of jntelljgent J * O U J J ~ mulattoes, much superior in average intelligence to the negroes. We might expect the throne for some generations to be occupied by a more OF less yellow king; but
gercdit y.
280 ~
can any oiie believe that tlie wliolc islaiid \rill gradually acqiiirc L: white or evxi a yel!om popnlatioii? “D3irwiii says that in t h e struggle for life a grain may torii tlie balance i n favor of a giveii strtictiire, wliicli will then be lmxmGi. But one of the weights in the aailc of natnre is due to tlie nurnlcr of n giren tribc. Let tlierc he 7000 A’s and 7000 E’P, representing two rarietics of it gircn animal, and let all tlie B‘s, in virtue of n diglit difference of strncture, l i n tlic ~ better cliance of life by a T&T part. We must allow that there is a sliglitp~ob:~bility that tlic descendants of B will supplant t h e cicscendnnts of A; but let thcre be only 7001 A’s :tg:iinst 7000 B’s a t first, oiid tlic cliaiices are once more eqiial, vhile if there be 7002 A’s t o start, tlie odds wonld be h i d on the A’s. Tme, t l q st:ind a g i m t e r cliancc of being liilled, but tlicn they can better afford to bc killcd. The grain will oiily turn tlie scales when tliesc :ire .;err nicely b:ilmiced, aiid an ad\ anlage i n nnmbrrs counts for ivciglit, ercn as an adr-mitage in structnre. As the numbers of t h e favored .i-:iricty diminish, so ninst its rc1:itire adrantagcs inelrase, if tlic cliance of its existence is to suqass the chance of itsextinction. until hardly any conceivable adv:uit:ige would enable tlic descendants of a single pair t o externiinnte tlic dcsccnclatits of many thousands, if they and their descend:mts are supposed to breed freely with the inferior rariety, and so gradually lose tlieir asccndancy. ” Darwin acknowledges t h a t t h e justice of thesc remarks cannot hc dispnted, and there is 110 cscnpe from t h c conclusion tliat if variations do not appear fimultaneonsly i n n gi‘eat nnniber of Iiidividnals, the theory of niitural srlcction fails t o explain the origin of epcico. B u t tlie tlieory itself is so firmly estabished by otlicr
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facts, that thc logical conclusion seems to be, not that natural selection is a t fault, but that Darwin’s opinion, that variations are fortuitous, is an error. According to our view of the cause of variation, it is plain that a change in tlie environment, affecting many individuals of a species in the same wuy, will cause, in succeeding generations, variation of the same cells in all or nearly all of them. I t is also clear that since a change in one cell of an organism will disturb the harmonious adjustmcnt of all adjaccnt or related cells, any variation which makes its appea,rance will become more marked instcad of bcing obliterated, in the o8spring of successive gcnerations. I think it is clcar, without further discussion, that our theory of heredity entirely does away with this very serious difficnlty, and furnislies a firmer basis for the theory of natural selection. I t is also clear that this cannot be said of the Pangenesis hypothesis, or of any other hypothesis which has been proposed.
The Forination of Complicated Organs by the Xiatural Selection of Fortuitous Vuriations demands U?&titited Time. There is another objcction of nearly the same character, which must hare struck every thinker with more or less force. H o w are the various organs of a highly complicated organism, or the various structures which enter iiito thc formation of a complicntcd organ, kept in harmonious adjnstmejit to each other by the selection of variations which are, in Darwin’s sense, fortuitous? It is plain that, as soon as one part has varied in any direction, the harmonious adjustment of related parts will be disturbed, and that thcy too must vary correspondingly in order to restore the proper tone to the whole, and it is equally clear that eyen a slight change in a compli-
2s2
Heredity.
cated organ will thus, if the various modifications are really fortuitous, require i~ 7u ery great number of geiierations to supply the necessary variations. There does not seem to be any logical ground for doubting that any of tIic adaptations of nature n7ight have been produced by the natural selection, from an indefinite number of fortuitgus variations, of those wliicli happened to be favorable; but in the case of any complex adaptation, an indefinite and almost infinite period of time would be required. Darwin says (Origin of Species, p. 143) that reason tells us that if numeroux gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful t o its possessor, as is certainly the case; if further the eye ever raries, and these variations be inherited, as is likewise certainly the case; and if such variations should be nseful to any mima1 under changed conditions of life, then the difficulty of believing that a perfect and complex eye could bs fornzed by natural selection, though insuperable by our jniagination, sliould not be considered as subvrrsive of the theory. Before we can accept as possible this view of the evolution of the eye “ we must siippose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, aiid then the old ones to be all destroyed. . Let this process go on for millions of years; and during each year 011 millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass as the works of the Creator are to those of man?” To show that complex adaptations might have been produced by the selection of fortuitous variations is by no means to prove that they have thus been produced, and we may well doubt whether life has existed long
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enough q o n earth, to aliom all the harmonious adjustments of living things to be slowly perfected in this way. Thc vast number of changes wliicli must be co-ordinated in order to produce any considerable modification of one of tlie higlier animals, and the length of time which must be necessary if the snccessive steps are purely fortuitous, are points which must have attracted the notice of every one who has read the “Origin of Species.” The difficulty is obvious, and it has been noticed by many writers, but Murphy, in his discussion of tlie evolution of the vertebrate eye (Habit atid Infelligeme, p. 319), has stated it with great force. He says: “ T h e higher the organization, whether of an entiie organism or of a single organ, the greater is the number of the parts that co-operate, and the more perfect is their cooperation; and consequently tlie more necessity there is for corrcsponding variations to take place in all the cooperating parts at, once, and the more useless mill be any variation whatever unless it is accompaiiied by corivspondiiig variations in the co-opcrating parts; mliilc it is obvious that the greater the number of variakions which are needed in order t o effect ail improvement, the less will be tlie probability of their all occurring at once. I t is no reply to this to say, what no doubt is abstractly true, that whatever is possible becomes probable, if only time enough is allowed. There are improbabilitics so great that the common-sense of mankind treats them as impossibilities. It is not, for instance, in tlie strictest sense of the word, inipossihle that :I poem and a mathematical proposition should be obtained by the process of Qhaking letters out of a box; but it is improbable to a dcgrce that cannot be distinguished from impossibility; and tlie improbability of obtaining an improvement in an organ by means of several spontaneous variations, all occurring together, is an improbability of the same
Heredity.
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kind. If we suppose that any single variation occurs on the average once in nz times, the probability of that 1 variation occurring in any individual mill be -; and snp11L
pose that x variations must concur in order to make an improvement, then the probability of the necessary variations all occurring together will be L.; Now suppose, mx
what I think a moderate proposition, that the value of 1 1 rn is 1000, and the value of x is 10, then - = __ !mX looolo 1 30 = A number about ten thousand times as
-.
1o30 great as the number of waves of light that hare fallen on the earth since historical time began. And i t is to be
further observed that no improvement will give its possessor a certainty of surviving and leaving offspring, but only an extra chance, thc vaIue of whi.ch it is quite impossible to estimate.” No one can be more firmly convinced of the great potency of natural selection than I am, but I am sure every one will feel that the problem of the origin of species would be greatly simplified if i t conld be shown that variations are not fortuitous in Darwin’s sense of tlie word, but that natural selection is in some way provided with rariation in those parts where change is needed. Mivart has discussed this subject at considerable length. He points out that tlie modification of dornesticated animals by the continued selection of slight rariations, is a very slow process, and after quoting Darwin’s statement that mild species probably change much more slowly than domesticated forms, lie continues as follows: “Let us take for an example the proboscis monkey of Borneo. According to Mr. Darwin’s own opinion, this form might have been sensibly changed in
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the course of two or three centnries. According to this, to evolve it as a true and perfect species one thousand ye:~,rsmonld be a very nioder:tte period. Let ten thousand ye:vs be taken to represent approximately the period of sobstantidly ccnstant conditions, during which no consider&le change would be brought about. Now, if one thousand years may represent the period required for the evolution of this species and of the other species of the genus, ten times that period should, I think, be allowed for tlie differentiation of that genus, the African Circopithecns, and the other genera of the family Simiidae, the differences between the genera being certainly more than tcnfold greater than those between the species of the enme genus. ". For the diff~~entintion of the families Simiidae and Cebida+---so very much more distinct and different that any two genera of either family-a period ten times greater should, I believe, be allomcd than that reqaired for the erolution of the subordinate gronps. A similarly increasing ratio should be granted for tlie snccessi ve deyelopments of. the difference between the Leniiiroid and the higher forms of primates; for those between the original primates and other root-forms of placental mammals; for those between primary placental and implacental mammals; and perhaps, also, for the divergence of the most nncieiit stock of tliese and of the moiiotremes, for in all these cases modifications of structure appear to increase in complexity in at lenst that ratio. Finally, a vast period mnst be granted for the development of the lowest mammalian type from the primitivestock of the whole vertebrate sub-kingdom. Supposing this primitit- stock to have arisen diryctly from a very lowly original animal indeed (snch as a nematoid worm, an ascidian, or a jelly-fish), yet it is not
..
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Heredity.
easy to believe that less than two tlionsand million years would be reqiiired for tlic totality of animal clevelopment by no otlicr means tliari minnte, furtuitons, occasional and intermitting variations in all consic1cr;ible structures. If this be even an apl~roximationto the truth, tlien tlierc seem to be strong rtasons for believing tliat geologicti1 time is not suficient for such a process. . . , . “Now, it will be a moderate computation to allow 25,000,000 years for tlie deposition of the strata down to and including the Upper Silnrian. If, then, the evolationory work done during this deposition only reprevelits R Iiundredtli pait of tlie sum total, me sholl reqniie 2,500,000,000 (two tlionsand five liundrcd million) years for the complete development of the whole animiil kingdom to its present state. Exen one quarter of this, liorever, mould far exceed the time wliicli pliysics and nstronomy seem able to allow for the completion of tlie process. Now all these difficultits are avoided if we adm i t that new forms of animal life of all degrees of eomldexity appear from time to time with comparative snddenness, b e h g evolved according to laws i n part depending on surrounding conditions, in part internal, similar to the way in whicli c ~ y s t a l s(and perhaps, from recent researches, the lowest forms of life) build themselves up according t o t h e interim1 laws of their coniponent snbstance, and i n harmony and coricspondence with all enironing influences and conditions.” Darwin himself seems to believe that i n order to explain tlie lprmonious co-ordination of all t h e inter-related parts of acomplicated animal, we must believe that n a t t b l selection is greatly aided by otlier influences, such as t h e inherited effect of use and disease, the di-
“. . .
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rcct action of external conditions, and especially tlie law of correlated variability. Our theory of heredity furiiishcs exactly what me need t o csc:ipe tliis difficulty, for me can uiidcrstatid tliat a change in any part of the body, disturbing, as it niust, the harmonious adjustment of related parts, acts directly to produce variations in these parts i n succeeding generations, by causing the transmission of gcmmules. The time wliicli is needed for the evolution of a complicated orgin by natural selection is thus brought within reasonable limits, and one of tlic niost serious and fundamental objections to Darwin's explanation of tlie origin of species is complctely done away w i t h He says: " We may borrow an illustration from Mr. Herbert Spenccr, who remarks that when tlie Irish elk acquired its gigantic horns, weighing above oiie hundred poiind~, nnnierons co-ordinated clianges of structure would liare been indispensable, namely, a tliickcned skull t o carry the horns; strengtliened ccrvicd vertebrz with strengthcued ligmwnts; enlarged dorsal vertebra to support the neck, with powerful fore-legs and feet; all these parts being supplicd with proper blood-vessels, muscles and nerves. How, then, could these admirably co-ordinatcd structures liarc been acquired? According to the doctrine whicli I maintain, the horns of the male elk were slowly gained through sexual selection, that is, by the best armed males conquering the worse armed, and leaviiig a greater number of descendants. But it is not a t all necessary that tlie several parts of the body should have simultaneously varied. Each stag presents individual differences, and i n the same district those whicli had diglitly heavier horns, or stronger necks. or stronger bodies, or were the niost couragcous, would serve the greatest number of does, and consequently have the
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greatest nnmber of offspring. The offsl"'ing monld inherit in a greater or less degree these same qnalities; would occasionally intercross wit11 each other, or with other individnals varying in some favorable manner; and of their offspring, those mliich were the best endowed in any respect woiild continue multiplying: and SO onwards, always progressing, sometimes in one direction, and sometimes in another, towards the present excellent co-ordinated structure.of the male elk. " To make this clear, let us reflect on the probable steps, as shown in the twentieth chapter, by which our race and dray horses have arrived at their present state of excellence: if we could Tiew tlie whole series of intermediate forms between one of these animals and an carly iinimproved progenitor, me shonId behold a vast number of animals not equally improved in each generation throughout their entire strnctnre, but sometirncs a little more in one point, and sometimes i n another, yet on the wlmle gradually approaching in character to our present race or dray horses, which are so admirably fitted i n the one case for fleetness, and in the other for draught. I' Although natural selection would thus tend to gire t o the male elk its prescnt structure, yet it is probable that the inherited infliience of use has played an equal or more important part. As the horns gradntilly increased in weight, the mnscles of the neck, with the bones to wliich they are attaehcd, mould increase in size and strength; and these parts would react on the body and Icgs. Nor must we overlook the fact that certain parts of the skull and extrcmities mo~dd,judging from analogy, tend from the first to w r y in a correlated manner. The increased weight of the horns would also act directly on the skull in the same manner as
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when one bone is removed in the leg of the dog, the other bone, which lins to carry the whole weight of the body, increases i n thickness. But from the facts given with respect to horned and hornless cattle, it is probable tliat the horns and skull would immediately act on each other through the principle of correlation. Lastly, the growth and subsequent wear and tear of the augmented muscles and bones mould require an increased supply of blood, and consequently an increased snpply of food; and this again mould reqnire increased power of mastication, digestion, respiration and excretion.” It will be seen by a careful examination of this extract that Darwin is compelled, by cases of this kind, t o believe that other influences have played a part equal to or more important than that of natural selection, and he is compelled to attribute the co-ordinated modification of related parts to the action of the law of correlated variability. I have already called atten tion to the fact that this law of correlated \-ariation is a necessary result of our view Qf the nature of heredity, for a change in one part must cause variation in co-ordinated parts; and gemniules thrown off by a certain organ of the body may cause co-ordinated variation in all the homologous parts of a descendant. I believe that it will be clear to every one, without further explanation, that the acceptance of our theory will greatly simplify our conception of the action of natural selection, and will enable us to understand the rapid evolution of co-ordinated structures, without being compelled to attribute them to other influences. Darwin appcars to have felt the need of sometliing of the kind, for we find evidence that lie has hunted long and faithfully, but in vain, for something to show that changed conditions produce, directly, the proper modi-
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fications, and failing to find any such proof, hc lias accepted, as tlie only alternative, the view that variations are fortuitous. This is not the only alternative, for we see that there is a third view, namely, that changed conditions cause the variation, but do not determine its character. I n his exliaustive essay on Variation, Darwin has discussed the question whether the external conditions of life haue such a direct and definite influence that tlie exposure of many individuals for many generations to any cliange in their pliysical conditions will rksnit in the modification of a11 or nearly all of them in the same direction, thus producing a new sub-variety without the aid of selection. He points out that many animals and plants which range widely and are exposed to great diversity of conditions remain nearly the same in character; that the two hundred plants which are distributed orer every Englisli coiinty, and which must have bcen exposed for an immense period to considerable differences of climate and soil, are uniform throughout the whole area; and that certain birds, insects and plants which range over large portions of the world, nevertheless retain the same character. He calls attention to the fact that fowls and pigeons have varied, and will no doubt go on varying, in directly opposite ways, though kept during many generations under nearly tlie same Conditions; and he tlierefore concludes that the amount of modification which animals and plants have undergone under domestication does not correspond with the degree to which they have been exposed to changed circumstances. l i e lays especial stress, in this connection, upon the phenomena of budvariation, and s a p : “It is well worth while to reflect
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maturely on some striking case of bnd-vnriation-for instance, that of the peach. This tree has been cultivated by the million in various parts of the world, has been treated differently, grown on its own roots znd grafted on various stocks, planted as a standard against a wall, and under glass; yet each bud of each sub-variety keeps true to its kind. But occczsionally, at long intervals of time, a tree in England, or under the widely different climate of Virginia, produces a single bud, and this yields a branch which ever after bears ncctarines. . Now is it possible to conceive of conditions more exactly alike than these to wliich the buds on the same tree are exposed? Yet one bud alone, oat of the many tliousands borne by the same tree, has suddenly, without any apparent cause, produceda nectarine. But the case is even stronger than this, for the same flomer-bud has yielded a fruit, one half or onc quarter a nectarine, and the other half or three quarters a peach. Agnin, seren or eight varieties of thc peach have yielded, by bud-mriation, nectarines; the nectarines thus produced, no doubt differ a little from each other, but still they are nectarines. Of course there must be some cause, internal or external, to excite the pcach-bud to change its nature; but I cannot imagine a class of facts better adapted to force on our minds the conviction that what we call the external conditions of life are quite insignificant, in relation to any particular variation, in comparison with the organization or constitution of the being which varies. We are thus driven to conclude that in most cases the conditions of life play a subordinate part in cansing any particular modification; like that which a sixirk plays, when a mass of combustibles bursts into flame, the nature of the flame depending on the combustible matter and not on the spark. Hence, although it must,
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be admitted that new conditions of life do sometimes definitely affcct organic beings, it may be donbtcd whether mell-marked races have often been produced by the direct action of clianged conditions, without the aid of selection, either by man or nature” (Variatiow, 347-352). While we acknowledge the great weight of this rensoning we must bear in mind that evidence to show that new forms of life are not produced, without the aid of selection, by direct modification, is not neccesarily proof that the causes of variation have no relation to the p r pose of the modificatioii-that variations are, so fdr as their use goes, purely fortuitous. Even if external changes do not give rise to useful modifications, unlcss they are aided by natural wlection, i t may still be true that they play an impwtant and essential part. I t may be true that n change of condititins does not cecessarily produce a change of structure, and yet true that when a change of structure does takb place it is due t o the changed conditions. It may be true that an unfavorablb change in the environment has no power to produce a compensating change of hereditary structure without the aid of natural selection, and yet true that this externaI change may be the cause of variation in the part affected. If this latter supposition be a fact the work of natural selection will be almost infinitely simplified, for in place of an indefinite number of fortuitous variations, it will be furnished with variation of the part in which change is needed, nnd i t is only an even chance whether a change in a part whicli is out of harmony mitli its environment be favorable or unfavorable. According t o our theory of heredity, when an srgan-
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ism, placed under new conditions, becomes modified to meet the change in its environment, the existence of the internal change is cansed by the external change, while its precise character is determined by other factors, chiefly by the hereditary cliaracteristics of the corresponding part, in both parents. As long as the harmony which has been gradually established, by natural selection, between any particular cell and its conditions of life, remains undisturbed, this cell will continue to perform its function as a part of the body, and will have little tendency to give rise t o gemmules. When through any change, either in the conditions of life external to the organism, or in other parts of the body, this cell comes to be placed in circnmstances which are unfavorable to the performance of its function, it will exert the tendency to throw off gemmules; for each cell being, in a morphological sense, an independent organism, possesses this power, by inheritance, although natnral selection has gradnally acted, during the past history of the evolution of life, to prevent the useless manifestation of the tendency, as long as snrrounding conditions are favorable and no change is needed. These gemmules, when transmitted t o the egg, by impregnation, mill, by sexual union with the corresponding parts of the egg, cause variation in the homologous cells' of the offspring, and will thus produce a congenital hereditary change at the very time when, and in the very part where, such change is needed. Instead of being purely fortuitous on the one hand, or due on the other hand to the direct modifying influence of external conditions, congenital variations are due to the manifestation of a general law, which has gradually become established, during the evolution of
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life, for this very purpose. I believe that the gradual establishment of this law of heredity is due to the action of n a t a d selection; to the divergent specialization of the two sexual elements; and to a physiological division of labor, each step in the production of which has been advantageous, and has therefore been perpetuated like any other useful variation. According to this view, we must recognize in the law of natural selection, not simply a great nieans of modification, but the agcncy to which organic evolution is almost exclusively due; but we must also believe that, in the higher multicellular organisms it acts indirectly, and is subordinate to another law, the law of heredity, which itself OWCS existence to the law of natural selection.
Oejection fo the T%w thirt the Variatioiz qf any Part is Caused by the l’mitsmission qf Gemnaules, d L i c k owe their Ezistence to the Action of Upifai*orable Coisditions upon tlie Corresponding Part of tlie Parent. Mr. H. W. Conn has called my attention to the f a d that in many cases it is difficult to sce any connection between tlie function of a new variation and a failure to perform that function in the p:ireut. He instances the mimetic colors of insects, and the long neck of the giraffe, and says that i t is difficult to see how the action of unfavorable conditions upon the parents could have given rise to these variations. H e says that if an insect were dangerously conspicuous its unfavor:ible conditions of life would not affect tlie cells to which its color is due, in any especial way, but would lead to tlie destruction of tlie entire animal. So, too, if a series of dry seasons should place the giraffe under conditions of hardship, the individuals
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with the shortcst necks would suffer most, from inability to reach their food, but he says that this would not affect the cells of their necks especially, but would result in general disadvantage to the mliole body. The validity of this objection cannot be denicd, and I do not think it would be difficult to find many instances which aie much more striking than the two which have been referred to. I t is very difficult to understand how our explanation of tlie origin of variation can apply to instances of modification in animals which, like worker bees, do not produce descendants. I t is proper to point out, however, that these cases are no more difficult to explain after our theory of heredity is accepted than witliout it. Its acceptance does, in many cases, greatly sidplify our conception of natural selection, and the fact that it still leaves difficnlties nnexplained, is no reason for rejecting it, provided it does not add to these difficulties. I n tlie case of the giraffe it is not difticult to understand that if circumstances should compel this animal to stretch frequently after foliage almost beyond its reach, this might cause hardship in tlie cells of the neck, and thus result in the production of gemmules, and in conscqaent variation of this part of the body. As sterile insects are simply sexual insects which have riot become perfectly dereloped, me must believe that all their characteristics are shared by the sexual insects, and there is therefore no great difficulty in understanding how the action of unfavorable conditions upon the sexual form might cause variation in the sterile form. The various cells of the body stand in such intimate relations to each other, and are mutually dependent upon each other in so many ways, that it is quite impos-
Heredity. sible to trace out in its completeness the effect of an external influence. A change outside the body may have an obvious and direct effect upon the cells of a certain part, and these cells may influence other cells and so on indefinitely. Any of the cells which arc thus affected may give rise to gemmules, and may thus result in a favorable rariation which will be seized upon-and perpetuated by natural selection. A new variation may therefore follow from an external cliange which has no direct influence upon the part in which the vari at’1011 occurs. This would be an apparent but not a real objection to our view that the cause of a variation is to be songht in the unfavorable action of changed conditions upon tlic part in mliich the variation occurs, but our inability to trace the connection betreen a rariation and the external change to which it is due, is no reason for doubting the reality of the connection.
Saltatory Evolution. The origin of species by the natural selection of minute fortuitous variations, demands time which is so long that it is practically infinite, and many naturalists have accordingly held that the successive clianges may possibly not be so minute as Darwin believes. Thus Huxley says: “We greatly suspect that Nature doesmake considerable jumps in the way of variation now and then, and that these saltations give rise to some of the gaps which appear t o exist in the series of known forms.” Galton compares the evolution of an organism to the rolling of a rough stone, mliicli has, in consequence of its roughness, a vast number of natural facets on any one of which it might rest in stable equilibrium. When pushed, this stone would yield a little; but it mould fall back again on the withdrawal of the pressure, unless this
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was great enough to overpass the limit of the facet on which it has been resting. I n this case it would ,tumble over into a new position of stability, which it will retain until the pressure again becomes great enough to dislodge i t and roll i t another step onwards. He says, 6‘ The various positions of stable equilibrium may be looked upon as 80 many typical attitudes of the stone, the type being more durable as the limits of its stabilihy are wider. We also see clearJy that there is no violation of the law of continuity in the movements of the stone, though it can only repose in certain widely separated positions.” Mivart, who has discussed this subject a t some length, has given many reasons for believiug, in opposition to Darwin, that such sudden jumps do occur, and that evolution is not always by minute changes. It is clcar to every one that any theory of the cause of variation, which recognized the possibility of sudden and extensive modification, would very greatly diminish the time which is demanded for the origiii of species by natural selection, and would thus greatly simplify OUT conception of tlie working of this law. We liave just seen that as our theory of heredity explains how a mriation in one part causes related parts to vary, it removes one great objection to the theory of natural selection, and I wish now to call attention to the fact that, since a change in any part will disturb the harmony of related parts, thus causing their cells to tlirow off gemmules, a slight change in one generation may bccomc, in following generations, a very considerable modification. There is therefore no reason why natural selcctioii should not often be presented with great and extended variations-the saltations which Mivart belicvcs iu-and the evolution of organisms may
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therefore be a much more rapid process than Darwin believes. We will now examine the evidence to show that sndden changes of this kind do sometimes occur. This evidence is of necessity drawn almost entirely from our domesticated animals and plants. A great gap between fossil forms might be attributed to the imperfection of the record, and if a wild form were to come into existence suddeqly it would simply be recorded as a very rare species, and there mould be no way to tell whether it is the first or the last of its race. If a considerable modification of a well-known mild species should appear suddenly in a region mliich is ~vellknowii and thoroiiglily explored, we might 11n~esufficient evidence to be certain that it is due to recent Taristion: a n d there arc a few instances of this kind, the spike-horned buck of the Adiroiidncks being the most conspicuous oiie with mliich I am acquainted. I n Dec., 1869, a writer in thc American Naturabist says that lie has hunted in the Adirondacks where the %ervus w ~ i a n u sabounds for tlie last twenty-one years. Abont fourteen jearspgo he first heard of spike-horn bucks. These bccame'from year to year niore common; about five years ago he shot one, and fiubseqnently another, and now they are frequently killed. He says that tlie spike-horn differs greatlyfrom the common antler of C. Virgininnus. It consists of a single spike, more slender than the antlcr, and scarcely half as long, projccting forward from tlie brow, and termiiiiting in a very sharp point. IIe beliel-es that it gives a considerable advantage to its possessor over the common buck, as it is a more effective weapon tliaii tlie common antler, a t the same time that it enables him to run more swiftly than the common buck through thick woods and underbrush.
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This certainly seems to be an instance of the sudden appearance, i n a wild species, of a very considerable modification, and although i t is true that fern similiar instances have been recorded, the study of variation in domesticated aziiinals leads us to believe that rnaiiy similar cases mnst occur in wild forms, altliough our means of observation do not allow us to prove that this is tho case. In 1791 a ram lamb was horii i n Massachusetts, having short crooltcd Iegs and a long back, like a twnspit dog. From tliis one lamb tlic well-known ancon breed of sheep was raised (Darwin, Varintioi~,I. 1). 126). Darwin says that in 1828 a single ram-lamb was born on the Manchamp farm with long, smooth, straight silky wool. Tlie ram was of small size, with a large Iieatl, long neck, narrow cliest, and long flanks. This one ram is the founder of tlio hfauch:tnil?-n~eriiiobreed of sheep, and has transmitted all his desirable peculiarities to a ~vhole race of descendants, althongli certain undesirable pecnh r i t i e s liare becn eliminated by judicious selection. Darwin says (VaYiati072, I. 11. 350): “Tliere is one strange fact with resliect to tlie peacock, namely, the occasional appearance in England of the ‘ japanned’ or ‘blacli-sliouldercd kind.’ This form has lately been named on tlic liigli authority of hfr. Sclater as a distinct species, Pavo ~zigrt$enwis, which lie believes will liereafter be found wild in some country, but not in India, where it is certainly nnknomn. Tligse japanned birds differ consideralsly from tlie common enco cock i n the color of their second:try wing-fentliers, scapnlars, wing coverts and thighs; the femdes are much paler, and tlic young, as I hear from Mr. Bartlett, likewise difler. They can be propagated perfectly trim Althongh they do not resemble the Iijbrids which have been raised between
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P. crisfatus and I I ~ U ~ ~ C Znevcrtlieless IS, they are in some respects intermediate in cliaracter between these two species; and this fact favors, as Mr. Sclater believes, tlie view tliat they form a distincf, and natural species. On the otlicr Iiand, Sir H. I-Ieron states that this breed suddenly appeared within liis memory in Lord Brownlow’s large stock of pied, white, and common peacocks. The same thing occnrred in Sir J. Trcvclyan’s flock composed of co~iinionand pied peacocks. I t is remarkable that in these two latter instances t h e black-shouldered kind increased to the extinction of tlie lmviously existing breed. I liave also received, tlirougli Mr. Sclater, 8 statement from MY.Hudson Gurney that he mired, many years ago, a pair of black-dionldered pexocks from tlie common kind, and aiiotlier orinthologist, Prof. A. Newton, states that, five or six years ago, a female bird, in all respects similar to the female of tlie black-shouldered kind, was produced from a stock of common peacocks in his possession, which, during more tlian twenty years, had not been crossed with birds of any other strain. Here we have five distinct cases of j;ipanned birds suddenly appearing in flocks of the coninion kind kept in England. Better evidence of tlie first appearance of a new variety could hardly be desired. If me reject this evidence, and believe that tlie jiipannId peacock is a distinct species, we must suppose in all these cases that the common breed had at some former period been crossed with the supposed P. niyripennis, but liad lost every trace of the cross, yet tliat the birds occasionally produced offspring which snddenly and completely reacquired, through reversion, the charactcrs of P. nigripennis. I have heard of no other such case in the animal or veget;ible kingdom. . . So remarkable a form as P. nigvi=penwis,when first imported, would have
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realized a large price; it is therefore improbable that it s1louli.i liavc been silently introduced, and its history subsequeiitly lost. On the whole the evidence seems to nip, as it did to Sir H. Heron, to preponderate strongly in favor of the black-shouldered breed being a variation, indnced either by the climate of England or by some uiiknomn cause, such as reversion to a premordial and extinch condition of the species. On the view that the black slioulclered peacock is a variety, tlie case is the niost remarkable ever recordcd of the thbrupt appearance of a new form mliicli so closely resembles a true species tli:tt i t has deceived one of the most experienced of liuing ornithologists.” Aiivart quotes from Naudin, Godron, and others, several very similar cases in plants. From the seeds of a poppy, which snddcnly took on a remarkable variation in its fruit, a crown of secondary capsules being added to the nornial central capsule, a field of poppies was grown. These resembled the form from which the seed was taken, and gave seed wliicli agnin reproduced the variation. In 1861 Godron “observed among a sowing of Datura fatula, the fruits of which are very spinous, a single individual of which the capsule was perfectly smooth. The seeds talien from this plant all furnished plants having tlie character of this individual.” These seeds were cultivated up t o the fifth and sixth generations, and the latest descendants did not exhibit the least tendency to revert to tlie spinous f o r m These cases show us that very considerable variations may suddenly appear in cultivated plants and domesticated animals, and that these sudden modifications may be strongly inherited, and may thus give rise to new races by sudden jumps. The analogy of domesticated forms would lead us to
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believe that the same thing sometimes occurs i n nature, and that Darwin lias over-estimated the niiiinteness of the changes in mild organisms, and lias 1hus failed to see that natural selection may give rise to new and wellmarked races in a fcw generations. O u r theory of heredity would lead us to expect mnch of this sudden modification, and i t gives 11s a simple explanation of its origin, mid tlins gives to the law of natural selection a much simpler and therefore a much more probable form than that in which it presented itself to the mind of its discoverer.
Parallel Vwiat io It. According t o the \ - i s tliat -ariatiom are ptirely fortnitons, the chances are almost inconceinbly grcnt agliinst the independent modification of sewral forms along par;illcl lines, by the action of natural selection, yet Darwin gives many instances in mhich this has actually occurred. H e says that by the term " analogous or parallel variation" h e mjshes to cxpress that similar characters occasionally makc tlicir appearance in the several rarieties or raccs descended from the same species, and more rarcly in the offspring of widely distinct species. For instance the nectarine is the offspring of the peach; and the varieties of both these trees offer a remarkable 1mralielism in the fruit being white, red or ycllow-fleshed, clingstone or freestone, in the flowers being large or sma11, in the leaves being serrated or crenated, furnished with globose or reniforin glands, or quitc destitute of glands. I n this case me know that tlic two forms hare independently varied in parallel lines, and that each variety of the nectarine has not derived its character from a corresponding kariety of the peach. The several varieties of
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the apricot, which belongs to a closely allied genus, differ from each otlicr in nearly the same parallel manner. Darwin gives many similar instances, and we must acknowledge that in these cases we have homologies which are not due to inheritance from a common ancestor, but to secondary modification. I t is true that, in all the cases which Darwin gives, the parallelism exists between forms wliich are very much alike, and wliicli lime quite recently diverged from a common ancestor, but there is reason to believe tliat this is not always tlie case. Morphologists assume that homology or morphological resemblance is, in itself, evidence of community of descent, and when two widely separated organisms present features which show fundamental similarity of plan, they take it for granted tliat they owe their resemblances to inheritance frqm a common ancestor, which exhibited all the characteristics which they share in common. This is no doubt true as a general rule, and even if it were not true it would usually be extremely difficult to prove its falsity; but there are a few cases where great groups of animals are related to each other in sucli a peculiar way that tlie view that all their homologies are due to dcscent is untenable. The Medusae present such a case. These animals resemble each other in many particulars. They have a muscular contractile gelatinous umbrella by the pulsations of wliich they swim through the water. The digestive organs are suspended from the concave centre of the umbrella, and they give rise to diverticula which penetrate its gelatinous substance. Their reprodiictive organs are developed upon the digestive tract or upon i t s diverticula, while their organs of sense are placed around thc margin of the umbrella. They usually pass
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through a fixed polyp-like larval stage before maturity is reached, and this polyp larva is destitute of a swimming nmbrella, and of organs of special sense. I t has an elongated cylindrical body, by one end of which it is attached, while the mouth is placed a t the opposite end and is snrrounded by a crown of tentacles. The group is divided into two grand divisions, tlie medusae with a veil or diaphragm across the opening of the umbrella, and the medusre without a ~ e i l . The two gronps resemble each other in all essential particulars, and no naturalist has doubted that they are truly homologous with each other, but they present certain constant differences, such as the presence of a Feil and the absence of gastric filaments in the one group, and the absence of a veil and the presence of gastric filaments in the second group. The larva of a veiled mednsae is a hydroid-polyp, which has a simple digestive cavity, and the power of mnltiplicatioii by lateral budding, while the polyp-larva in the veilless mednsa is known as a wyphistoma. It has gastric filaments iu its digestive cavity, and it multiplies by terminal budding or fiLwon. . I n other respects, the two kinds of I a r w Ehow a close homology with each other, but the points of rescmblance are not the same as those which unite tlie two gronps of mature medusa. Haeckel has devoted many years t o the study of the medusa, and his opinion is entitled to .very great weight, and he believes that the resemblances between the larva are due to community of descent, but that the resemblances between the adults are not. IIe believes that the remote ancestor of all the medusa was a polyp which nnited in itself the features which now distinguish the hydra-larva of the veiled medusa from the scyphostoma larvae of the veilless forms, and that these
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two larval €orms have diverged in two directions from this anccstor, from wliicli t h y inherit all that they have in common. IIacclicl believes that after this separation took place, the veiled medusae were developed from hydroid polyps, while tlie veilless medusae mere developed from scyphistoma polyps. The many.i>oints of resemblance betv,een tlie two forms of nieilnsae are, therefore, not due to Common inhcritancc, but liave been secondarily acquired. They are due to the f w t t h a t the two groups of mednsae have been evolved along parallel but distinct lines. Haccltel's familiarity with the medusae entitles him to speak with great authority; but still he may possibly be wrong, and the origin of the two groups may not have been as he supposes. There are four possible hypotheses as to the origin of the medusae, in favor of each of which something may be said. We may hold with Haeckel that the two larval p"1yps are the divergent descendants of a common anccstral polyp, which had no medusa stage, and that each has subsequently devcloped medusae, or me may bel i e w t l ~ tlie t common ancestor was a medusa without a p01.q~larva, and that the hydra larva and the scyphistoma larva have been independently acqnired, or we may believe that the ancestral form liad both a Iarv:il polyp-like stage, and an adult medusa stage, or finally we may assume what seems to 11s the most probable riew, that the ancestral form was neither a true swimming medusa nor a true sedentary polyp, but something half-way between, like the actinula of Tnbularia or the euibrgo of Cnnina. I do not see any fifth alternative, and one of these four suppositions must correspond with the actual evolution of tlie group. Now
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whicherer one we accept, TVC are compelled to believe that there lias been parallel evolution, and that certain homologies between thc various forms are not due to inlie1 itance, but to independent modification. Haeckcl’s view ,compels us to believe that the yesemblaiices between the two groups of medusae hare been independently acquired. If we accept the second alternative, and derive the two forms from an ancestral medusae, the resemblances between the larvae must be due to parallel variation. Suppose, then, that we accept the third or the fourth view, and derive both groups from an ancestral form which had a polyp-like l a r d stage, and a medusa-like adult stage, or else from an ancestral form wliicli nnited in itself certain of the larval and certain of the adult characters. Among the veiled medusae there are some which, like Liriope, do not pass through a hydra stage, but lay eggs, which develop diyectly into m e d u s q and there aye also forms which, like the fresh water hjdra, hare no medusae stage. Among the veillcss forms there are also some which have no medusa stage, but which, like Luceriiaria and the Tesseridae, remain permanently as scypliistoma-_polyps, and i t is probable that in others, as the Charybdeadze, the eggs hatch into medusae, as thcy do in Liriope, without the intervention of alarval polyp stage. 1 t . k therefore impossible to frame any hypothesis as to the origin of the medusae, which will do away with the necessity for the belief that parallel modificatioii, along independent lines, has occurred in the different subdivisions of the group. If we accept Darwin’s view of the origin of variation, parallel modification is not absolutely impossible, although the chances against it are very great indeed.
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The cases where i t can be proved to have occurred are not very numerous, it is trnc, but there are enough of them to present a serious difficulty. On our view that an external change which acts upon a certain part of tlie body may cause variation i n that psrticnlar part, the climices against the parallel modification of allied organisms are very grcatly diminislied, so much so that the occasional occurrence of such modifications might be expected. If such cases were the rule they mould be equally fatal to the theory of natural &election, wlicthcr our theory of heredity were accepted or not; bnt the cases are very far from frequent. Geiieral and Special Homology : and the Signijcancs of Serial Homology, Symmetry and Polymorphism. We have, eo far, been occupied in studying tlie evidcnce for tlie lam of heredity which is affordwl by the slight and recently acquired differences between the sexes of the same species, between the young and the adnlt, between domesticated and wild races, between the Iiyhrid offspring of allied species, be tween reciprocal hybrids, etc. Thc bearing of this law upon the more profonnd problems of morphology has hardly been referred to, for the field wliicli we have examined, although me h:Lve passed over it very rapidly, has furnished material for a treatise of considerable length. The discussion of the general problems of morphology mould require another voIunie of even greatcr length, for I believe, and hope to show in another place, that the acceptance of my view will lead to considerable change in our manner of liandling these problems ; and will so shift our view as to remodel some of the fundamenta1 principles of tlie science. I believe that it will throw light upon many obscure and perplexing qnestions, such as the significance of
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symmetry and general homology, the origin of polxmorphism, the definition of an individual or person, etc.; but the end of a book is not the place to enter upon a a new field, and I am forced to reserve this subject for future discussion, although I mill now indicate very briefly the nature of this explanation. The basis of modern morphology i5 the doctrine that homology indicates genetic relntionsliip. Homology is fundamental similarity ofaplan, as distinguished from difference or similarity in physiological €unction. For example a man’s arm and hand are fitted for grasping, atid a bird’s wing for flight, and their different uscs rcnder them unlike ench otlicr in a superficial view, althoagh there is below :ind behind tliis obvious differcnce a more deep-seated resemblance. The feathers which cover the bird’s wing have the same histological structure and the same origin as the hairs upon the human arm; the skin which covers the limb has the same character in both cases; the wing, like the arm, has a siipporting skclcton, which consists of a shoulder and upper arm, a forearm, a wrist, and a hand; the muscles have the same structure and the same generd arrangement, and the way in wliich they are supplied with nerves and blood-vessels is the same. This fundamental itientityof strncture which is obscured, but not dcstroyed by the difference of use, is homology. I n a superficinlpiew the wing of a bird resembles the wing of a dragon-fly more closely than it resembles a man’s arm, but careful examination shows that the insect’s wing is not a limb n t all, but npeculiar outgrowth from thebody. The resemblance between a hird’s wing and an insect‘s wing is not an homology, and it has no morphological significance : it does not indicate that there is any close relationship between a bird and an insect.
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I t is sometimes dificnlt to determine whetliera resemblance is an homology or not, and in simple cases we decide by asking whether it can be due to similarity of use. We know that a bird’s wing is more closely rclated to a man’s arm than it is to an insect’s wing, bccause the resemblance between the two wings is 110 more than we should expect in organs adapted to the same purpose; but there is nothing in the use of the arm and wing to explain what they have in common. I n cases too complicated to be settled in this simple way we appeal to embryology, and ask mlietlicr the resemblance becomes more marked or lcss marlied when we study it in its younger stagcs. The arm and tlie wing arc more :dike in the embryo tlian they are in thc adults, and the features wliicli they share in comnion make tlicir appearance earlier than their distinctive characteristics. An homology then is a resemblance w11ic:h is not due to similarity of use, and which is more conspicuous in the embryo than in the adult. This is the doctrine of homology considered from its structural side; historically considered, an homology is a resemblance doe to community of descent, as distinguished from one due to recent modification. The modcrn morphologist believes that the resemblances between a bird’s wing and a man’s arm are due to inheritance from a remote ancestor in which the limb had all the characteristics which arc common to the wing and arm; that during the evolution of birds and mammals along two divergent lines from this ancestral form, the distinctive features which fit the wing for flight and the hand for grasping have been gradually acqnired. The doctrine that homology is an indication of ancestral relationship, and that the past history of organisms
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can be traced by studying their anatomy and embryology, is tlie basis of the modern science of morphology. Now there are two kinds of ho:noIogy, special homology and generd liomology. Homologies between corresponding parts of different aiiiniuls are known ns special homologies, and those between different parts of the same animal as general homologies. As examples of general homology wc may instance tlic serial homology of a way-fish, the bilaterzll symmetry of mammals, and the radial symmetry of a star-fish. So far as structure goes tlie lion~ulogybetween a man’s arm and a man’s leg is precisely like the liomology between his arm and a bird’s wing. It is a resemblance wliicli is not due to similarity of use, but to fundamental resemblance, and it is more marked in the embryo than it is in the adult, and me seem, at first sight, to be jnstified in concluding that, if special homologies indicate genetic descent, general homologies must also; and that if general homologics cannot be explained iu this may, tlie explanation of special liomologies cannot be accepted. Mivart has pointed out that it is impossible to explain general homologies by attributing them to inlieritnnce from a common ancestor, and he therefore concludes that special liomologies do not prove genetic evolution. On tlie othcr hand many authors have licld that since special homologies indicate descent, general homologies must have the same meaning, and this belief has led to such speculations as the attempt to trace the vertebrates back to an annelid with a number of equivalent segments, to trace the echinoderms back to a commnnity of worms, and to trace tlie polymorphic siphonophores back to unspecialized commnnities of hydroids. I hope to show in another place that the acceptance
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of my view of the natnre of heredity enables us to ayoid both of these results, since i t shows that special homologies may be due to lieredi ty of one sort, and general homologies to lieredity of another sort. Since corresponding cells in tlie homologous parts of the body of any individual are derived from closely related parts of the egg, t h y may be affected by similar gemmules and may thus give rise to what Darwin calls analogous variations. This form of inheritance I propose to call ontogenetic heredity, to distinguish i t from ordinary inheritance from an ancestor. I shall point out, in another place, that while special homologies are due to ordinary or phylogenetic heredity, that is, to descent from a common ancestor, general homologies are, in many cases, due to ontogenetic liercdity ; that special homologies are old, and that they jiidicate genetic relationship, and thus cnable us to trace the origin and history of animals, while general liomologits arc, in many capes, new, and recently acquired by secondary modification, and they do not indicate ancestry.
CHAPTER XII. RECAPITULhTION AND CONCLUSION.
THEobscurity and complexity of the phenomena af heredity afford no ground for tlie belief that the subject is outside tlie legitimate province of scientific inquiry. The existence, in a simple and unspecialized egg, of the potentiality of a highly organized and delicately adjnsted animal, with special functions, instincts and powers of adaptation, with tlie capacity for establishing and perpetuating harmonious relations to the clitlnging conditions of the world around it, is certainly one of the most profonnd problems of the material universe. The fertilized egg is one of the greatest wonders within oiir knowledge, but this is no reason for refraining from studying it. If we believe that living things lime become wliat thcy now are by a process of gradual evolution, and that tliey owe their cliaracteristics to the influences to which'they have been exposed in tlie past, we must believe that tlie properties of the egg arc capable of explanation, as far as these determining causes are open to study. If we accept tlie generalizations of modern science, and hold that an unicellular ovum is homologous with and is descended from a remote ancestral unicellular organism, and that its properties have been gradiially acquired by the natural selection of favorable variations, we must believe that the origin of its properties is 3s much within our reach as the origin of species. Tlic most proniiiient characteristic of heredity is that
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it may be brought about not only by tlie varions forms of asexual reproduction, but also by tlie sexual union of two reproductive elements, each of which is liomologous with the other cells of the body. In the lower aninials and plants the cells mliich thus unite with each other, or conjugate, are similar in form, and probably in function also; but i n a11 the higher organisms the male cell is very different from the ovum in form, size, and structure, as well as its mode of origin. The prcsent structure of each organism is the resnltant of two factors, wliicli we niay call adlierelice to type aiid adnl3titt ion to iiem conditions, or if the nse of t e r m withoiit teleological implicatioas is desired, we may speak of them :IS heredity and variibtion, or we may follow Haecliel and call tlieni memory of past experiences, and perception of new relations. The precise ternis to be used is a matter of little consequence. Tlie cssential thing is the recognition nf the fact that each organism is tlie resultant of two factors, and tliet evolution is two-sided. An animal is wliat it is because i t has the power to hold on to tile cxperiences and adaptations which fitted its parents for their place i n nature, and the parents acquired those peculiaritics in virtue of their pomcrs to gradually adjust their structure and habits to their environment. Tliis is the morphological side of erolution. Looking at it from its dyiiariiical or functional side, we notice that each step in tlic process of advancement has been remliccl by divergent specialization and by p11ysiologic:~l division of labor. Animals diverge from each other by acqniring the power to occupy different fields, to procure and use different kiuds of food, to exist in different media, etc., and the organs and tissues and cells of a highly specialized animal or plant are adapted to perform
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definite, restricted functions exactly and efficiently, while each part of a low organism fills maiiy offices, but fills them all imperfectly. We find in all except the lowest organisms that heredity is brought about by two dissimilar reproductive elements, and we find that each organism is the resultant of two factors-heredity and variation. It is natuxal to inquire whether there may not be some connection between these two relations; wlietlier the natural selection of favorable variations has not acted upon the reproductive elements as it has upon the mature organisms; mlietlier it has not brought about a pliysiological division of labor between these elements; whether their originally similar functions have not gradiially become specialize$ until one has become tlie conserrati\ e medium, and tlie other the agent of progress in Iiercdity. According t o tlie view advocated in this book, such has actually been the history of the evolution of sex, and natural selection has evolved, in all the higher organisms, a secondary law of heredity, mliicli enables it to do its morlr rapidly and effectirely, with little waste. I n the metazon and in the higher plants, natural selcction is not a crude, rough ‘‘ hit or miss” method of evolution, for the law of heredity, itself a result of the law of natural selection, is that the ovum is tlie vehicle of heredity, while gemmnles or cell germs from cells in all parts of the body, are transmitted to the ovum by the male cell, thus causing variation when and where it is needed. This view is opposed to the conclusion of many high antliorities that there is no difference in the functions of the sexual elements, but exaniination shows that the reasons which they have given for this conclusion admit of another simple explanation.
Recapilulation and Conclusion. -
31.5
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Darwin’s rcason for his Etatcmcnt that each sexual elcrnent has tlte power to ti-ansniit every singlc chwracteristic of the parent form, and that it is an error to suppose that the male transmits certain chnrnctrrs and tlic fcjrnde other chmictcrs, is that when lijbrids arc paired and bred inter so, the characters of either grandparent often reappear in the progeny. A little thought willsliow that it is impossible to prove any such conclnsion in this may. If two animals which differ from each other in cvcry rcspect could be made to cross, the m u l t mould fnrnish concltisire eyidcnce as to the correctness or incor~ectncssof Darwin’s statement, but in any possible cross the parents are essentially alike, and they diEer only in minor features of recent acqnisition. The possibility of parthenogenesis proves that the ovum does transmit the entire orgauizatim, bnt it isimpo3siblc to show, from the plienoniena of crossing, that the male elcrnent has the same power. Tlie reason given by HUXICY for his opinion that an animal inherits every characteristic of each parent, is that the ovum and the male cell are homologons with each other, and that all the cells of the body are descended, by a process of division, from the compound germ which is formea by their nnion. Homology, or similarity of origin, is no ground for assuming similarity of function, and the fact that the male cell and the egg are homologous with each other is no reason ~vhateverfor a belief that their parts in heredity are alike. The fact that either sex ay, under certain circumstances, acquire t hc secondary sexual characters of the otlier, seems at first sight to show tliat the mhole organization of the male exists in a potential and latent state in the body of every female, and that the whole organi-
...
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zation of the female is latcnt i n ewry male; that each individual is a complete double person. If we accept this conclusion i t is only logical to conclude that tlie power to revert or acqiiire tlie characteristics of remote ancestors provcs the existence, in a latent state, in encll individnal, of the coriiplete organization of each of a long series of ancestors of both sexes. This snttle metaphysical conception is so foreign to t h e methods and tendencies of modem thought, that when we conipnre i t with Hunter’s simple and definite statement that the natural history characteristics of any species of animal are to be found in those properties that are common to both sexes, there does not seem to be any room for choice. Tlie view that each individnal inlicrits all tlie characteristics of the species, and tliat tlie distinctive characteristics of tlie male are arrested in ccrtain ones, while tlie distinctive features of the female remain latent in others, furnislies a simple and adcqnate explanation of the facts, and remoyes all neccssity for tile subtle, complex and unthinkable, compouiid personality ligpothesis. In tliis connection the interesting and practical question, wlist determines the sex of the embryo, can hardly fail to suggest itself to the reader. I have rchaincd from a discussion of this important paint in the body of this work, as i t has no direct bearing upon our ni~gumcnt and I have no solntion to oeer. A s I liavc so far omi ttctl all reference to tlie subject, 1 will take occasion now to call attention. in tliis connection, t o the facts detailed on pp. 55-69. Tlie render will see tliat all female bces arc born from fertilized eggs, and all male bees from nnfertilized eggs; wliile tlie unfertilized eggs of t3aplinia gire rise to females only, and in many of the gall wasps both males and females are born from parthonogenetic eggs. There-
Recapitulation and Conclusion.
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is no necessary or constant connection between the fertilization of the egg and the sex of the embryo, and the conclusion which I have reached from the study of these cases and of oiir scanty information upon the subject from other sources, is that sex is not determined by any constant law; that in certain animals and plttnts the sex of the embryo is determined by certain conditions, while in other groups it is determined by quite different conditions. However this may be, it is obvious that since perfect males and perfect females may arise from eggs which are fertilized, and also from eggs which are not fertilized, the necessity for fertilization does not come from the necessity for transmitting to the offspring the organization of each parent. A review of the opinions and reasoning of various authors shows that there is no good ground for believing that the two reproductive elements play simjlar parts in heredity and transmit every cliaracteristic of each parent. I t is impossible to prove i t by the phenomena of crossing, since the only animals which can be made to cross are essentially alike, and differ only in minor points. The homology between the ovum and the male cell is no reason. for supposing that their functions are similar. There is no reason for assuming that each sex transmits its entire organization to the offspring, since the latent transmission of secondary sexual characters can be wore simply explained by assuming that each embryo inherits, but does not necessarily develop, all the characteristics of its species. Reversion and alternation of generations admit of a similar explanation, and we may conclude that there is and can be no proof that each sexual element transmits all the characteristics of the parent. There is therefore
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no a priori absurdity in the hypothesis that the ovnni and the male cell fill different offices. While there is 110 reason for believing that the functions of these elements are alike, there are ninny reasons for believing that this is not the case; for example, the almost universal occurrence of differences in f&m, size, and structure; the possibility of pdrthenogenesis; the differences between reciprocal hybrids; the fact that the offspring of a male hybrid and a female of a pure species is much more variable than the offspring of a female hybrid by the male of apure species; and the fact that a part Fhich is more developed or is of more functional importance in the male parent than it is in the female parent, is much more apt to vary in the offspring than n part which is more developed or more important in the mother than it is in the father. I n the absence of all evidence to the contrary I think we may safely conclude from this positive evidence that a division of physiological labor has arisen during the evolution of life, and that the functions of the reproductive elements have became specialized in divergent directions. The only way to discover the exact nature of this specialization is to study the influence of each element separately, and the comparison of sexual with asexual reproduction is the best available method of doing this, since asexual reproduction is essentially reproduction without a male element, while sexual reproduction is reproduction with a male element. Organisms produced from fertilized ova differ from those produced asexually only in their greater tendency to vary, and the hypothesis that the male element has become specialized for the transmission of n tendency to vary naturally suggests itself. Variation is not dependent upon fertilization, for plants produced from buds
Recapitulation and Conclusion.
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vary as well as those born from fertilized seeds, although bud variatioiis are extremely rare as compared with seedling variations. I n any attempt to frame an hypothesis of heredity we must therefore recognize all the following facts : that the two reproductive elements are homologous, and that their functions were originally alike; that the possibility of parthenogenesis, together with many other well ascertained facts, shows that their functions are not alike, in the higher orgaiiisms, a t present; that their present functions are due to divergent specialization or physiological division of labor; that variation is possible without sexual union, but that the introduction of a male element in reproduction greatly increases the frequency of its occurrence. Among tho unicellular organisms variability is provided for by conjugation, or the fusion of two entire individuals so that the new generation is derived from 8 compound germ which contains particles to represent all the parts of the body of each parent. I n the metazoa and the many celled plants the reproductive bodies are localized and they are single cells, and there must therefore be some mechanism or organization in virtue of which they represent cells from all parts of the body, and thus provide for further variation. These various considerations have led us to believe that each cell of the organism inherits from its unicellular ancestow the power to throw off cell germs or gemmules; that these germs penetrate to all parts of the body, and that those which thus reach the developing reproductive elements insure variation, in the next generation, in the cells which they represent; that originally the two sexual elements were alike in function; that each inherited from the fertilized ovum o€ the p r e
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ceding generation the power to give rise to a new organism with all tlie established hereditary cliaracteristics of the race; and that each element also had, by virtue of its contained gemmnles, the power to transmit variabili ty. The existence, in each element, of the power to transmit the hereditary clinracteristics of the species is obviously superfluous, since the object of sexual union, the transmission of a tendency to vary, wonld be equally well secured if only one element hsd the power to transmit the common characteristics of both parents. I therefore believe that, as organisms gradiially increased in size, as tlie number of cells in their bodics grew greater, and as the differentiationand specialization of these cells became more and more marked, one element, the male cell, became adapted for storing up gemmnles, and, at thesame time, gradually lost its unnecessary and useless power to transmit hereditary characteristics. This process was gradual, and eveu in the highest animals the power of the male cell to transmit heredicary characters does not seem to be completely lost, although few traces of it remain. I also suppose that natural selection has acted upon the various cells of the body to restrain them from throwing off unnecessary gemmnles, and that this power is exercised only when a change in the surrounding world renders variation necessary. After framing this hypothesis the next step is to tesC it by applying it to the various observed phenomena of heredity in order to see how far it explains and interprets them. I have attempted to do this in chapters VI. to X. of this book, and I think we are jtistified in conclnding, as the result of this review, that, while there are many facts which the hypothesis does not explain, they are not of such a character as to directly contradict it, while it
RecapituZation and Conclusion.
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does group and illuminate many classes of facts which are quite inexplicable without it. The evidence from hybrids seems to be strongly in its favor, and it presents niany features wliich are perfectly simplc and natural, according to onr view of heredity, altliougli no oth'er explanation of them has ever been offered. Hybrids and mongrels are highly variable, as we should expect from the fact that many of the cells of their bodies must be placed under unnatural conditions, and must therefore have a tendency to throw off gemmules. Darwin's pangenesis hypothesis accounts for the variability of hybrids, but it does not account for the very remarkable fact that hybrids from forms which have loiig been cultivated or domesticated are more variiable than those from wild species or varieties, or for the fact that the children of hybrids are more variable than the hybrids themselves. Our view not only explains the variability of hybrids, but it also accounts for the excessive variability of hybrids from domesticated forms, and of the children of hybrids, for domesticated animals and plants live under unnatural conditions, and they are therefore more prolific of gemmnles than wild species, and as the body of a male hybrid is a new thing tlic cells will be much more likely than those of the pure parent t o throw off gem?ules. The fact that variation is due to the male influence, and that the action upon the male parent of unnatural or changed conditions results in the variability of the child, is well shown by crossing the hybrid with the pure species, for when the male hybrid is crossed with a pure female the children are much more variable than those bornfram a hybrid mother by a pure father.
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The remarkable history of reciprocal crosses is, on the whole, exactly what we should expect, and although there are many difficulties, they are no greater than the complexity of the subject would lead us to anticipate. The study of variation brings out a number of secondary laws, all of which might have been derived from our view of the nature of heredity. The law that sexual offspring are more variable than those produced asexually has just been discussed, and i t is clearly in perfect accordance with our view. Another most interesting and remarkable law-that changed conditions do not act directly, but that they cause subsequent generations t o vary-receives a simple explanation as soon as me recognize that variation is due to the transmission of gemmules, not t o the direct modifying influence of external conditions. We can also understand why variation should itself be hereditary, why specific characters should be more variable than generic characters, why species of large genera should vary more than species of small genera, why a part developed to an unusual degree or in an unusual way should also be extremely variable, and why secondary sexual characters should show a marked tendency to vary. The study of secondary sexual characters aids us, like the study of hybrids and of variation, to analyze or disentangle the influences of the two sexes in heredity. These characters, therefore, possess especial interest in connection with our subject. They are found, upon examination, to present many striking peculiarities which might have been directly deduced from our view of the nature of heredity. As gemmules which w e formed in the male body am much more likely to be transmitted to descendants, and
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thus to give rise to variation, than gemmules which are formed in the female body, we should expect to find, in a variation which first appears in a male, much more tendency t o become hereditary than in a variation which first appears in a female. For thesame reason we should expect t o find an organ which is confined to males much more likely than one confined to females to give rise t o hereditary modifications. For a similar reason we should expect the males of unisexual animals to vary more than the females. We can form some conception of the amount of modification which a n animal has recently undergone by comparing the adults with the young, and by comparing allied species with each other. When we make comparisons of this kind we find that throughout the animal kingdom, with Fery few exceptions, wherever the sexes are separate and differ from each other, the males of allied species differ from each other more than the females do, and the adult male differs more than the adult female from the young. This law ia ao pronounced and conspicuous that its existence has long been recognized by all naturalists. We also find that organs which are confined to males, or which are of more importance or are more perfectly developed in the males than they are in the females, are very much more likely to give rise to hereditary modifications than parts which are confined to or are most developed in females; that a part which is tlius confined to males is much more likely to vary than a similar female part; that males are, as a rule, more variable than females; and that the male leads and the female follows in the evolution of new races. The scientific accuracy of most of these generalizations regarding secondary sexual characters has long been
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recognized, although no one, so far as I know, has attempted to trace them back to a fundamental law of heredity. On the contrary, most of the authors who have discussed them have treated them rather as special cases than as the results of a general principle, and analysis shows that none of the explantitions which have been. advanced are sufficiently broad to cover the whole ground. Daines Barrington and Wallace have held that the explanation of the fact that male birdsand male insects are often so much more brilliantly colored and conspicuously ornamented than the females is to be found in the fact that the female, while laying her eggs or while incubating, is much more exposed to the attaclisof enemies than the male, and that inasmuch as the perpetuation of the race depends upon the safety of the females at this time, natural selection has gradually exterminated the conspicuous females, and has preserved those with the least striking colors. We know, however, that the male is usually mora brjlliantly colored than the female among reptiles which do not incubate, and evcn among certain fishes where the male attends to the eggs and young. It is therefore clear that Wallace’s explanation stops short of the whole trnth, and Darwin’s exhaustive review of the subject seems to prove tliat amon$ birds i t is the male and not the female which has been directly modified. Darwin believes that the greater modification of the males as compared with the females is due to sexual selection. The males have struggled with each other for the possession of the females, or have been selected by the females, and this process long continned is believed by him to have resulted in the perpetuation of the strongest, best armed, or most attractivc males.
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It is plain that sexual selection must have the effect which Darwin attributes to it, but the fact that even in choice breeds of domesticated aninials which are mated according to the wislies of tlie breeder, and not according to their own selection, the males are more modified than the females, shows that behind the action of sexual selection some more profonnd 1.mmust exist. Darwin believes that tliia explanation is to be found in the fact that the male usually has stronger passions than the female, aud is consequently more exposed to the action of natural selection. IZe says that the perpetnation of the race depends upon the existence of the sexual passion, and that, since the male mnst in most cases seek the female, the most eager males have left the greatest number of offspring, and have thus become selected. Wlicn me bear in mind the fact that the parental instinct is fully as important to the race as the sexual instinct, and that this is usually most developed in the female, we see that the failure of tlie female to undergo modification for the good of the species as frequently as the maIe cannot be explained without the recognition of some more general law. The singular history of secondary scxual characters receives a ready explanation by the lam of heredity, for this law leads 11s to look to the cells of the male body for the origin of most of the variations through which the species has attained to its 'present organization. Since gemmules which originate in a male body are more likely to be transmitted than those formed in a female body, and since gemmules are most likely to be formed in the sex in whichan organis of most functional importance, and therefore most subject to disturbing influences, we can readily see why a part which is im-
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portant to males should vary more than a part which is important to females. We are thus able to understand tlie great difference in the males of allied species, the difference between the adult male and tlic female or young, and the great diversity and variability of secondary male characters, and we should expect to find, what actually is the case, that among the higher animals, when the sexes have long been separated, the males are more variable than the females. I n the chapter on the intellectual differences betmeen men and women, I have shown that those philosophical writers who have devoted especial attention to the subject have reached conclusions which are exactly what our 4iypothesis would lead us to expect. The viev that the male mind is the progressive element in intellectual development, and the female mind the conservative element,. accords with the views which have been generally recognized and accepted by the common consent of mankind, and although our opinions upon this very complicated subject may possibly be very far from accurate, a certain conformation to the demands of our hypothesis cannot be denied. The facts relating to hybrids, to variation, to the secondary sexual characters of animals, and to the intellectual differences between men and women, which are stated at some length in chapters V. to IX., cover a very wide and diversified field, and any law mliich groups and explains them all is certainly worthy of careful examination. The most candid review which I am able to give to the evidence from all these sources, convinces me that tlie explanation which I have offered in this book is at least a step in the right direction, and that whether it be accepted in its present form or not, it does serve to en-
RecapituZation and ConcZusion.
827
large our insight into the hidden relations between the phenomena of nature. Chapter XI. is devoted to ail examination of the law of natnral sclcction, as modified by the law of heredity, s l i d I have liere atfeniptcd to sliow that the acceptaiice of this secondary law will remove the most serious objections to the view that our present f o r m of life have been brought into existence through the survival of the fittest variations, qnd I have also called attention to the fact that tlic law of heredity is itself a result of the b w of na t u r d select ion. No oiie can deny that tliere are grave objections to the law of natural selection in its origiiial form. Darlyin admits this in many places, and ttble but dissenting critics h a w stated most of tliese objections with great ability. The evidence for tlie 1:im of natural selection ia so many sided, so extensive, niid so satisfactory, that we may fairly conclude that the dificultics will disappear with greater knomlcdgc, and as none of its hostile critica have proposcd anything mliatever to take its place, the difficulties which they have pointed ont have liardly ieceivcd from naturalists the attention which they deserve. One of tlie most serious objections is that natnral selection cmnot effect any permanent modification of a race, unless g e n t numbers of individuals vary in essentially the same may at nearly the same time, and that tlic chances against this arc great beyond computation if variations are purely fortuitous in Darmin’s sense of the word. Darwin lins acknowledged the weight o € this objection, and tliere is no escape from the conclusion that natural sclection fails to account for thc origin of species, unless we can show that many ii1dividn:ils tend to vary at tlie same time. According to our view, the production of
328
Heredity.
gcnimulcs and the consequent variations are due to the dircct action of changed conditions upon certain cells of the body, and any cliangcvvliicli affects all tlie individuals of a specics will cause tlic same part to vary i n all of tlicm a t the same time. This objcction to tlie law of natural selection is thus entirely removed. The evolution of a complicated organism, or tlie modification of any part which includes a number of complicated structures, without destroying tlicir liarmonioue adjustment to each other, dcmands a w r y greet number of variations, and if t h e are fortuitous, we may well doubt mlietlier tliere has bccn time enough for tlic cvolution of life by nataral selection. Acearcling to our theory of hercdity, a change in one part of the body is i n itself a cause of variation in rclatcd parts; and as cliariges thus tcnd to occur wlierc and when they are needed, the time wliicli is dcmandcd for tlie cvolntion of a complicated organ by iiatural selcction is brought mitliin reasonable limits, :md one of tlie most fundamental objections is thus completely remorcd. There are many reasons for believing that vari at'ions under nature may not be so minute as Darwin snpposes, but that evolution may take place by jumps or saltations. According to our view a cliangc in one part will disturb tlie harmony of related parts, and will cause their cells to throw off gemmules. A slight change in one generation may thus become in following generations a very considerable modification, and tliere is no reason why natural selection should not be occasionally presented with great and important saltations. The lam of heredity also enables us t o understand the occasional occurrence of parallel or analogous variation, and tlie parallcl evolution of polyphylletic-groups.
INDEX. Adjustment between internal and external relations, 40 Adler on parthenogenesis in' gall-wasps, 6!! Adult organism, 6 Albrecht on parthenogenesis,
56, 58 Alcippe, 183 Alternation of generations, 115 American naturalist on spikehorn deer, 298 Anchorella, 179 Ancon sheep, 299 Adimdculist, 22 Annelids, 173 Anolis, 198 Antirlhinum, 139 Apple, 90 Apus, 58 Arbacia, 66 Argus pheasant, 201 Aristotle on parthenogenesis, 55 Aristotle on latent sexual characters, 85, 105 Artemia, 58, 91 Artliropoda, sexual diff ersnces in, 173 Asexual reproduction, 11, 17, 143, 249 Ass and horse, hybrids from, 197
Babyrusa, 205 Bachman on variation of turkey, 150 Balfour on polar globules, 71 Barnacle, sexual differences in, 179, 181 Barrington on colors of birds, 207 Basset on parthenogenesis, 62 Beclistein on spurred hens, 210 Bee, reproduction of, 9 " partlienogenesis in, 60 " variation of, 145 Beetle, sexual differences in, 191 Bell-bird, sexual differences in, 202 Birds, female modification i4
203 Birds, sexual differences in, 199 " weapons, etc., originally acquired by male, 199 Bischoff on parthenogenesis,Blumenbacli on Polishfow1,226 Bom byx, 58 Bonnet on evolution, 20, 85 Branchippus, 173 Buceros, 201 Budding in hybrids, 11 Bud-variation, 84, 144 Buffon on development] 26,85
330
Index.
Buffon on effect of castration, 106 Butterffies, sexual differences in, 196 Callionymus, 197 Cattle, hybrids from, 180 Cause of sex, 316 Ceratophara, 198 Chamelion, 399 Changed conditions cause snbsequent generations to vary, 149 Cherry, variation of, 149 Cladocera, sexual differences in, 175 Claus on parthenogenesis, 68 Cohn on parthenogenesis, 66 Color changed by a change of food, 90 Congenital characters not always hereditary, 93 Conn on cause of variation, 294 Copepoda, sexual differences in, 175 Correllated Variation, 84, 157 Crabs, sexual differences in, 1S6 Crayfish, dimorpliism in, 188 Crossingas a cause of reversion, 132 Crossing as a cause of variation, 119 Cryptophyalus, 183 Cultivated plants, variation of, 146 Cyclops, sexual differences in,
175 Dall on saltatory evolution, 88 Daphnia, parthenogenesis in, 67
Darwin, 85 'L on bud variation of of orange, 147 'I on causes of variation,
275 '6
.' '1
.I
.I
,' I 6
dd
., .#
I'
I,
81
dI ,d
Id
Id
'd
on compound personality, 114 on eomplexity of the germ, 114 on correllated variation, 84 on direct action of cxternal conditions,QO on evolution of eye, 282 japanned peacock, 299 on latent transmission of sexual characters,
105 on modification of male butterflies, 196 on pangenesis, 48 . on parallel or analogous variation, 302 on reciprocal hybrids, 129 on reversion, 10, 115, 132 on reversion in horse, 133 on selection, 12 on sexual charactera of birds, 199 on sexual selection, 169, 212 on transmission by each sex, 100 on transmission without fusion, 131
Indez
332
Darwin, on variation, 141,146
Fish-lice, sexual differences in,
149,153 on variation fron crossing, 120, 122
177 Fowls, hybrids from, 129, 131 " modification of female, 222 " reversion in, 135
"
on variation of specie of large genera, 15 Development often indirect, 2 Dianthus, variability of hybrid "
124
Gall-wasp, parthenogenesis in,
62
Direct influence of external con ditions, 89 Dog, reversion in, 10 " hybrids.from, 131 " variation of, 91, 150 Domesticated animals more va riable than wild ones, 142 Edrnnndstoii on sea-gull, 93 Education and culture, 272 Elephant, 205 Epigenesis, 20, 27 " and evolution, 24 Evadne, 58 Evolution and division of labor,
Gallus banlriva, reversion to,
135 Galton on pangenesis, 53 " on saltatory evolution,
296 Giirtner on variability of bybrids, 120, 124 Gegenbauer on nature of ovum, 28 Gelassimus, 189 Gemmules, 82 Gerstaecker on parthenogene. sis, 56 Godine on hybrid sheep, 129 Goose, inflexibility of, 142
313 " "
"
" "
"
"
Bonnet on, 20 of coniplicated or. gans, 156 definitions of, 20 Huxley on, 20 hypothesis logically imperfect, 82 morphological aspect of, 313 saltatory, 157,296
Falconer on variation of dog and goat, 91 Female modification, 235 Fishes, sexual differences in,
196
Haeclrel on perigenesis, 33, 45 '' on phylogeny of Me. dusze, 304 '' on significance of ontiw y , 30 Hagen on dimorphism of cray. fish, 188 Haller on evolution, 20 Huxley on evolution, 20 Harvey, 22 Hemp, variation of, 90 Hen, parthenogenesis in, 67 Xeredity, theory of, 16, 80 " ontogenetic aiid phylogenetic, 311 " and memory, 37
332
InBex.
Heredity, speculations on, 18 " meaning of word, 6 '' proper subject for study, 12 '' how caused, 81 Hinney, 127 Hippocrates on viragines, 105 Holmgrim on pigeon, 93 Homology of ovum and male cell, 102 '' significance of, 307 Hornbill. 201 IIorse and ass, hybrid from, 127 " in mines, 91 " reversion in, 10, 133 Humming bird, 201 Hunter on Intent transmission,
109 on sea-gull, 92 Huxley ou evolution and epigenesis, 46 " on reciprocal crosses, 127 '' on saltatory evolution, 83, 296 on transmission by each sex, 100, 102, 103 Hybrids, argument from, 99 " * evidence from, 118 " summary of chapter on, 137 " variation of, 18 Hydroids, budding in, 11 '' heredity in, 113 "
.I
Intellectual differences between men and women, 242 Jgger on heredity, 41, 45, 85 Jurine on parthenogenesis, 57 Kipp on parthenogenesis, 59 Kirtland on variation of cherry,
IS9 Kiilreuter on hybrids of mirabiIis, 120 Latent transmission of sexual characters, 104, 117 Leckey on male and female mind, 269 Leptodora, parthenogenesis in,
57 Leydig on rotifera, 171 Life, definition of, 39 Life and memory, 38 Lion, hybrid from with tiger,
130 'Lizards, sexual differences in, 198 Lyell on variation of dog, 150 Leeuwenhoek, discovery of spermatozoa, 21 Lubbock on parthenogenesis,
57 Lucifer, sexual differences in, 184 Male mind progressive, 257 Male more eager than female,
230 Ibla, 182 Inheritance of a tendency, 88 Insects, sexual differences in,
189
Male and female types of character, 259 Male cell, properties of, 81 " functions of, 84
Ingex, Male cell, discovery of, 21 Itlale more variable than female, 160 Male fish originally varied, 198 Male more modified than female, 170 Manx cat, hybrid from, 128 Many individuals must vary together, 155, 277 Mammals, sexual differences in, 204 &hn,sexual differences in, 204 BIaiicliamp merino sheep, 299 3IcCrady on polar cells, 70 Yednss, parallel variation in, 303 Mice, hybrid, 131 . Mill on subjcction of women, 2G4 Milabilis, variation in, 120, 121 Mivart on Iiomology, 310 " on requisites of a theory of heredity, 86 " on saltatory evolution, 83, 297 'I on sudden variation of plants, 301 " on time needed forevo. lotion, 284 Molliensia, 197 ItIoquin-Tandon on variation of plants, 94 Moths, pnrtlienogenesis in, 69 Mule, 127, 134 Milller on dimorphism i n crustacea, 188 Natural selection, 12, 14, 275 Narwhal, 205
333
Nervous system, development of, 23 Neuroterus, 63 Niata cattle, hybrids from, 130 North British Review, argument against natural selection, 277 Notedelphys, 177 Oelacher on parthenogenesis, 67 Onitis, 193 Orange, bud-variation in, 147 Orcliestia, 187 Origin of sex, 314, 316 " of complicated organs, 281 Ornitliorinchus, 205 Ostracodu, 175 Ovist, 22 Ovum, complexity 'of, 87 " development of, 23 " function of, 84 " homologous with other cells, 23 " properties of, 8, 81 " structure of, 22 Ovum and male cell, difference in fnnction of, 53, 74 Ovum and male cell, honiologous, 17 Pander, 22 Pangenesis hypothesis, Dar. win's, 50, 121, 124 Pangenesis hypothesis, Qalton's objection to, 53 Pangenesis hypothesis, objections to, 53 Pangenesis hypothesis, remodelled, 80
Index.
334
Paradise birds, 201 Par:illel or analogous variation, 302 Parthenogenesis, 55 Peacock, variation in, 299 Perigenesis, 33 Pheasant, 203 Pigeon, direct modification of, 94 " modification of male, 218 Polar cells, 70 Polisli fowl, 225 Pollicipes, 181 Psyche, 61 Pterotnalus, 65 Reciprocal hybrids, 125 Reproduction, 19 " by immature animals, 43 " of bees, 9 " of marine animals, 9 " sexual and asexURI, 11,17,249 Requisites o€ theory of lieredity, 16 Reversion, 17 " by crossing, 132 '' Darwin on, 10, 115 " in dogs, 10 '' i n horses, 10 " and .Jilger's hypotheEis, 44 " two kinds, 133 I' in offspring of hybrids, 136 " of lost instincts, 135 Rhodites, 65
-
Rotifera, 66, 171 Ruminants, 206 Saltatoryevolution. 83,157,296 Salter on variation of strawberry, 91 Sapliirrina, 176 Sassafras in Europe, 90 Scalpellum, 181 Sclisfferon parthenogenesis, 56 Sea-gull, 92 Sea urchin, 66 Secondaiy sexual characters,
iGG Semper on Lyninaeus, 92 Sex of parent, influence of, 18, 123 Sexnnl differences in Agrion, 239 Alcippe, 183 ancliorella, 179 argonauta, 237 artliropoda, 173 barn:iclw, 179, 181 beetles, 191 braiicliippus, 173 call duck, 236 cave beetles, 237 cicada, 188 clndocera, 175 copepoda. 175, 177 crabs, 186 crayfish, 188 cryptophyalus, 183 cyclops, 175 gelassinius, 189 ibln, 182 insects, 188, 236 lernentoma, 176 locusticlae, 188
Index. Sexual differences in lucifer,
184 notodelphys, 177 p i t i s , 193 orcliestia, 1S7 ostracoda, 175 papilio turnus, 237 paradise birds, 237 pliasmidae, 236 pollicipes, 181 rotifera, 171 snphirrina, 176 scalpellum, 181 shrike, 237 social insects, 237 tanais, 188 Sexual dimorphism, 187 " reproduction, 11,17,143,
289 selection, 169,212 Sheep, hybrid, 129,131 " modification of male, "
222 '' variation in, 297 Siebold on parthenogenesis, 55 Simpson on hermaphroditism,
109 "
on latent transmission, 106
Sitana, 198 Smerinthus, 59 Solenobia, 61 Sow, parthenogenesis in, 67 Spurred hen, 210 Spathogaster, Species of large genera, 153 Speculations on heredity, 18 Spermatozoa, discovery of, 21 Spencer, definition of life, 38 '' on Irish elk, 287
826
Spike-horn deer, 298 Staphylinid3c, 194 Strongylocentrotus, 66 Sweet pea, hybrid, 132 Tanais, 188 Theory of heredity, 319,16 Tiger, liybrid,.l30 Transmission without fusion, 131 Turkey, variation of, 150 Uhler on polymorphism, 238 Variability, how caused, 82 " of.offspring of hybrids, 122 " of sexual characters, 154 Variations, 13, 17 cau'jes of, 140,275,293 caused by climate, 142. " change of food,142 " crossing, 119 " excessof food, 143 correllated, 157 of exceptional parts, 153 of homologous parts, 158 of hybrids, 18 law of equable, 154 of male butterflies, 195 of organisms produced sexually, 143,249 parallel or analogous, 302 and sexual reproduction, 249 of species of large genera, 153 summary of chapter on, 161 Viragines, 105 Von Baer, 22
336
Index.
birds, 208 on polymorphism, 238 Walrus, 205 Walsh, law of equable variation, 154 Wart hog, 205 Waterton on hen with male characters, 106 Weismann on parthenogenesis, "
1
brid willow, 124 Wilson on parthenogenesis, 66 Wollff on heredity, 22 Xiphophorus, 198 Parrell on removal of oviduct,
''
105 on variation of dog, 150
