The Descent of Man and Selection in Relation to Sex
by
Charles Darwin
Part 5 out of 17
susceptible to any change of climate: in fact, take them away from their
island homes, and they are almost certain to die, and that independently of
diet or extraneous influences." He further states that the inhabitants of
the Valley of Nepal, which is extremely hot in summer, and also the various
hill-tribes of India, suffer from dysentery and fever when on the plains;
and they die if they attempt to pass the whole year there.
We thus see that many of the wilder races of man are apt to suffer much in
health when subjected to changed conditions or habits of life, and not
exclusively from being transported to a new climate. Mere alterations in
habits, which do not appear injurious in themselves, seem to have this same
effect; and in several cases the children are particularly liable to
suffer. It has often been said, as Mr. Macnamara remarks, that man can
resist with impunity the greatest diversities of climate and other changes;
but this is true only of the civilised races. Man in his wild condition
seems to be in this respect almost as susceptible as his nearest allies,
the anthropoid apes, which have never yet survived long, when removed from
their native country.
Lessened fertility from changed conditions, as in the case of the
Tasmanians, Maories, Sandwich Islanders, and apparently the Australians, is
still more interesting than their liability to ill-health and death; for
even a slight degree of infertility, combined with those other causes which
tend to check the increase of every population, would sooner or later lead
to extinction. The diminution of fertility may be explained in some cases
by the profligacy of the women (as until lately with the Tahitians), but
Mr. Fenton has shewn that this explanation by no means suffices with the
New Zealanders, nor does it with the Tasmanians.
In the paper above quoted, Mr. Macnamara gives reasons for believing that
the inhabitants of districts subject to malaria are apt to be sterile; but
this cannot apply in several of the above cases. Some writers have
suggested that the aborigines of islands have suffered in fertility and
health from long continued inter-breeding; but in the above cases
infertility has coincided too closely with the arrival of Europeans for us
to admit this explanation. Nor have we at present any reason to believe
that man is highly sensitive to the evil effects of inter-breeding,
especially in areas so large as New Zealand, and the Sandwich archipelago
with its diversified stations. On the contrary, it is known that the
present inhabitants of Norfolk Island are nearly all cousins or near
relations, as are the Todas in India, and the inhabitants of some of the
Western Islands of Scotland; and yet they seem not to have suffered in
fertility. (45. On the close relationship of the Norfolk Islanders, Sir
W. Denison, 'Varieties of Vice-Regal Life,' vol. i. 1870, p. 410. For the
Todas, see Col. Marshall's work 1873, p. 110. For the Western Islands of
Scotland, Dr. Mitchell, 'Edinburgh Medical Journal,' March to June, 1865.)
A much more probable view is suggested by the analogy of the lower animals.
The reproductive system can be shewn to be susceptible to an extraordinary
degree (though why we know not) to changed conditions of life; and this
susceptibility leads both to beneficial and to evil results. A large
collection of facts on this subject is given in chap. xviii. of vol. ii. of
my 'Variation of Animals and Plants under Domestication,' I can here give
only the briefest abstract; and every one interested in the subject may
consult the above work. Very slight changes increase the health, vigour,
and fertility of most or all organic beings, whilst other changes are known
to render a large number of animals sterile. One of the most familiar
cases, is that of tamed elephants not breeding in India; though they often
breed in Ava, where the females are allowed to roam about the forests to
some extent, and are thus placed under more natural conditions. The case
of various American monkeys, both sexes of which have been kept for many
years together in their own countries, and yet have very rarely or never
bred, is a more apposite instance, because of their relationship to man.
It is remarkable how slight a change in the conditions often induces
sterility in a wild animal when captured; and this is the more strange as
all our domesticated animals have become more fertile than they were in a
state of nature; and some of them can resist the most unnatural conditions
with undiminished fertility. (46. For the evidence on this head, see
'Variation of Animals,' etc., vol. ii. p. 111.) Certain groups of animals
are much more liable than others to be affected by captivity; and generally
all the species of the same group are affected in the same manner. But
sometimes a single species in a group is rendered sterile, whilst the
others are not so; on the other hand, a single species may retain its
fertility whilst most of the others fail to breed. The males and females
of some species when confined, or when allowed to live almost, but not
quite free, in their native country, never unite; others thus circumstanced
frequently unite but never produce offspring; others again produce some
offspring, but fewer than in a state of nature; and as bearing on the above
cases of man, it is important to remark that the young are apt to be weak
and sickly, or malformed, and to perish at an early age.
Seeing how general is this law of the susceptibility of the reproductive
system to changed conditions of life, and that it holds good with our
nearest allies, the Quadrumana, I can hardly doubt that it applies to man
in his primeval state. Hence if savages of any race are induced suddenly
to change their habits of life, they become more or less sterile, and their
young offspring suffer in health, in the same manner and from the same
cause, as do the elephant and hunting-leopard in India, many monkeys in
America, and a host of animals of all kinds, on removal from their natural
conditions.
We can see why it is that aborigines, who have long inhabited islands, and
who must have been long exposed to nearly uniform conditions, should be
specially affected by any change in their habits, as seems to be the case.
Civilised races can certainly resist changes of all kinds far better than
savages; and in this respect they resemble domesticated animals, for though
the latter sometimes suffer in health (for instance European dogs in
India), yet they are rarely rendered sterile, though a few such instances
have been recorded. (47. 'Variation of Animals,' etc., vol. ii. p. 16.)
The immunity of civilised races and domesticated animals is probably due to
their having been subjected to a greater extent, and therefore having grown
somewhat more accustomed, to diversified or varying conditions, than the
majority of wild animals; and to their having formerly immigrated or been
carried from country to country, and to different families or sub-races
having inter-crossed. It appears that a cross with civilised races at once
gives to an aboriginal race an immunity from the evil consequences of
changed conditions. Thus the crossed offspring from the Tahitians and
English, when settled in Pitcairn Island, increased so rapidly that the
island was soon overstocked; and in June 1856 they were removed to Norfolk
Island. They then consisted of 60 married persons and 134 children, making
a total of 194. Here they likewise increased so rapidly, that although
sixteen of them returned to Pitcairn Island in 1859, they numbered in
January 1868, 300 souls; the males and females being in exactly equal
numbers. What a contrast does this case present with that of the
Tasmanians; the Norfolk Islanders INCREASED in only twelve and a half years
from 194 to 300; whereas the Tasmanians DECREASED during fifteen years from
120 to 46, of which latter number only ten were children. (48. These
details are taken from 'The Mutineers of the "Bounty,"' by Lady Belcher,
1870; and from 'Pitcairn Island,' ordered to be printed by the House of
Commons, May 29, 1863. The following statements about the Sandwich
Islanders are from the 'Honolulu Gazette,' and from Mr. Coan.)
So again in the interval between the census of 1866 and 1872 the natives of
full blood in the Sandwich Islands decreased by 8081, whilst the half-
castes, who are believed to be healthier, increased by 847; but I do not
know whether the latter number includes the offspring from the half-castes,
or only the half-castes of the first generation.
The cases which I have here given all relate to aborigines, who have been
subjected to new conditions as the result of the immigration of civilised
men. But sterility and ill-health would probably follow, if savages were
compelled by any cause, such as the inroad of a conquering tribe, to desert
their homes and to change their habits. It is an interesting circumstance
that the chief check to wild animals becoming domesticated, which implies
the power of their breeding freely when first captured, and one chief check
to wild men, when brought into contact with civilisation, surviving to form
a civilised race, is the same, namely, sterility from changed conditions of
life.
Finally, although the gradual decrease and ultimate extinction of the races
of man is a highly complex problem, depending on many causes which differ
in different places and at different times; it is the same problem as that
presented by the extinction of one of the higher animals--of the fossil
horse, for instance, which disappeared from South America, soon afterwards
to be replaced, within the same districts, by countless troups of the
Spanish horse. The New Zealander seems conscious of this parallelism, for
he compares his future fate with that of the native rat now almost
exterminated by the European rat. Though the difficulty is great to our
imagination, and really great, if we wish to ascertain the precise causes
and their manner of action, it ought not to be so to our reason, as long as
we keep steadily in mind that the increase of each species and each race is
constantly checked in various ways; so that if any new check, even a slight
one, be superadded, the race will surely decrease in number; and decreasing
numbers will sooner or later lead to extinction; the end, in most cases,
being promptly determined by the inroads of conquering tribes.
ON THE FORMATION OF THE RACES OF MAN.
In some cases the crossing of distinct races has led to the formation of a
new race. The singular fact that the Europeans and Hindoos, who belong to
the same Aryan stock, and speak a language fundamentally the same, differ
widely in appearance, whilst Europeans differ but little from Jews, who
belong to the Semitic stock, and speak quite another language, has been
accounted for by Broca (49. 'On Anthropology,' translation,
'Anthropological Review,' Jan. 1868, p. 38.), through certain Aryan
branches having been largely crossed by indigenous tribes during their wide
diffusion. When two races in close contact cross, the first result is a
heterogeneous mixture: thus Mr. Hunter, in describing the Santali or hill-
tribes of India, says that hundreds of imperceptible gradations may be
traced "from the black, squat tribes of the mountains to the tall olive-
coloured Brahman, with his intellectual brow, calm eyes, and high but
narrow head"; so that it is necessary in courts of justice to ask the
witnesses whether they are Santalis or Hindoos. (50. 'The Annals of Rural
Bengal,' 1868, p. 134.) Whether a heterogeneous people, such as the
inhabitants of some of the Polynesian islands, formed by the crossing of
two distinct races, with few or no pure members left, would ever become
homogeneous, is not known from direct evidence. But as with our
domesticated animals, a cross-breed can certainly be fixed and made uniform
by careful selection (51. 'The Variation of Animals and Plants under
Domestication,' vol. ii. p. 95.) in the course of a few generations, we may
infer that the free intercrossing of a heterogeneous mixture during a long
descent would supply the place of selection, and overcome any tendency to
reversion; so that the crossed race would ultimately become homogeneous,
though it might not partake in an equal degree of the characters of the two
parent-races.
Of all the differences between the races of man, the colour of the skin is
the most conspicuous and one of the best marked. It was formerly thought
that differences of this kind could be accounted for by long exposure to
different climates; but Pallas first shewed that this is not tenable, and
he has since been followed by almost all anthropologists. (52. Pallas,
'Act. Acad. St. Petersburg,' 1780, part ii. p. 69. He was followed by
Rudolphi, in his 'Beytrage zur Anthropologie,' 1812. An excellent summary
of the evidence is given by Godron, 'De l'Espece,' 1859, vol. ii. p. 246,
etc.) This view has been rejected chiefly because the distribution of the
variously coloured races, most of whom must have long inhabited their
present homes, does not coincide with corresponding differences of climate.
Some little weight may be given to such cases as that of the Dutch
families, who, as we hear on excellent authority (53. Sir Andrew Smith, as
quoted by Knox, 'Races of Man,' 1850, p. 473.), have not undergone the
least change of colour after residing for three centuries in South Africa.
An argument on the same side may likewise be drawn from the uniform
appearance in various parts of the world of gipsies and Jews, though the
uniformity of the latter has been somewhat exaggerated. (54. See De
Quatrefages on this head, 'Revue des Cours Scientifiques,' Oct. 17, 1868,
p. 731.) A very damp or a very dry atmosphere has been supposed to be more
influential in modifying the colour of the skin than mere heat; but as
D'Orbigny in South America, and Livingstone in Africa, arrived at
diametrically opposite conclusions with respect to dampness and dryness,
any conclusion on this head must be considered as very doubtful. (55.
Livingstone's 'Travels and Researches in S. Africa,' 1857, pp. 338, 339.
D'Orbigny, as quoted by Godron, 'De l'Espece,' vol. ii. p. 266.)
Various facts, which I have given elsewhere, prove that the colour of the
skin and hair is sometimes correlated in a surprising manner with a
complete immunity from the action of certain vegetable poisons, and from
the attacks of certain parasites. Hence it occurred to me, that negroes
and other dark races might have acquired their dark tints by the darker
individuals escaping from the deadly influence of the miasma of their
native countries, during a long series of generations.
I afterwards found that this same idea had long ago occurred to Dr. Wells.
(56. See a paper read before the Royal Soc. in 1813, and published in his
Essays in 1818. I have given an account of Dr. Wells' views in the
Historical Sketch (p. xvi.) to my 'Origin of Species.' Various cases of
colour correlated with constitutional peculiarities are given in my
'Variation of Animals and Plants under Domestication,' vol. ii. pp. 227,
335.) It has long been known that negroes, and even mulattoes, are almost
completely exempt from the yellow-fever, so destructive in tropical
America. (57. See, for instance, Nott and Gliddon, 'Types of Mankind,' p.
68.) They likewise escape to a large extent the fatal intermittent fevers,
that prevail along at least 2600 miles of the shores of Africa, and which
annually cause one-fifth of the white settlers to die, and another fifth to
return home invalided. (58. Major Tulloch, in a paper read before the
Statistical Society, April 20, 1840, and given in the 'Athenaeum,' 1840, p.
353.) This immunity in the negro seems to be partly inherent, depending on
some unknown peculiarity of constitution, and partly the result of
acclimatisation. Pouchet (59. 'The Plurality of the Human Race'
(translat.), 1864, p. 60.) states that the negro regiments recruited near
the Soudan, and borrowed from the Viceroy of Egypt for the Mexican war,
escaped the yellow-fever almost equally with the negroes originally brought
from various parts of Africa and accustomed to the climate of the West
Indies. That acclimatisation plays a part, is shewn by the many cases in
which negroes have become somewhat liable to tropical fevers, after having
resided for some time in a colder climate. (60. Quatrefages, 'Unite de
l'Espece Humaine,' 1861, p. 205. Waitz, 'Introduction to Anthropology,'
translat., vol. i. 1863, p. 124. Livingstone gives analogous cases in his
'Travels.') The nature of the climate under which the white races have
long resided, likewise has some influence on them; for during the fearful
epidemic of yellow fever in Demerara during 1837, Dr. Blair found that the
death-rate of the immigrants was proportional to the latitude of the
country whence they had come. With the negro the immunity, as far as it is
the result of acclimatisation, implies exposure during a prodigious length
of time; for the aborigines of tropical America who have resided there from
time immemorial, are not exempt from yellow fever; and the Rev. H.B.
Tristram states, that there are districts in Northern Africa which the
native inhabitants are compelled annually to leave, though the negroes can
remain with safety.
That the immunity of the negro is in any degree correlated with the colour
of his skin is a mere conjecture: it may be correlated with some
difference in his blood, nervous system, or other tissues. Nevertheless,
from the facts above alluded to, and from some connection apparently
existing between complexion and a tendency to consumption, the conjecture
seemed to me not improbable. Consequently I endeavoured, with but little
success (61. In the spring of 1862 I obtained permission from the
Director-General of the Medical department of the Army, to transmit to the
surgeons of the various regiments on foreign service a blank table, with
the following appended remarks, but I have received no returns. "As
several well-marked cases have been recorded with our domestic animals of a
relation between the colour of the dermal appendages and the constitution;
and it being notorious that there is some limited degree of relation
between the colour of the races of man and the climate inhabited by them;
the following investigation seems worth consideration. Namely, whether
there is any relation in Europeans between the colour of their hair, and
their liability to the diseases of tropical countries. If the surgeons of
the several regiments, when stationed in unhealthy tropical districts,
would be so good as first to count, as a standard of comparison, how many
men, in the force whence the sick are drawn, have dark and light-coloured
hair, and hair of intermediate or doubtful tints; and if a similar account
were kept by the same medical gentlemen, of all the men who suffered from
malarious and yellow fevers, or from dysentery, it would soon be apparent,
after some thousand cases had been tabulated, whether there exists any
relation between the colour of the hair and constitutional liability to
tropical diseases. Perhaps no such relation would be discovered, but the
investigation is well worth making. In case any positive result were
obtained, it might be of some practical use in selecting men for any
particular service. Theoretically the result would be of high interest, as
indicating one means by which a race of men inhabiting from a remote period
an unhealthy tropical climate, might have become dark-coloured by the
better preservation of dark-haired or dark-complexioned individuals during
a long succession of generations."), to ascertain how far it holds good.
The late Dr. Daniell, who had long lived on the West Coast of Africa, told
me that he did not believe in any such relation. He was himself unusually
fair, and had withstood the climate in a wonderful manner. When he first
arrived as a boy on the coast, an old and experienced negro chief predicted
from his appearance that this would prove the case. Dr. Nicholson, of
Antigua, after having attended to this subject, writes to me that dark-
coloured Europeans escape the yellow fever more than those that are light-
coloured. Mr. J.M. Harris altogether denies that Europeans with dark hair
withstand a hot climate better than other men: on the contrary, experience
has taught him in making a selection of men for service on the coast of
Africa, to choose those with red hair. (62. 'Anthropological Review,'
Jan. 1866, p. xxi. Dr. Sharpe also says, with respect to India ('Man a
Special Creation,' 1873, p. 118), "that it has been noticed by some medical
officers that Europeans with light hair and florid complexions suffer less
from diseases of tropical countries than persons with dark hair and sallow
complexions; and, so far as I know, there appear to be good grounds for
this remark." On the other hand, Mr. Heddle, of Sierra Leone, "who has had
more clerks killed under him than any other man," by the climate of the
West African Coast (W. Reade, 'African Sketch Book,' vol. ii. p. 522),
holds a directly opposite view, as does Capt. Burton.) As far, therefore,
as these slight indications go, there seems no foundation for the
hypothesis, that blackness has resulted from the darker and darker
individuals having survived better during long exposure to fever-generating
miasma.
Dr. Sharpe remarks (63. 'Man a Special Creation,' 1873, p. 119.), that a
tropical sun, which burns and blisters a white skin, does not injure a
black one at all; and, as he adds, this is not due to habit in the
individual, for children only six or eight months old are often carried
about naked, and are not affected. I have been assured by a medical man,
that some years ago during each summer, but not during the winter, his
hands became marked with light brown patches, like, although larger than
freckles, and that these patches were never affected by sun-burning, whilst
the white parts of his skin have on several occasions been much inflamed
and blistered. With the lower animals there is, also, a constitutional
difference in liability to the action of the sun between those parts of the
skin clothed with white hair and other parts. (64. 'Variation of Animals
and Plants under Domestication,' vol. ii. pp. 336, 337.) Whether the
saving of the skin from being thus burnt is of sufficient importance to
account for a dark tint having been gradually acquired by man through
natural selection, I am unable to judge. If it be so, we should have to
assume that the natives of tropical America have lived there for a much
shorter time than the Negroes in Africa, or the Papuans in the southern
parts of the Malay archipelago, just as the lighter-coloured Hindoos have
resided in India for a shorter time than the darker aborigines of the
central and southern parts of the peninsula.
Although with our present knowledge we cannot account for the differences
of colour in the races of man, through any advantage thus gained, or from
the direct action of climate; yet we must not quite ignore the latter
agency, for there is good reason to believe that some inherited effect is
thus produced. (65. See, for instance, Quatrefages ('Revue des Cours
Scientifiques,' Oct. 10, 1868, p. 724) on the effects of residence in
Abyssinia and Arabia, and other analogous cases. Dr. Rolle ('Der Mensch,
seine Abstammung,' etc., 1865, s. 99) states, on the authority of Khanikof,
that the greater number of German families settled in Georgia, have
acquired in the course of two generations dark hair and eyes. Mr. D.
Forbes informs me that the Quichuas in the Andes vary greatly in colour,
according to the position of the valleys inhabited by them.)
We have seen in the second chapter that the conditions of life affect the
development of the bodily frame in a direct manner, and that the effects
are transmitted. Thus, as is generally admitted, the European settlers in
the United States undergo a slight but extraordinary rapid change of
appearance. Their bodies and limbs become elongated; and I hear from Col.
Bernys that during the late war in the United States, good evidence was
afforded of this fact by the ridiculous appearance presented by the German
regiments, when dressed in ready-made clothes manufactured for the American
market, and which were much too long for the men in every way. There is,
also, a considerable body of evidence shewing that in the Southern States
the house-slaves of the third generation present a markedly different
appearance from the field-slaves. (66. Harlan, 'Medical Researches,' p.
532. Quatrefages ('Unite de l'Espece Humaine,' 1861, p. 128) has collected
much evidence on this head.)
If, however, we look to the races of man as distributed over the world, we
must infer that their characteristic differences cannot be accounted for by
the direct action of different conditions of life, even after exposure to
them for an enormous period of time. The Esquimaux live exclusively on
animal food; they are clothed in thick fur, and are exposed to intense cold
and to prolonged darkness; yet they do not differ in any extreme degree
from the inhabitants of Southern China, who live entirely on vegetable
food, and are exposed almost naked to a hot, glaring climate. The
unclothed Fuegians live on the marine productions of their inhospitable
shores; the Botocudos of Brazil wander about the hot forests of the
interior and live chiefly on vegetable productions; yet these tribes
resemble each other so closely that the Fuegians on board the "Beagle" were
mistaken by some Brazilians for Botocudos. The Botocudos again, as well as
the other inhabitants of tropical America, are wholly different from the
Negroes who inhabit the opposite shores of the Atlantic, are exposed to a
nearly similar climate, and follow nearly the same habits of life.
Nor can the differences between the races of man be accounted for by the
inherited effects of the increased or decreased use of parts, except to a
quite insignificant degree. Men who habitually live in canoes, may have
their legs somewhat stunted; those who inhabit lofty regions may have their
chests enlarged; and those who constantly use certain sense-organs may have
the cavities in which they are lodged somewhat increased in size, and their
features consequently a little modified. With civilised nations, the
reduced size of the jaws from lessened use--the habitual play of different
muscles serving to express different emotions--and the increased size of
the brain from greater intellectual activity, have together produced a
considerable effect on their general appearance when compared with savages.
(67. See Prof. Schaaffhausen, translat., in 'Anthropological Review,' Oct.
1868, p. 429.) Increased bodily stature, without any corresponding
increase in the size of the brain, may (judging from the previously adduced
case of rabbits), have given to some races an elongated skull of the
dolichocephalic type.
Lastly, the little-understood principle of correlated development has
sometimes come into action, as in the case of great muscular development
and strongly projecting supra-orbital ridges. The colour of the skin and
hair are plainly correlated, as is the texture of the hair with its colour
in the Mandans of North America. (68. Mr. Catlin states ('N. American
Indians,' 3rd ed., 1842, vol. i. p. 49) that in the whole tribe of the
Mandans, about one in ten or twelve of the members, of all ages and both
sexes, have bright silvery grey hair, which is hereditary. Now this hair
is as coarse and harsh as that of a horse's mane, whilst the hair of other
colours is fine and soft.) The colour also of the skin, and the odour
emitted by it, are likewise in some manner connected. With the breeds of
sheep the number of hairs within a given space and the number of excretory
pores are related. (69. On the odour of the skin, Godron, 'Sur l'Espece,'
tom. ii. p. 217. On the pores in the skin, Dr. Wilckens, 'Die Aufgaben der
Landwirth. Zootechnik,' 1869, s. 7.) If we may judge from the analogy of
our domesticated animals, many modifications of structure in man probably
come under this principle of correlated development.
We have now seen that the external characteristic differences between the
races of man cannot be accounted for in a satisfactory manner by the direct
action of the conditions of life, nor by the effects of the continued use
of parts, nor through the principle of correlation. We are therefore led
to enquire whether slight individual differences, to which man is eminently
liable, may not have been preserved and augmented during a long series of
generations through natural selection. But here we are at once met by the
objection that beneficial variations alone can be thus preserved; and as
far as we are enabled to judge, although always liable to err on this head,
none of the differences between the races of man are of any direct or
special service to him. The intellectual and moral or social faculties
must of course be excepted from this remark. The great variability of all
the external differences between the races of man, likewise indicates that
they cannot be of much importance; for if important, they would long ago
have been either fixed and preserved, or eliminated. In this respect man
resembles those forms, called by naturalists protean or polymorphic, which
have remained extremely variable, owing, as it seems, to such variations
being of an indifferent nature, and to their having thus escaped the action
of natural selection.
We have thus far been baffled in all our attempts to account for the
differences between the races of man; but there remains one important
agency, namely Sexual Selection, which appears to have acted powerfully on
man, as on many other animals. I do not intend to assert that sexual
selection will account for all the differences between the races. An
unexplained residuum is left, about which we can only say, in our
ignorance, that as individuals are continually born with, for instance,
heads a little rounder or narrower, and with noses a little longer or
shorter, such slight differences might become fixed and uniform, if the
unknown agencies which induced them were to act in a more constant manner,
aided by long-continued intercrossing. Such variations come under the
provisional class, alluded to in our second chapter, which for want of a
better term are often called spontaneous. Nor do I pretend that the
effects of sexual selection can be indicated with scientific precision; but
it can be shewn that it would be an inexplicable fact if man had not been
modified by this agency, which appears to have acted powerfully on
innumerable animals. It can further be shewn that the differences between
the races of man, as in colour, hairiness, form of features, etc., are of a
kind which might have been expected to come under the influence of sexual
selection. But in order to treat this subject properly, I have found it
necessary to pass the whole animal kingdom in review. I have therefore
devoted to it the Second Part of this work. At the close I shall return to
man, and, after attempting to shew how far he has been modified through
sexual selection, will give a brief summary of the chapters in this First
Part.
NOTE ON THE RESEMBLANCES AND DIFFERENCES IN THE STRUCTURE AND THE
DEVELOPMENT OF THE BRAIN IN MAN AND APES BY PROFESSOR HUXLEY, F.R.S.
The controversy respecting the nature and the extent of the differences in
the structure of the brain in man and the apes, which arose some fifteen
years ago, has not yet come to an end, though the subject matter of the
dispute is, at present, totally different from what it was formerly. It
was originally asserted and re-asserted, with singular pertinacity, that
the brain of all the apes, even the highest, differs from that of man, in
the absence of such conspicuous structures as the posterior lobes of the
cerebral hemispheres, with the posterior cornu of the lateral ventricle and
the hippocampus minor, contained in those lobes, which are so obvious in
man.
But the truth that the three structures in question are as well developed
in apes' as in human brains, or even better; and that it is characteristic
of all the Primates (if we exclude the Lemurs) to have these parts well
developed, stands at present on as secure a basis as any proposition in
comparative anatomy. Moreover, it is admitted by every one of the long
series of anatomists who, of late years, have paid special attention to the
arrangement of the complicated sulci and gyri which appear upon the surface
of the cerebral hemispheres in man and the higher apes, that they are
disposed after the very same pattern in him, as in them. Every principal
gyrus and sulcus of a chimpanzee's brain is clearly represented in that of
a man, so that the terminology which applies to the one answers for the
other. On this point there is no difference of opinion. Some years since,
Professor Bischoff published a memoir (70. 'Die Grosshirn-Windungen des
Menschen;' 'Abhandlungen der K. Bayerischen Akademie,' B. x. 1868.) on the
cerebral convolutions of man and apes; and as the purpose of my learned
colleague was certainly not to diminish the value of the differences
between apes and men in this respect, I am glad to make a citation from
him.
"That the apes, and especially the orang, chimpanzee and gorilla, come very
close to man in their organisation, much nearer than to any other animal,
is a well known fact, disputed by nobody. Looking at the matter from the
point of view of organisation alone, no one probably would ever have
disputed the view of Linnaeus, that man should be placed, merely as a
peculiar species, at the head of the mammalia and of those apes. Both
shew, in all their organs, so close an affinity, that the most exact
anatomical investigation is needed in order to demonstrate those
differences which really exist. So it is with the brains. The brains of
man, the orang, the chimpanzee, the gorilla, in spite of all the important
differences which they present, come very close to one another" (loc. cit.
p. 101).
There remains, then, no dispute as to the resemblance in fundamental
characters, between the ape's brain and man's: nor any as to the
wonderfully close similarity between the chimpanzee, orang and man, in even
the details of the arrangement of the gyri and sulci of the cerebral
hemispheres. Nor, turning to the differences between the brains of the
highest apes and that of man, is there any serious question as to the
nature and extent of these differences. It is admitted that the man's
cerebral hemispheres are absolutely and relatively larger than those of the
orang and chimpanzee; that his frontal lobes are less excavated by the
upward protrusion of the roof of the orbits; that his gyri and sulci are,
as a rule, less symmetrically disposed, and present a greater number of
secondary plications. And it is admitted that, as a rule, in man, the
temporo-occipital or "external perpendicular" fissure, which is usually so
strongly marked a feature of the ape's brain is but faintly marked. But it
is also clear, that none of these differences constitutes a sharp
demarcation between the man's and the ape's brain. In respect to the
external perpendicular fissure of Gratiolet, in the human brain for
instance, Professor Turner remarks: (71. 'Convolutions of the Human
Cerebrum Topographically Considered,' 1866, p. 12.)
"In some brains it appears simply as an indentation of the margin of the
hemisphere, but, in others, it extends for some distance more or less
transversely outwards. I saw it in the right hemisphere of a female brain
pass more than two inches outwards; and on another specimen, also the right
hemisphere, it proceeded for four-tenths of an inch outwards, and then
extended downwards, as far as the lower margin of the outer surface of the
hemisphere. The imperfect definition of this fissure in the majority of
human brains, as compared with its remarkable distinctness in the brain of
most Quadrumana, is owing to the presence, in the former, of certain
superficial, well marked, secondary convolutions which bridge it over and
connect the parietal with the occipital lobe. The closer the first of
these bridging gyri lies to the longitudinal fissure, the shorter is the
external parieto-occipital fissure" (loc. cit. p. 12).
The obliteration of the external perpendicular fissure of Gratiolet,
therefore, is not a constant character of the human brain. On the other
hand, its full development is not a constant character of the higher ape's
brain. For, in the chimpanzee, the more or less extensive obliteration of
the external perpendicular sulcus by "bridging convolutions," on one side
or the other, has been noted over and over again by Prof. Rolleston, Mr.
Marshall, M. Broca and Professor Turner. At the conclusion of a special
paper on this subject the latter writes: (72. Notes more especially on
the bridging convolutions in the Brain of the Chimpanzee, 'Proceedings of
the Royal Society of Edinburgh,' 1865-6.)
"The three specimens of the brain of a chimpanzee, just described, prove,
that the generalisation which Gratiolet has attempted to draw of the
complete absence of the first connecting convolution and the concealment of
the second, as essentially characteristic features in the brain of this
animal, is by no means universally applicable. In only one specimen did
the brain, in these particulars, follow the law which Gratiolet has
expressed. As regards the presence of the superior bridging convolution, I
am inclined to think that it has existed in one hemisphere, at least, in a
majority of the brains of this animal which have, up to this time, been
figured or described. The superficial position of the second bridging
convolution is evidently less frequent, and has as yet, I believe, only
been seen in the brain (A) recorded in this communication. The
asymmetrical arrangement in the convolutions of the two hemispheres, which
previous observers have referred to in their descriptions, is also well
illustrated in these specimens" (pp. 8, 9).
Even were the presence of the temporo-occipital, or external perpendicular,
sulcus, a mark of distinction between the higher apes and man, the value of
such a distinctive character would be rendered very doubtful by the
structure of the brain in the Platyrrhine apes. In fact, while the
temporo-occipital is one of the most constant of sulci in the Catarrhine,
or Old World, apes, it is never very strongly developed in the New World
apes; it is absent in the smaller Platyrrhini; rudimentary in Pithecia (73.
Flower, 'On the Anatomy of Pithecia Monachus,' 'Proceedings of the
Zoological Society,' 1862.); and more or less obliterated by bridging
convolutions in Ateles.
A character which is thus variable within the limits of a single group can
have no great taxonomic value.
It is further established, that the degree of asymmetry of the convolution
of the two sides in the human brain is subject to much individual
variation; and that, in those individuals of the Bushman race who have been
examined, the gyri and sulci of the two hemispheres are considerably less
complicated and more symmetrical than in the European brain, while, in some
individuals of the chimpanzee, their complexity and asymmetry become
notable. This is particularly the case in the brain of a young male
chimpanzee figured by M. Broca. ('L'ordre des Primates,' p. 165, fig. 11.)
Again, as respects the question of absolute size, it is established that
the difference between the largest and the smallest healthy human brain is
greater than the difference between the smallest healthy human brain and
the largest chimpanzee's or orang's brain.
Moreover, there is one circumstance in which the orang's and chimpanzee's
brains resemble man's, but in which they differ from the lower apes, and
that is the presence of two corpora candicantia--the Cynomorpha having but
one.
In view of these facts I do not hesitate in this year 1874, to repeat and
insist upon the proposition which I enunciated in 1863: (74. 'Man's Place
in Nature,' p. 102.)
"So far as cerebral structure goes, therefore, it is clear that man differs
less from the chimpanzee or the orang, than these do even from the monkeys,
and that the difference between the brain of the chimpanzee and of man is
almost insignificant when compared with that between the chimpanzee brain
and that of a Lemur."
In the paper to which I have referred, Professor Bischoff does not deny the
second part of this statement, but he first makes the irrelevant remark
that it is not wonderful if the brains of an orang and a Lemur are very
different; and secondly, goes on to assert that, "If we successively
compare the brain of a man with that of an orang; the brain of this with
that of a chimpanzee; of this with that of a gorilla, and so on of a
Hylobates, Semnopithecus, Cynocephalus, Cercopithecus, Macacus, Cebus,
Callithrix, Lemur, Stenops, Hapale, we shall not meet with a greater, or
even as great a, break in the degree of development of the convolutions, as
we find between the brain of a man and that of an orang or chimpanzee."
To which I reply, firstly, that whether this assertion be true or false, it
has nothing whatever to do with the proposition enunciated in 'Man's Place
in Nature,' which refers not to the development of the convolutions alone,
but to the structure of the whole brain. If Professor Bischoff had taken
the trouble to refer to p. 96 of the work he criticises, in fact, he would
have found the following passage: "And it is a remarkable circumstance
that though, so far as our present knowledge extends, there IS one true
structural break in the series of forms of Simian brains, this hiatus does
not lie between man and the manlike apes, but between the lower and the
lowest Simians, or in other words, between the Old and New World apes and
monkeys and the Lemurs. Every Lemur which has yet been examined, in fact,
has its cerebellum partially visible from above; and its posterior lobe,
with the contained posterior cornu and hippocampus minor, more or less
rudimentary. Every marmoset, American monkey, Old World monkey, baboon or
manlike ape, on the contrary, has its cerebellum entirely hidden,
posteriorly, by the cerebral lobes, and possesses a large posterior cornu
with a well-developed hippocampus minor."
This statement was a strictly accurate account of what was known when it
was made; and it does not appear to me to be more than apparently weakened
by the subsequent discovery of the relatively small development of the
posterior lobes in the Siamang and in the Howling monkey. Notwithstanding
the exceptional brevity of the posterior lobes in these two species, no one
will pretend that their brains, in the slightest degree, approach those of
the Lemurs. And if, instead of putting Hapale out of its natural place, as
Professor Bischoff most unaccountably does, we write the series of animals
he has chosen to mention as follows: Homo, Pithecus, Troglodytes,
Hylobates, Semnopithecus, Cynocephalus, Cercopithecus, Macacus, Cebus,
Callithrix, Hapale, Lemur, Stenops, I venture to reaffirm that the great
break in this series lies between Hapale and Lemur, and that this break is
considerably greater than that between any other two terms of that series.
Professor Bischoff ignores the fact that long before he wrote, Gratiolet
had suggested the separation of the Lemurs from the other Primates on the
very ground of the difference in their cerebral characters; and that
Professor Flower had made the following observations in the course of his
description of the brain of the Javan Loris: (75. 'Transactions of the
Zoological Society,' vol. v. 1862.)
"And it is especially remarkable that, in the development of the posterior
lobes, there is no approximation to the Lemurine, short hemisphered brain,
in those monkeys which are commonly supposed to approach this family in
other respects, viz. the lower members of the Platyrrhine group."
So far as the structure of the adult brain is concerned, then, the very
considerable additions to our knowledge, which have been made by the
researches of so many investigators, during the past ten years, fully
justify the statement which I made in 1863. But it has been said, that,
admitting the similarity between the adult brains of man and apes, they are
nevertheless, in reality, widely different, because they exhibit
fundamental differences in the mode of their development. No one would be
more ready than I to admit the force of this argument, if such fundamental
differences of development really exist. But I deny that they do exist.
On the contrary, there is a fundamental agreement in the development of the
brain in men and apes.
Gratiolet originated the statement that there is a fundamental difference
in the development of the brains of apes and that of man--consisting in
this; that, in the apes, the sulci which first make their appearance are
situated on the posterior region of the cerebral hemispheres, while, in the
human foetus, the sulci first become visible on the frontal lobes. (76.
"Chez tous les singes, les plis posterieurs se developpent les premiers;
les plis anterieurs se developpent plus tard, aussi la vertebre occipitale
et la parietale sont-elles relativement tres-grandes chez le foetus.
L'Homme presente une exception remarquable quant a l'epoque de l'apparition
des plis frontaux, qui sont les premiers indiques; mais le developpement
general du lobe frontal, envisage seulement par rapport a son volume, suit
les memes lois que dans les singes:" Gratiolet, 'Memoire sur les plis
cerebres de l'Homme et des Primateaux,' p. 39, Tab. iv, fig. 3.)
This general statement is based upon two observations, the one of a Gibbon
almost ready to be born, in which the posterior gyri were "well developed,"
while those of the frontal lobes were "hardly indicated" (77. Gratiolet's
words are (loc. cit. p. 39): "Dans le foetus dont il s'agit les plis
cerebraux posterieurs sont bien developpes, tandis que les plis du lobe
frontal sont a peine indiques." The figure, however (Pl. iv, fig. 3),
shews the fissure of Rolando, and one of the frontal sulci plainly enough.
Nevertheless, M. Alix, in his 'Notice sur les travaux anthropologiques de
Gratiolet' ('Mem. de la Societe d'Anthropologie de Paris,' 1868, page 32),
writes thus: "Gratiolet a eu entre les mains le cerveau d'un foetus de
Gibbon, singe eminemment superieur, et tellement rapproche de l'orang, que
des naturalistes tres-competents l'ont range parmi les anthropoides. M.
Huxley, par exemple, n'hesite pas sur ce point. Eh bien, c'est sur le
cerveau d'un foetus de Gibbon que Gratiolet a vu LES CIRCONVOLUTIONS DU
LOBE TEMPORO-SPHENOIDAL DEJA DEVELOPPEES LORSQU'IL N'EXISTENT PAS ENCORE DE
PLIS SUR LE LOBE FRONTAL. Il etait donc bien autorise a dire que, chez
l'homme les circonvolutions apparaissent d'a en w, tandis que chez les
singes elles se developpent d'w en a."), and the other of a human foetus at
the 22nd or 23rd week of uterogestation, in which Gratiolet notes that the
insula was uncovered, but that nevertheless "des incisures sement de lobe
anterieur, une scissure peu profonde indique la separation du lobe
occipital, tres-reduit, d'ailleurs des cette epoque. Le reste de la
surface cerebrale est encore absolument lisse."
Three views of this brain are given in Plate II, figs. 1, 2, 3, of the work
cited, shewing the upper, lateral and inferior views of the hemispheres,
but not the inner view. It is worthy of note that the figure by no means
bears out Gratiolet's description, inasmuch as the fissure (antero-
temporal) on the posterior half of the face of the hemisphere is more
marked than any of those vaguely indicated in the anterior half. If the
figure is correct, it in no way justifies Gratiolet's conclusion: "Il y a
donc entre ces cerveaux [those of a Callithrix and of a Gibbon] et celui du
foetus humain une difference fondamental. Chez celui-ci, longtemps avant
que les plis temporaux apparaissent, les plis frontaux, ESSAYENT
d'exister."
Since Gratiolet's time, however, the development of the gyri and sulci of
the brain has been made the subject of renewed investigation by Schmidt,
Bischoff, Pansch (78. 'Ueber die typische Anordnung der Furchen und
Windungen auf den Grosshirn-Hemispharen des Menschen und der Affen,'
'Archiv fur Anthropologie,' iii. 1868.), and more particularly by Ecker
(79. 'Zur Entwicklungs Geschichte der Furchen und Windungen der Grosshirn-
Hemispharen im Foetus des Menschen.' 'Archiv fur Anthropologie,' iii.
1868.), whose work is not only the latest, but by far the most complete,
memoir on the subject.
The final results of their inquiries may be summed up as follows:--
1. In the human foetus, the sylvian fissure is formed in the course of the
third month of uterogestation. In this, and in the fourth month, the
cerebral hemispheres are smooth and rounded (with the exception of the
sylvian depression), and they project backwards far beyond the cerebellum.
2. The sulci, properly so called, begin to appear in the interval between
the end of the fourth and the beginning of the sixth month of foetal life,
but Ecker is careful to point out that, not only the time, but the order,
of their appearance is subject to considerable individual variation. In no
case, however, are either the frontal or the temporal sulci the earliest.
The first which appears, in fact, lies on the inner face of the hemisphere
(whence doubtless Gratiolet, who does not seem to have examined that face
in his foetus, overlooked it), and is either the internal perpendicular
(occipito-parietal), or the calcarine sulcus, these two being close
together and eventually running into one another. As a rule the occipito-
parietal is the earlier of the two.
3. At the latter part of this period, another sulcus, the "posterio-
parietal," or "Fissure of Rolando" is developed, and it is followed, in the
course of the sixth month, by the other principal sulci of the frontal,
parietal, temporal and occipital lobes. There is, however, no clear
evidence that one of these constantly appears before the other; and it is
remarkable that, in the brain at the period described and figured by Ecker
(loc. cit. pp. 212-213, Taf. II, figs. 1, 2, 3, 4), the antero-temporal
sulcus (scissure parallele) so characteristic of the ape's brain, is as
well, if not better developed than the fissure of Rolando, and is much more
marked than the proper frontal sulci.
Taking the facts as they now stand, it appears to me that the order of the
appearance of the sulci and gyri in the foetal human brain is in perfect
harmony with the general doctrine of evolution, and with the view that man
has been evolved from some ape-like form; though there can be no doubt that
form was, in many respects, different from any member of the Primates now
living.
Von Baer taught us, half a century ago, that, in the course of their
development, allied animals put on at first, the characters of the greater
groups to which they belong, and, by degrees, assume those which restrict
them within the limits of their family, genus, and species; and he proved,
at the same time, that no developmental stage of a higher animal is
precisely similar to the adult condition of any lower animal. It is quite
correct to say that a frog passes through the condition of a fish, inasmuch
as at one period of its life the tadpole has all the characters of a fish,
and if it went no further, would have to be grouped among fishes. But it
is equally true that a tadpole is very different from any known fish.
In like manner, the brain of a human foetus, at the fifth month, may
correctly be said to be, not only the brain of an ape, but that of an
Arctopithecine or marmoset-like ape; for its hemispheres, with their great
posterior lobster, and with no sulci but the sylvian and the calcarine,
present the characteristics found only in the group of the Arctopithecine
Primates. But it is equally true, as Gratiolet remarks, that, in its
widely open sylvian fissure, it differs from the brain of any actual
marmoset. No doubt it would be much more similar to the brain of an
advanced foetus of a marmoset. But we know nothing whatever of the
development of the brain in the marmosets. In the Platyrrhini proper, the
only observation with which I am acquainted is due to Pansch, who found in
the brain of a foetal Cebus Apella, in addition to the sylvian fissure and
the deep calcarine fissure, only a very shallow antero-temporal fissure
(scissure parallele of Gratiolet).
Now this fact, taken together with the circumstance that the antero-
temporal sulcus is present in such Platyrrhini as the Saimiri, which
present mere traces of sulci on the anterior half of the exterior of the
cerebral hemispheres, or none at all, undoubtedly, so far as it goes,
affords fair evidence in favour of Gratiolet's hypothesis, that the
posterior sulci appear before the anterior, in the brains of the
Platyrrhini. But, it by no means follows, that the rule which may hold
good for the Platyrrhini extends to the Catarrhini. We have no information
whatever respecting the development of the brain in the Cynomorpha; and, as
regards the Anthropomorpha, nothing but the account of the brain of the
Gibbon, near birth, already referred to. At the present moment there is
not a shadow of evidence to shew that the sulci of a chimpanzee's, or
orang's, brain do not appear in the same order as a man's.
Gratiolet opens his preface with the aphorism: "Il est dangereux dans les
sciences de conclure trop vite." I fear he must have forgotten this sound
maxim by the time he had reached the discussion of the differences between
men and apes, in the body of his work. No doubt, the excellent author of
one of the most remarkable contributions to the just understanding of the
mammalian brain which has ever been made, would have been the first to
admit the insufficiency of his data had he lived to profit by the advance
of inquiry. The misfortune is that his conclusions have been employed by
persons incompetent to appreciate their foundation, as arguments in favour
of obscurantism. (80. For example, M. l'Abbe Lecomte in his terrible
pamphlet, 'Le Darwinisme et l'origine de l'Homme,' 1873.)
But it is important to remark that, whether Gratiolet was right or wrong in
his hypothesis respecting the relative order of appearance of the temporal
and frontal sulci, the fact remains; that before either temporal or frontal
sulci, appear, the foetal brain of man presents characters which are found
only in the lowest group of the Primates (leaving out the Lemurs); and that
this is exactly what we should expect to be the case, if man has resulted
from the gradual modification of the same form as that from which the other
Primates have sprung.
PART II. SEXUAL SELECTION.
CHAPTER VIII.
PRINCIPLES OF SEXUAL SELECTION.
Secondary sexual characters--Sexual selection--Manner of action--Excess of
males--Polygamy--The male alone generally modified through sexual
selection--Eagerness of the male--Variability of the male--Choice exerted
by the female--Sexual compared with natural selection--Inheritance, at
corresponding periods of life, at corresponding seasons of the year, and as
limited by sex--Relations between the several forms of inheritance--Causes
why one sex and the young are not modified through sexual selection--
Supplement on the proportional numbers of the two sexes throughout the
animal kingdom--The proportion of the sexes in relation to natural
selection.
With animals which have their sexes separated, the males necessarily differ
from the females in their organs of reproduction; and these are the primary
sexual characters. But the sexes often differ in what Hunter has called
secondary sexual characters, which are not directly connected with the act
of reproduction; for instance, the male possesses certain organs of sense
or locomotion, of which the female is quite destitute, or has them more
highly-developed, in order that he may readily find or reach her; or again
the male has special organs of prehension for holding her securely. These
latter organs, of infinitely diversified kinds, graduate into those which
are commonly ranked as primary, and in some cases can hardly be
distinguished from them; we see instances of this in the complex appendages
at the apex of the abdomen in male insects. Unless indeed we confine the
term "primary" to the reproductive glands, it is scarcely possible to
decide which ought to be called primary and which secondary.
The female often differs from the male in having organs for the nourishment
or protection of her young, such as the mammary glands of mammals, and the
abdominal sacks of the marsupials. In some few cases also the male
possesses similar organs, which are wanting in the female, such as the
receptacles for the ova in certain male fishes, and those temporarily
developed in certain male frogs. The females of most bees are provided
with a special apparatus for collecting and carrying pollen, and their
ovipositor is modified into a sting for the defence of the larvae and the
community. Many similar cases could be given, but they do not here concern
us. There are, however, other sexual differences quite unconnected with
the primary reproductive organs, and it is with these that we are more
especially concerned--such as the greater size, strength, and pugnacity of
the male, his weapons of offence or means of defence against rivals, his
gaudy colouring and various ornaments, his power of song, and other such
characters.
Besides the primary and secondary sexual differences, such as the
foregoing, the males and females of some animals differ in structures
related to different habits of life, and not at all, or only indirectly, to
the reproductive functions. Thus the females of certain flies (Culicidae
and Tabanidae) are blood-suckers, whilst the males, living on flowers, have
mouths destitute of mandibles. (1. Westwood, 'Modern Classification of
Insects,' vol. ii. 1840, p. 541. For the statement about Tanais, mentioned
below, I am indebted to Fritz Muller.) The males of certain moths and of
some crustaceans (e.g. Tanais) have imperfect, closed mouths, and cannot
feed. The complemental males of certain Cirripedes live like epiphytic
plants either on the female or the hermaphrodite form, and are destitute of
a mouth and of prehensile limbs. In these cases it is the male which has
been modified, and has lost certain important organs, which the females
possess. In other cases it is the female which has lost such parts; for
instance, the female glow-worm is destitute of wings, as also are many
female moths, some of which never leave their cocoons. Many female
parasitic crustaceans have lost their natatory legs. In some weevil-
beetles (Curculionidae) there is a great difference between the male and
female in the length of the rostrum or snout (2. Kirby and Spence,
'Introduction to Entomology,' vol. iii. 1826, p. 309.); but the meaning of
this and of many analogous differences, is not at all understood.
Differences of structure between the two sexes in relation to different
habits of life are generally confined to the lower animals; but with some
few birds the beak of the male differs from that of the female. In the
Huia of New Zealand the difference is wonderfully great, and we hear from
Dr. Buller (3. 'Birds of New Zealand,' 1872, p. 66.) that the male uses
his strong beak in chiselling the larvae of insects out of decayed wood,
whilst the female probes the softer parts with her far longer, much curved
and pliant beak: and thus they mutually aid each other. In most cases,
differences of structure between the sexes are more or less directly
connected with the propagation of the species: thus a female, which has to
nourish a multitude of ova, requires more food than the male, and
consequently requires special means for procuring it. A male animal, which
lives for a very short time, might lose its organs for procuring food
through disuse, without detriment; but he would retain his locomotive
organs in a perfect state, so that he might reach the female. The female,
on the other hand, might safely lose her organs for flying, swimming, or
walking, if she gradually acquired habits which rendered such powers
useless.
We are, however, here concerned only with sexual selection. This depends
on the advantage which certain individuals have over others of the same sex
and species solely in respect of reproduction. When, as in the cases above
mentioned, the two sexes differ in structure in relation to different
habits of life, they have no doubt been modified through natural selection,
and by inheritance limited to one and the same sex. So again the primary
sexual organs, and those for nourishing or protecting the young, come under
the same influence; for those individuals which generated or nourished
their offspring best, would leave, ceteris paribus, the greatest number to
inherit their superiority; whilst those which generated or nourished their
offspring badly, would leave but few to inherit their weaker powers. As
the male has to find the female, he requires organs of sense and
locomotion, but if these organs are necessary for the other purposes of
life, as is generally the case, they will have been developed through
natural selection. When the male has found the female, he sometimes
absolutely requires prehensile organs to hold her; thus Dr. Wallace informs
me that the males of certain moths cannot unite with the females if their
tarsi or feet are broken. The males of many oceanic crustaceans, when
adult, have their legs and antennae modified in an extraordinary manner for
the prehension of the female; hence we may suspect that it is because these
animals are washed about by the waves of the open sea, that they require
these organs in order to propagate their kind, and if so, their development
has been the result of ordinary or natural selection. Some animals
extremely low in the scale have been modified for this same purpose; thus
the males of certain parasitic worms, when fully grown, have the lower
surface of the terminal part of their bodies roughened like a rasp, and
with this they coil round and permanently hold the females. (4. M.
Perrier advances this case ('Revue Scientifique,' Feb. 1, 1873, p. 865) as
one fatal to the belief in sexual election, inasmuch as he supposes that I
attribute all the differences between the sexes to sexual selection. This
distinguished naturalist, therefore, like so many other Frenchmen, has not
taken the trouble to understand even the first principles of sexual
selection. An English naturalist insists that the claspers of certain male
animals could not have been developed through the choice of the female!
Had I not met with this remark, I should not have thought it possible for
any one to have read this chapter and to have imagined that I maintain that
the choice of the female had anything to do with the development of the
prehensile organs in the male.)
When the two sexes follow exactly the same habits of life, and the male has
the sensory or locomotive organs more highly developed than those of the
female, it may be that the perfection of these is indispensable to the male
for finding the female; but in the vast majority of cases, they serve only
to give one male an advantage over another, for with sufficient time, the
less well-endowed males would succeed in pairing with the females; and
judging from the structure of the female, they would be in all other
respects equally well adapted for their ordinary habits of life. Since in
such cases the males have acquired their present structure, not from 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, sexual selection must here have
come into action. It was the importance of this distinction which led me
to designate this form of selection 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 perfected through sexual
selection, that is by the advantage acquired by certain individuals over
their rivals. But in most cases of this kind it is impossible to
distinguish between the effects of natural and sexual selection. Whole
chapters could be filled with details on the differences between the sexes
in their sensory, locomotive, and prehensile organs. As, however, these
structures are not more interesting than others adapted for the ordinary
purposes of life I shall pass them over almost entirely, giving only a few
instances under each class.
There are many other structures and instincts which must have been
developed through sexual selection--such as the weapons of offence and the
means of defence of the males for fighting with and driving away their
rivals--their courage and pugnacity--their various ornaments--their
contrivances for producing vocal or instrumental music--and their glands
for emitting odours, most of these latter structures serving only to allure
or excite the female. It is clear that these characters are the result of
sexual and not of ordinary selection, since unarmed, unornamented, or
unattractive males would succeed equally well in the battle for life and in
leaving a numerous progeny, but for the presence of better endowed males.
We may infer that this would be the case, because the females, which are
unarmed and unornamented, are able to survive and procreate their kind.
Secondary sexual characters of the kind just referred to, will be fully
discussed in the following chapters, as being in many respects interesting,
but especially as depending on the will, choice, and rivalry of the
individuals of either sex. When we behold two males fighting for the
possession of the female, or several male birds displaying their gorgeous
plumage, and performing strange antics before an assembled body of females,
we cannot doubt that, though led by instinct, they know what they are
about, and consciously exert their mental and bodily powers.
Just as man can improve the breeds of his game-cocks by the selection of
those birds which are victorious in the cockpit, 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. A slight degree of variability leading to some
advantage, however slight, in reiterated deadly contests would suffice for
the work of sexual selection; and it is certain that secondary sexual
characters are eminently variable. Just as man can give beauty, according
to his standard of taste, to his male poultry, or more strictly can modify
the beauty originally acquired by the parent species, can give to the
Sebright bantam a new and elegant plumage, an erect and peculiar carriage--
so it appears that female birds in a state of nature, have by a long
selection of the more attractive males, added to their beauty or other
attractive qualities. No doubt this implies powers of discrimination and
taste on the part of the female which will at first appear extremely
improbable; but by the facts to be adduced hereafter, I hope to be able to
shew that the females actually have these powers. When, however, it is
said that the lower animals have a sense of beauty, it must not be supposed
that such sense is comparable with that of a cultivated man, with his
multiform and complex associated ideas. A more just comparison would be
between the taste for the beautiful in animals, and that in the lowest
savages, who admire and deck themselves with any brilliant, glittering, or
curious object.
From our ignorance on several points, the precise manner in which sexual
selection acts is somewhat uncertain. Nevertheless if those naturalists
who already believe in the mutability of species, will read the following
chapters, they will, I think, agree with me, that sexual selection has
played an important part in the history of the organic world. It is
certain that amongst almost all animals there is a struggle between the
males for the possession of the female. This fact is so notorious that it
would be superfluous to give instances. Hence the females have the
opportunity of selecting one out of several males, on the supposition that
their mental capacity suffices for the exertion of a choice. In many cases
special circumstances tend to make the struggle between the males
particularly severe. Thus the males of our migratory birds generally
arrive at their places of breeding before the females, so that many males
are ready to contend for each female. I am informed by Mr. Jenner Weir,
that the bird-catchers assert that this is invariably the case with the
nightingale and blackcap, and with respect to the latter he can himself
confirm the statement.
Mr. Swaysland of Brighton has been in the habit, during the last forty
years, of catching our migratory birds on their first arrival, and he has
never known the females of any species to arrive before their males.
During one spring he shot thirty-nine males of Ray's wagtail (Budytes Raii)
before he saw a single female. Mr. Gould has ascertained by the dissection
of those snipes which arrive the first in this country, that the males come
before the females. And the like holds good with most of the migratory
birds of the United States. (5. J.A. Allen, on the 'Mammals and Winter
Birds of Florida,' Bulletin of Comparative Zoology, Harvard College, p.
268.) The majority of the male salmon in our rivers, on coming up from the
sea, are ready to breed before the females. So it appears to be with frogs
and toads. Throughout the great class of insects the males almost always
are the first to emerge from the pupal state, so that they generally abound
for a time before any females can be seen. (6. Even with those plants in
which the sexes are separate, the male flowers are generally mature before
the female. As first shewn by C.K. Sprengel, many hermaphrodite plants are
dichogamous; that is, their male and female organs are not ready at the
same time, so that they cannot be self-fertilised. Now in such flowers,
the pollen is in general matured before the stigma, though there are
exceptional cases in which the female organs are beforehand.) The cause of
this difference between the males and females in their periods of arrival
and maturity is sufficiently obvious. Those males which annually first
migrated into any country, or which in the spring were first ready to
breed, or were the most eager, would leave the largest number of offspring;
and these would tend to inherit similar instincts and constitutions. It
must be borne in mind that it would have been impossible to change very
materially the time of sexual maturity in the females, without at the same
time interfering with the period of the production of the young--a period
which must be determined by the seasons of the year. On the whole there
can be no doubt that with almost all animals, in which the sexes are
separate, there is a constantly recurrent struggle between the males for
the possession of the females.
Our difficulty in regard to sexual selection lies in understanding how it
is that the males which conquer other males, or those which prove the most
attractive to the females, leave a greater number of offspring to inherit
their superiority than their beaten and less attractive rivals. Unless
this result does follow, the characters which give to certain males an
advantage over others, could not be perfected and augmented through sexual
selection. When the sexes exist in exactly equal numbers, the worst-
endowed males will (except where polygamy prevails), ultimately find
females, and leave as many offspring, as well fitted for their general
habits of life, as the best-endowed males. From various facts and
considerations, I formerly inferred that with most animals, in which
secondary sexual characters are well developed, the males considerably
exceeded the females in number; but this is not by any means always true.
If the males were to the females as two to one, or as three to two, or even
in a somewhat lower ratio, the whole affair would be simple; for the
better-armed or more attractive males would leave the largest number of
offspring. But after investigating, as far as possible, the numerical
proportion of the sexes, I do not believe that any great inequality in
number commonly exists. In most cases sexual selection appears to have
been effective in the following manner.
Let us take any species, a bird for instance, and divide the females
inhabiting a district into two equal bodies, the one consisting of the more
vigorous and better-nourished individuals, and the other of the less
vigorous and healthy. The former, there can be little doubt, would be
ready to breed in the spring before the others; and this is the opinion of
Mr. Jenner Weir, who has carefully attended to the habits of birds during
many years. There can also be no doubt that the most vigorous, best-
nourished and earliest breeders would on an average succeed in rearing the
largest number of fine offspring. (7. Here is excellent evidence on the
character of the offspring from an experienced ornithologist. Mr. J.A.
Allen, in speaking ('Mammals and Winter Birds of E. Florida,' p. 229) of
the later broods, after the accidental destruction of the first, says, that
these "are found to be smaller and paler-coloured than those hatched
earlier in the season. In cases where several broods are reared each year,
as a general rule the birds of the earlier broods seem in all respects the
most perfect and vigorous.") The males, as we have seen, are generally
ready to breed before the females; the strongest, and with some species the
best armed of the males, drive away the weaker; and the former would then
unite with the more vigorous and better-nourished females, because they are
the first to breed. (8. Hermann Muller has come to this same conclusion
with respect to those female bees which are the first to emerge from the
pupa each year. See his remarkable essay, 'Anwendung der Darwin'schen
Lehre auf Bienen,' 'Verh. d. V. Jahrg.' xxix. p. 45.) Such vigorous pairs
would surely rear a larger number of offspring than the retarded females,
which would be compelled to unite with the conquered and less powerful
males, supposing the sexes to be numerically equal; and this is all that is
wanted to add, in the course of successive generations, to the size,
strength and courage of the males, or to improve their weapons.
But in very many cases the males which conquer their rivals, do not obtain
possession of the females, independently of the choice of the latter. The
courtship of animals is by no means so simple and short an affair as might
be thought. The females are most excited by, or prefer pairing with, the
more ornamented males, or those which are the best songsters, or play the
best antics; but it is obviously probable that they would at the same time
prefer the more vigorous and lively males, and this has in some cases been
confirmed by actual observation. (9. With respect to poultry, I have
received information, hereafter to be given, to this effect. Even birds,
such as pigeons, which pair for life, the female, as I hear from Mr. Jenner
Weir, will desert her mate if he is injured or grows weak.) Thus the more
vigorous females, which are the first to breed, will have the choice of
many males; and though they may not always select the strongest or best
armed, they will select those which are vigorous and well armed, and in
other respects the most attractive. Both sexes, therefore, of such early
pairs would as above explained, have an advantage over others in rearing
offspring; and this apparently has sufficed during a long course of
generations to add not only to the strength and fighting powers of the
males, but likewise to their various ornaments or other attractions.
In the converse and much rarer case of the males selecting particular
females, it is plain that those which were the most vigorous and had
conquered others, would have the freest choice; and it is almost certain
that they would select vigorous as well as attractive females. Such pairs
would have an advantage in rearing offspring, more especially if the male
had the power to defend the female during the pairing-season as occurs with
some of the higher animals, or aided her in providing for the young. The
same principles would apply if each sex preferred and selected certain
individuals of the opposite sex; supposing that they selected not only the
more attractive, but likewise the more vigorous individuals.
NUMERICAL PROPORTION OF THE TWO SEXES.
I have remarked that sexual selection would be a simple affair if the males
were considerably more numerous than the females. Hence I was led to
investigate, as far as I could, the proportions between the two sexes of as
many animals as possible; but the materials are scanty. I will here give
only a brief abstract of the results, retaining the details for a
supplementary discussion, so as not to interfere with the course of my
argument. Domesticated animals alone afford the means of ascertaining the
proportional numbers at birth; but no records have been specially kept for
this purpose. By indirect means, however, I have collected a considerable
body of statistics, from which it appears that with most of our domestic
animals the sexes are nearly equal at birth. Thus 25,560 births of race-
horses have been recorded during twenty-one years, and the male births were
to the female births as 99.7 to 100. In greyhounds the inequality is
greater than with any other animal, for out of 6878 births during twelve
years, the male births were to the female as 110.1 to 100. It is, however,
in some degree doubtful whether it is safe to infer that the proportion
would be the same under natural conditions as under domestication; for
slight and unknown differences in the conditions affect the proportion of
the sexes. Thus with mankind, the male births in England are as 104.5, in
Russia as 108.9, and with the Jews of Livonia as 120, to 100 female births.
But I shall recur to this curious point of the excess of male births in the
supplement to this chapter. At the Cape of Good Hope, however, male
children of European extraction have been born during several years in the
proportion of between 90 and 99 to 100 female children.
For our present purpose we are concerned with the proportions of the sexes,
not only at birth, but also at maturity, and this adds another element of
doubt; for it is a well-ascertained fact that with man the number of males
dying before or during birth, and during the first two years of infancy, is
considerably larger than that of females. So it almost certainly is with
male lambs, and probably with some other animals. The males of some
species kill one another by fighting; or they drive one another about until
they become greatly emaciated. They must also be often exposed to various
dangers, whilst wandering about in eager search for the females. In many
kinds of fish the males are much smaller than the females, and they are
believed often to be devoured by the latter, or by other fishes. The
females of some birds appear to die earlier than the males; they are also
liable to be destroyed on their nests, or whilst in charge of their young.
With insects the female larvae are often larger than those of the males,
and would consequently be more likely to be devoured. In some cases the
mature females are less active and less rapid in their movements than the
males, and could not escape so well from danger. Hence, with animals in a
state of nature, we must rely on mere estimation, in order to judge of the
proportions of the sexes at maturity; and this is but little trustworthy,
except when the inequality is strongly marked. Nevertheless, as far as a
judgment can be formed, we may conclude from the facts given in the
supplement, that the males of some few mammals, of many birds, of some fish
and insects, are considerably more numerous than the females.
The proportion between the sexes fluctuates slightly during successive
years: thus with race-horses, for every 100 mares born the stallions
varied from 107.1 in one year to 92.6 in another year, and with greyhounds
from 116.3 to 95.3. But had larger numbers been tabulated throughout an
area more extensive than England, these fluctuations would probably have
disappeared; and such as they are, would hardly suffice to lead to
effective sexual selection in a state of nature. Nevertheless, in the
cases of some few wild animals, as shewn in the supplement, the proportions
seem to fluctuate either during different seasons or in different
localities in a sufficient degree to lead to such selection. For it should
be observed that any advantage, gained during certain years or in certain
localities by those males which were able to conquer their rivals, or were
the most attractive to the females, would probably be transmitted to the
offspring, and would not subsequently be eliminated. During the succeeding
seasons, when, from the equality of the sexes, every male was able to
procure a female, the stronger or more attractive males previously produced
would still have at least as good a chance of leaving offspring as the
weaker or less attractive.
POLYGAMY.
The practice of polygamy leads to the same results as would follow from an
actual inequality in the number of the sexes; for if each male secures two
or more females, many males cannot pair; and the latter assuredly will be
the weaker or less attractive individuals. Many mammals and some few birds
are polygamous, but with animals belonging to the lower classes I have
found no evidence of this habit. The intellectual powers of such animals
are, perhaps, not sufficient to lead them to collect and guard a harem of
females. That some relation exists between polygamy and the development of
secondary sexual characters, appears nearly certain; and this supports the
view that a numerical preponderance of males would be eminently favourable
to the action of sexual selection. Nevertheless many animals, which are
strictly monogamous, especially birds, display strongly-marked secondary
sexual characters; whilst some few animals, which are polygamous, do not
have such characters.
We will first briefly run through the mammals, and then turn to birds. The
gorilla seems to be polygamous, and the male differs considerably from the
female; so it is with some baboons, which live in herds containing twice as
many adult females as males. In South America the Mycetes caraya presents
well-marked sexual differences, in colour, beard, and vocal organs; and the
male generally lives with two or three wives: the male of the Cebus
capucinus differs somewhat from the female, and appears to be polygamous.
(10. On the Gorilla, Savage and Wyman, 'Boston Journal of Natural
History,' vol. v. 1845-47, p. 423. On Cynocephalus, Brehm, 'Thierleben,'
B. i. 1864, s. 77. On Mycetes, Rengger, 'Naturgeschichte der Saugethiere
von Paraguay,' 1830, ss. 14, 20. On Cebus, Brehm, ibid. s. 108.) Little
is known on this head with respect to most other monkeys, but some species
are strictly monogamous. The ruminants are eminently polygamous, and they
present sexual differences more frequently than almost any other group of
mammals; this holds good, especially in their weapons, but also in other
characters. Most deer, cattle, and sheep are polygamous; as are most
antelopes, though some are monogamous. Sir Andrew Smith, in speaking of
the antelopes of South Africa, says that in herds of about a dozen there
was rarely more than one mature male. The Asiatic Antilope saiga appears
to be the most inordinate polygamist in the world; for Pallas (11. Pallas,
'Spicilegia Zoolog., Fasc.' xii. 1777, p. 29. Sir Andrew Smith,
'Illustrations of the Zoology of S. Africa,' 1849, pl. 29, on the Kobus.
Owen, in his 'Anatomy of Vertebrates' (vol. iii. 1868, p. 633) gives a
table shewing incidentally which species of antelopes are gregarious.)
states that the male drives away all rivals, and collects a herd of about a
hundred females and kids together; the female is hornless and has softer
hair, but does not otherwise differ much from the male. The wild horse of
the Falkland Islands and of the Western States of N. America is polygamous,
but, except in his greater size and in the proportions of his body, differs
but little from the mare. The wild boar presents well-marked sexual
characters, in his great tusks and some other points. In Europe and in
India he leads a solitary life, except during the breeding-season; but as
is believed by Sir W. Elliot, who has had many opportunities in India of
observing this animal, he consorts at this season with several females.
Whether this holds good in Europe is doubtful, but it is supported by some
evidence. The adult male Indian elephant, like the boar, passes much of
his time in solitude; but as Dr. Campbell states, when with others, "It is
rare to find more than one male with a whole herd of females"; the larger
males expelling or killing the smaller and weaker ones. The male differs
from the female in his immense tusks, greater size, strength, and
endurance; so great is the difference in these respects that the males when
caught are valued at one-fifth more than the females. (12. Dr. Campbell,
in 'Proc. Zoolog. Soc.' 1869, p. 138. See also an interesting paper by
Lieut. Johnstone, in 'Proceedings, Asiatic Society of Bengal,' May 1868.)
The sexes of other pachydermatous animals differ very little or not at all,
and, as far as known, they are not polygamists. Nor have I heard of any
species in the Orders of Cheiroptera, Edentata, Insectivora and Rodents
being polygamous, excepting that amongst the Rodents, the common rat,
according to some rat-catchers, lives with several females. Nevertheless
the two sexes of some sloths (Edentata) differ in the character and colour
of certain patches of hair on their shoulders. (13. Dr. Gray, in 'Annals
and Magazine of Natural History,' 1871, p. 302.) And many kinds of bats
(Cheiroptera) present well-marked sexual differences, chiefly in the males
possessing odoriferous glands and pouches, and by their being of a lighter
colour. (14. See Dr. Dobson's excellent paper in 'Proceedings of the
Zoological Society,' 1873, p. 241.) In the great order of Rodents, as far
as I can learn, the sexes rarely differ, and when they do so, it is but
slightly in the tint of the fur.
As I hear from Sir Andrew Smith, the lion in South Africa sometimes lives
with a single female, but generally with more, and, in one case, was found
with as many as five females; so that he is polygamous. As far as I can
discover, he is the only polygamist amongst all the terrestrial Carnivora,
and he alone presents well-marked sexual characters. If, however, we turn
to the marine Carnivora, as we shall hereafter see, the case is widely
different; for many species of seals offer extraordinary sexual
differences, and they are eminently polygamous. Thus, according to Peron,
the male sea-elephant of the Southern Ocean always possesses several
females, and the sea-lion of Forster is said to be surrounded by from
twenty to thirty females. In the North, the male sea-bear of Steller is
accompanied by even a greater number of females. It is an interesting
fact, as Dr. Gill remarks (15. 'The Eared Seals,' American Naturalist,
vol. iv. Jan. 1871.), that in the monogamous species, "or those living in
small communities, there is little difference in size between the males and
females; in the social species, or rather those of which the males have
harems, the males are vastly larger than the females."
Amongst birds, many species, the sexes of which differ greatly from each
other, are certainly monogamous. In Great Britain we see well-marked
sexual differences, for instance, in the wild-duck which pairs with a
single female, the common blackbird, and the bullfinch which is said to
pair for life. I am informed by Mr. Wallace that the like is true of the
Chatterers or Cotingidae of South America, and of many other birds. In
several groups I have not been able to discover whether the species are
polygamous or monogamous. Lesson says that birds of paradise, so
remarkable for their sexual differences, are polygamous, but Mr. Wallace
doubts whether he had sufficient evidence. Mr. Salvin tells me he has been
led to believe that humming-birds are polygamous. The male widow-bird,
remarkable for his caudal plumes, certainly seems to be a polygamist. (16.
'The Ibis,' vol. iii. 1861, p. 133, on the Progne Widow-bird. See also on
the Vidua axillaris, ibid. vol. ii. 1860, p. 211. On the polygamy of the
Capercailzie and Great Bustard, see L. Lloyd, 'Game Birds of Sweden,' 1867,
pp. 19, and 182. Montagu and Selby speak of the Black Grouse as polygamous
and of the Red Grouse as monogamous.) I have been assured by Mr. Jenner
Weir and by others, that it is somewhat common for three starlings to
frequent the same nest; but whether this is a case of polygamy or polyandry
has not been ascertained.
The Gallinaceae exhibit almost as strongly marked sexual differences as
birds of paradise or humming-birds, and many of the species are, as is well
known, polygamous; others being strictly monogamous. What a contrast is
presented between the sexes of the polygamous peacock or pheasant, and the
monogamous guinea-fowl or partridge! Many similar cases could be given, as
in the grouse tribe, in which the males of the polygamous capercailzie and
black-cock differ greatly from the females; whilst the sexes of the
monogamous red grouse and ptarmigan differ very little. In the Cursores,
except amongst the bustards, few species offer strongly-marked sexual
differences, and the great bustard (Otis tarda) is said to be polygamous.
With the Grallatores, extremely few species differ sexually, but the ruff
(Machetes pugnax) affords a marked exception, and this species is believed
by Montagu to be a polygamist. Hence it appears that amongst birds there
often exists a close relation between polygamy and the development of
strongly-marked sexual differences. I asked Mr. Bartlett, of the
Zoological Gardens, who has had very large experience with birds, whether
the male tragopan (one of the Gallinaceae) was polygamous, and I was struck
by his answering, "I do not know, but should think so from his splendid
colours."
It deserves notice that the instinct of pairing with a single female is
easily lost under domestication. The wild-duck is strictly monogamous, the
domestic-duck highly polygamous. The Rev. W.D. Fox informs me that out of
some half-tamed wild-ducks, on a large pond in his neighbourhood, so many
mallards were shot by the gamekeeper that only one was left for every seven
or eight females; yet unusually large broods were reared. The guinea-fowl
is strictly monogamous; but Mr. Fox finds that his birds succeed best when
he keeps one cock to two or three hens. Canary-birds pair in a state of
nature, but the breeders in England successfully put one male to four or
five females. I have noticed these cases, as rendering it probable that
wild monogamous species might readily become either temporarily or
permanently polygamous.
Too little is known of the habits of reptiles and fishes to enable us to
speak of their marriage arrangements. The stickle-back (Gasterosteus),
however, is said to be a polygamist (17. Noel Humphreys, 'River Gardens,'
1857.); and the male during the breeding-season differs conspicuously from
the female.
To sum up on the means through which, as far as we can judge, sexual
selection has led to the development of secondary sexual characters. It
has been shewn that the largest number of vigorous offspring will be reared
from the pairing of the strongest and best-armed males, victorious in
contests over other males, with the most vigorous and best-nourished
females, which are the first to breed in the spring. If such females
select the more attractive, and at the same time vigorous males, they will
rear a larger number of offspring than the retarded females, which must
pair with the less vigorous and less attractive males. So it will be if
the more vigorous males select the more attractive and at the same time
healthy and vigorous females; and this will especially hold good if the
male defends the female, and aids in providing food for the young. The
advantage thus gained by the more vigorous pairs in rearing a larger number
of offspring has apparently sufficed to render sexual selection efficient.
But a large numerical preponderance of males over females will be still
more efficient; whether the preponderance is only occasional and local, or
permanent; whether it occurs at birth, or afterwards from the greater
destruction of the females; or whether it indirectly follows from the
practice of polygamy.
THE MALE GENERALLY MORE MODIFIED THAN THE FEMALE.
Throughout the animal kingdom, when the sexes differ in external
appearance, it is, with rare exceptions, the male which has been the more
modified; for, generally, the female retains a closer resemblance to the
young of her own species, and to other adult members of the same group.
The cause of this seems to lie in the males of almost all animals having
stronger passions than the females. Hence it is the males that fight
together and sedulously display their charms before the females; and the
victors transmit their superiority to their male offspring. Why both sexes
do not thus acquire the characters of their fathers, will be considered
hereafter. That the males of all mammals eagerly pursue the females is
notorious to every one. So it is with birds; but many cock birds do not so
much pursue the hen, as display their plumage, perform strange antics, and
pour forth their song in her presence. The male in the few fish observed
seems much more eager than the female; and the same is true of alligators,
and apparently of Batrachians. Throughout the enormous class of insects,
as Kirby remarks, "the law is that the male shall seek the female." (18.
Kirby and Spence, 'Introduction to Entomology,' vol. iii. 1826, p. 342.)
Two good authorities, Mr. Blackwall and Mr. C. Spence Bate, tell me that
the males of spiders and crustaceans are more active and more erratic in
their habits than the females. When the organs of sense or locomotion are
present in the one sex of insects and crustaceans and absent in the other,
or when, as is frequently the case, they are more highly developed in the
one than in the other, it is, as far as I can discover, almost invariably
the male which retains such organs, or has them most developed; and this
shews that the male is the more active member in the courtship of the
sexes. (19. One parasitic Hymenopterous insect (Westwood, 'Modern Class.
of Insects,' vol. ii. p. 160) forms an exception to the rule, as the male
has rudimentary wings, and never quits the cell in which it is born, whilst
the female has well-developed wings. Audouin believes that the females of
this species are impregnated by the males which are born in the same cells
with them; but it is much more probable that the females visit other cells,
so that close inter-breeding is thus avoided. We shall hereafter meet in
various classes, with a few exceptional cases, in which the female, instead
of the male, is the seeker and wooer.)
The female, on the other hand, with the rarest exceptions, is less eager
than the male. As the illustrious Hunter (20. 'Essays and Observations,'
edited by Owen, vol. i. 1861, p. 194.) long ago observed, she generally
"requires to be courted;" she is coy, and may often be seen endeavouring
for a long time to escape from the male. Every observer of the habits of
animals will be able to call to mind instances of this kind. It is shewn
by various facts, given hereafter, and by the results fairly attributable
to sexual selection, that the female, though comparatively passive,
generally exerts some choice and accepts one male in preference to others.
Or she may accept, as appearances would sometimes lead us to believe, not
the male which is the most attractive to her, but the one which is the
least distasteful. The exertion of some choice on the part of the female
seems a law almost as general as the eagerness of the male.
We are naturally led to enquire why the male, in so many and such distinct
classes, has become more eager than the female, so that he searches for
her, and plays the more active part in courtship. It would be no advantage
and some loss of power if each sex searched for the other; but why should
the male almost always be the seeker? The ovules of plants after
fertilisation have to be nourished for a time; hence the pollen is
necessarily brought to the female organs--being placed on the stigma, by
means of insects or the wind, or by the spontaneous movements of the
stamens; and in the Algae, etc., by the locomotive power of the
antherozooids. With lowly-organised aquatic animals, permanently affixed
to the same spot and having their sexes separate, the male element is
invariably brought to the female; and of this we can see the reason, for
even if the ova were detached before fertilisation, and did not require
subsequent nourishment or protection, there would yet be greater difficulty
in transporting them than the male element, because, being larger than the
latter, they are produced in far smaller numbers. So that many of the
lower animals are, in this respect, analogous with plants. (21. Prof.
Sachs ('Lehrbuch der Botanik,' 1870, S. 633) in speaking of the male and
female reproductive cells, remarks, "verhalt sich die eine bei der
Vereinigung activ,...die andere erscheint bei der Vereinigung passiv.")
The males of affixed and aquatic animals having been led to emit their
fertilising element in this way, it is natural that any of their
descendants, which rose in the scale and became locomotive, should retain
the same habit; and they would approach the female as closely as possible,
in order not to risk the loss of the fertilising element in a long passage
of it through the water. With some few of the lower animals, the females
alone are fixed, and the males of these must be the seekers. But it is
difficult to understand why the males of species, of which the progenitors
were primordially free, should invariably have acquired the habit of
approaching the females, instead of being approached by them. But in all
cases, in order that the males should seek efficiently, it would be
necessary that they should be endowed with strong passions; and the
acquirement of such passions would naturally follow from the more eager
leaving a larger number of offspring than the less eager.
The great eagerness of the males has thus indirectly led to their much more
frequently developing secondary sexual characters than the females. But
the development of such characters would be much aided, if the males were
more liable to vary than the females--as I concluded they were--after a
long study of domesticated animals. Von Nathusius, who has had very wide
experience, is strongly of the same opinion. (22. 'Vortrage uber
Viehzucht,' 1872, p. 63.) Good evidence also in favour of this conclusion
can be produced by a comparison of the two sexes in mankind. During the
Novara Expedition (23. 'Reise der Novara: Anthropolog. Theil,' 1867, ss.
216-269. The results were calculated by Dr. Weisbach from measurements
made by Drs. K. Scherzer and Schwarz. On the greater variability of the
males of domesticated animals, see my 'Variation of Animals and Plants
under Domestication,' vol. ii. 1868, p. 75.) a vast number of measurements
was made of various parts of the body in different races, and the men were
found in almost every case to present a greater range of variation than the
women; but I shall have to recur to this subject in a future chapter. Mr.
J. Wood (24. 'Proceedings of the Royal Society,' vol. xvi. July 1868, pp.
519 and 524.), who has carefully attended to the variation of the muscles
in man, puts in italics the conclusion that "the greatest number of
abnormalities in each subject is found in the males." He had previously
remarked that "altogether in 102 subjects, the varieties of redundancy were
found to be half as many again as in females, contrasting widely with the
greater frequency of deficiency in females before described." Professor
Macalister likewise remarks (25. 'Proc. Royal Irish Academy,' vol. x.
1868, p. 123.) that variations in the muscles "are probably more common in
males than females." Certain muscles which are not normally present in
mankind are also more frequently developed in the male than in the female
sex, although exceptions to this rule are said to occur. Dr. Burt Wilder
(26. 'Massachusetts Medical Society,' vol. ii. No. 3, 1868, p. 9.) has
tabulated the cases of 152 individuals with supernumerary digits, of which
86 were males, and 39, or less than half, females, the remaining 27 being
of unknown sex. It should not, however, be overlooked that women would
more frequently endeavour to conceal a deformity of this kind than men.
Again, Dr. L. Meyer asserts that the ears of man are more variable in form
than those of a woman. (27. 'Archiv fur Path. Anat. und Phys.' 1871, p.
488.) Lastly the temperature is more variable in man than in woman. (28.
The conclusions recently arrived at by Dr. J. Stockton Hough, on the
temperature of man, are given in the 'Pop. Sci. Review,' Jan. 1st, 1874, p.
97.)
The cause of the greater general variability in the male sex, than in the
female is unknown, except in so far as secondary sexual characters are
extraordinarily variable, and are usually confined to the males; and, as we
shall presently see, this fact is, to a certain extent, intelligible.
Through the action of sexual and natural selection male animals have been
rendered in very many instances widely different from their females; but
independently of selection the two sexes, from differing constitutionally,
tend to vary in a somewhat different manner. The female has to expend much
organic matter in the formation of her ova, whereas the male expends much
force in fierce contests with his rivals, in wandering about in search of
the female, in exerting his voice, pouring out odoriferous secretions,
etc.: and this expenditure is generally concentrated within a short
period. The great vigour of the male during the season of love seems often
to intensify his colours, independently of any marked difference from the
female. (29. Prof. Mantegazza is inclined to believe ('Lettera a Carlo
Darwin,' 'Archivio per l'Anthropologia,' 1871, p. 306) that the bright
colours, common in so many male animals, are due to the presence and
retention by them of the spermatic fluid; but this can hardly be the case;
for many male birds, for instance young pheasants, become brightly coloured
in the autumn of their first year.) In mankind, and even as low down in
the organic scale as in the Lepidoptera, the temperature of the body is
higher in the male than in the female, accompanied in the case of man by a
slower pulse. (30. For mankind, see Dr. J. Stockton Hough, whose
conclusions are given in the 'Popular Science Review,' 1874, p. 97. See
Girard's observations on the Lepidoptera, as given in the 'Zoological
Record,' 1869, p. 347.) On the whole the expenditure of matter and force
by the two sexes is probably nearly equal, though effected in very
different ways and at different rates.
From the causes just specified the two sexes can hardly fail to differ
somewhat in constitution, at least during the breeding-season; and,
although they may be subjected to exactly the same conditions, they will
tend to vary in a different manner. If such variations are of no service
to either sex, they will not be accumulated and increased by sexual or
natural selection. Nevertheless, they may become permanent if the exciting
cause acts permanently; and in accordance with a frequent form of
inheritance they may be transmitted to that sex alone in which they first
appeared. In this case the two sexes will come to present permanent, yet
unimportant, differences of character. For instance, Mr. Allen shews that
with a large number of birds inhabiting the northern and southern United
States, the specimens from the south are darker-coloured than those from
the north; and this seems to be the direct result of the difference in
temperature, light, etc., between the two regions. Now, in some few cases,
the two sexes of the same species appear to have been differently affected;
in the Agelaeus phoeniceus the males have had their colours greatly
intensified in the south; whereas with Cardinalis virginianus it is the
females which have been thus affected; with Quiscalus major the females
have been rendered extremely variable in tint, whilst the males remain
nearly uniform. (31. 'Mammals and Birds of E. Florida,' pp. 234, 280,
295.)
A few exceptional cases occur in various classes of animals, in which the
females instead of the males have acquired well pronounced secondary sexual
characters, such as brighter colours, greater size, strength, or pugnacity.
With birds there has sometimes been a complete transposition of the
ordinary characters proper to each sex; the females having become the more
eager in courtship, the males remaining comparatively passive, but
apparently selecting the more attractive females, as we may infer from the
results. Certain hen birds have thus been rendered more highly coloured or
otherwise ornamented, as well as more powerful and pugnacious than the
cocks; these characters being transmitted to the female offspring alone.
It may be suggested that in some cases a double process of selection has
been carried on; that the males have selected the more attractive females,
and the latter the more attractive males. This process, however, though it
might lead to the modification of both sexes, would not make the one sex
different from the other, unless indeed their tastes for the beautiful
differed; but this is a supposition too improbable to be worth considering
in the case of any animal, excepting man. There are, however, many animals
in which the sexes resemble each other, both being furnished with the same
ornaments, which analogy would lead us to attribute to the agency of sexual
selection. In such cases it may be suggested with more plausibility, that
there has been a double or mutual process of sexual selection; the more
vigorous and precocious females selecting the more attractive and vigorous
males, the latter rejecting all except the more attractive females. But
from what we know of the habits of animals, this view is hardly probable,
for the male is generally eager to pair with any female. It is more
probable that the ornaments common to both sexes were acquired by one sex,
generally the male, and then transmitted to the offspring of both sexes.
If, indeed, during a lengthened period the males of any species were
greatly to exceed the females in number, and then during another lengthened
period, but under different conditions, the reverse were to occur, a
double, but not simultaneous, process of sexual selection might easily be
carried on, by which the two sexes might be rendered widely different.
We shall hereafter see that many animals exist, of which neither sex is
brilliantly coloured or provided with special ornaments, and yet the
members of both sexes or of one alone have probably acquired simple
colours, such as white or black, through sexual selection. The absence of
bright tints or other ornaments may be the result of variations of the
right kind never having occurred, or of the animals themselves having
preferred plain black or white. Obscure tints have often been developed
through natural selection for the sake of protection, and the acquirement
through sexual selection of conspicuous colours, appears to have been
sometimes checked from the danger thus incurred. But in other cases the
males during long ages may have struggled together for the possession of
the females, and yet no effect will have been produced, unless a larger
number of offspring were left by the more successful males to inherit their
superiority, than by the less successful: and this, as previously shewn,
depends on many complex contingencies.
Sexual selection acts in a less rigorous manner than natural selection.
The latter produces its effects by the life or death at all ages of the
more or less successful individuals. Death, indeed, not rarely ensues from
the conflicts of rival males. But generally the less successful male
merely fails to obtain a female, or obtains a retarded and less vigorous
female later in the season, or, if polygamous, obtains fewer females; so
that they leave fewer, less vigorous, or no offspring. In regard to
structures acquired through ordinary or natural selection, there is in most
cases, as long as the conditions of life remain the same, a limit to the
amount of advantageous modification in relation to certain special
purposes; but in regard to structures adapted to make one male victorious
over another, either in fighting or in charming the female, there is no
definite limit to the amount of advantageous modification; so that as long
as the proper variations arise the work of sexual selection will go on.
This circumstance may partly account for the frequent and extraordinary
amount of variability presented by secondary sexual characters.
Nevertheless, natural selection will determine that such characters shall
not be acquired by the victorious males, if they would be highly injurious,
either by expending too much of their vital powers, or by exposing them to
any great danger. The development, however, of certain structures--of the
horns, for instance, in certain stags--has been carried to a wonderful
extreme; and in some cases to an extreme which, as far as the general
conditions of life are concerned, must be slightly injurious to the male.
From this fact we learn that the advantages which favoured males derive
from conquering other males in battle or courtship, and thus leaving a
numerous progeny, are in the long run greater than those derived from
rather more perfect adaptation to their conditions of life. We shall
further see, and it could never have been anticipated, that the power to
charm the female has sometimes been more important than the power to
conquer other males in battle.
LAWS OF INHERITANCE.
In order to understand how sexual selection has acted on many animals of
many classes, and in the course of ages has produced a conspicuous result,
it is necessary to bear in mind the laws of inheritance, as far as they are
known. Two distinct elements are included under the term "inheritance"--
the transmission, and the development of characters; but as these generally
go together, the distinction is often overlooked. We see this distinction
in those characters which are transmitted through the early years of life,
but are developed only at maturity or during old age. We see the same
distinction more clearly with secondary sexual characters, for these are
transmitted through both sexes, though developed in one alone. That they
are present in both sexes, is manifest when two species, having strongly-
marked sexual characters, are crossed, for each transmits the characters
proper to its own male and female sex to the hybrid offspring of either
sex. The same fact is likewise manifest, when characters proper to the
male are occasionally developed in the female when she grows old or becomes
diseased, as, for instance, when the common hen assumes the flowing tail-
feathers, hackles, comb, spurs, voice, and even pugnacity of the cock.
Conversely, the same thing is evident, more or less plainly, with castrated
males. Again, independently of old age or disease, characters are
occasionally transferred from the male to the female, as when, in certain
breeds of the fowl, spurs regularly appear in the young and healthy
females. But in truth they are simply developed in the female; for in
every breed each detail in the structure of the spur is transmitted through
the female to her male offspring. Many cases will hereafter be given,
where the female exhibits, more or less perfectly, characters proper to the
male, in whom they must have been first developed, and then transferred to
the female. The converse case of the first development of characters in
the female and of transference to the male, is less frequent; it will
therefore be well to give one striking instance. With bees the pollen-
collecting apparatus is used by the female alone for gathering pollen for
the larvae, yet in most of the species it is partially developed in the
males to whom it is quite useless, and it is perfectly developed in the
males of Bombus or the humble-bee. (32. H. Muller, 'Anwendung der
Darwin'schen Lehre,' etc., Verh. d. n. V. Jahrg., xxix. p. 42.) As not a
single other Hymenopterous insect, not even the wasp, which is closely
allied to the bee, is provided with a pollen-collecting apparatus, we have
no grounds for supposing that male bees primordially collected pollen as
well as the females; although we have some reason to suspect that male
mammals primordially suckled their young as well as the females. Lastly,
in all cases of reversion, characters are transmitted through two, three,
or many more generations, and are then developed under certain unknown
favourable conditions. This important distinction between transmission and
development will be best kept in mind by the aid of the hypothesis of
pangenesis. According to this hypothesis, every unit or cell of the body
throws off gemmules or undeveloped atoms, which are transmitted to the
offspring of both sexes, and are multiplied by self-division. They may
remain undeveloped during the early years of life or during successive
generations; and their development into units or cells, like those from
which they were derived, depends on their affinity for, and union with
other units or cells previously developed in the due order of growth.
INHERITANCE AT CORRESPONDING PERIODS OF LIFE.
This tendency is well established. A new character, appearing in a young
animal, whether it lasts throughout life or is only transient, will, in
general, reappear in the offspring at the same age and last for the same
time. If, on the other hand, a new character appears at maturity, or even
during old age, it tends to reappear in the offspring at the same advanced
age. When deviations from this rule occur, the transmitted characters much
oftener appear before, than after the corresponding age. As I have dwelt
on this subject sufficiently in another work (33. The 'Variation of
Animals and Plants under Domestication,' vol. ii. 1868, p. 75. In the last
chapter but one, the provisional hypothesis of pangenesis, above alluded
to, is fully explained.), I will here merely give two or three instances,
for the sake of recalling the subject to the reader's mind. In several
breeds of the Fowl, the down-covered chickens, the young birds in their
first true plumage, and the adults differ greatly from one another, as well
as from their common parent-form, the Gallus bankiva; and these characters
are faithfully transmitted by each breed to their offspring at the
corresponding periods of life. For instance, the chickens of spangled
Hamburgs, whilst covered with down, have a few dark spots on the head and
rump, but are not striped longitudinally, as in many other breeds; in their
first true plumage, "they are beautifully pencilled," that is each feather
is transversely marked by numerous dark bars; but in their second plumage
the feathers all become spangled or tipped with a dark round spot. (34.
These facts are given on the high authority of a great breeder, Mr. Teebay;
see Tegetmeier's 'Poultry Book,' 1868, p. 158. On the characters of
chickens of different breeds, and on the breeds of the pigeon, alluded to
in the following paragraph, see 'Variation of Animals,' etc., vol. i. pp.
160, 249; vol. ii. p. 77.) Hence in this breed variations have occurred
at, and been transmitted to, three distinct periods of life. The Pigeon
offers a more remarkable case, because the aboriginal parent species does
not undergo any change of plumage with advancing age, excepting that at
maturity the breast becomes more iridescent; yet there are breeds which do
not acquire their characteristic colours until they have moulted two,
three, or four times; and these modifications of plumage are regularly
transmitted.
INHERITANCE AT CORRESPONDING SEASONS OF THE YEAR.
With animals in a state of nature, innumerable instances occur of
characters appearing periodically at different seasons. We see this in the
horns of the stag, and in the fur of Artic animals which becomes thick and
white during the winter. Many birds acquire bright colours and other
decorations during the breeding-season alone. Pallas states (35. 'Novae
species Quadrupedum e Glirium ordine,' 1778, p. 7. On the transmission of
colour by the horse, see 'Variation of Animals and Plants under
Domestication,' vol. i. p. 51. Also vol. ii. p. 71, for a general
discussion on 'Inheritance as limited by Sex.'), that in Siberia domestic
cattle and horses become lighter-coloured during the winter; and I have
myself observed, and heard of similar strongly marked changes of colour,
that is, from brownish cream-colour or reddish-brown to a perfect white, in
several ponies in England. Although I do not know that this tendency to
change the colour of the coat during different seasons is transmitted, yet
it probably is so, as all shades of colour are strongly inherited by the
horse. Nor is this form of inheritance, as limited by the seasons, more
remarkable than its limitation by age or sex.
INHERITANCE AS LIMITED BY SEX.
The equal transmission of characters to both sexes is the commonest form of
inheritance, at least with those animals which do not present strongly-
marked sexual differences, and indeed with many of these. But characters
are somewhat commonly transferred exclusively to that sex, in which they
first appear. Ample evidence on this head has been advanced in my work on
'Variation under Domestication,' but a few instances may here be given.
There are breeds of the sheep and goat, in which the horns of the male
differ greatly in shape from those of the female; and these differences,
acquired under domestication, are regularly transmitted to the same sex.
As a rule, it is the females alone in cats which are tortoise-shell, the
corresponding colour in the males being rusty-red. With most breeds of the
fowl, the characters proper to each sex are transmitted to the same sex
alone. So general is this form of transmission that it is an anomaly when
variations in certain breeds are transmitted equally to both sexes. There
are also certain sub-breeds of the fowl in which the males can hardly be
distinguished from one another, whilst the females differ considerably in
colour. The sexes of the pigeon in the parent-species do not differ in any
external character; nevertheless, in certain domesticated breeds the male
is coloured differently from the female. (36. Dr. Chapuis, 'Le Pigeon
Voyageur Belge,' 1865, p. 87. Boitard et Corbie, 'Les Pigeons de Voliere,'
etc., 1824, p. 173. See, also, on similar differences in certain breeds at
Modena, 'Le variazioni dei Colombi domestici,' del Paolo Bonizzi, 1873.)
The wattle in the English Carrier pigeon, and the crop in the Pouter, are
more highly developed in the male than in the female; and although these
characters have been gained through long-continued selection by man, the
slight differences between the sexes are wholly due to the form of
inheritance which has prevailed; for they have arisen, not from, but rather
in opposition to, the wish of the breeder.
Most of our domestic races have been formed by the accumulation of many
slight variations; and as some of the successive steps have been
transmitted to one sex alone, and some to both sexes, we find in the
different breeds of the same species all gradations between great sexual
dissimilarity and complete similarity. Instances have already been given
with the breeds of the fowl and pigeon, and under nature analogous cases
are common. With animals under domestication, but whether in nature I will
not venture to say, one sex may lose characters proper to it, and may thus
come somewhat to resemble the opposite sex; for instance, the males of some
breeds of the fowl have lost their masculine tail-plumes and hackles. On
the other hand, the differences between the sexes may be increased under
domestication, as with merino sheep, in which the ewes have lost their
horns. Again, characters proper to one sex may suddenly appear in the
other sex; as in those sub-breeds of the fowl in which the hens acquire
spurs whilst young; or, as in certain Polish sub-breeds, in which the
females, as there is reason to believe, originally acquired a crest, and
subsequently transferred it to the males. All these cases are intelligible
on the hypothesis of pangenesis; for they depend on the gemmules of certain
parts, although present in both sexes, becoming, through the influence of
domestication, either dormant or developed in either sex.
There is one difficult question which it will be convenient to defer to a
future chapter; namely, whether a character at first developed in both
sexes, could through selection be limited in its development to one sex
alone. If, for instance, a breeder observed that some of his pigeons (of
which the characters are usually transferred in an equal degree to both
sexes) varied into pale blue, could he by long-continued selection make a
breed, in which the males alone should be of this tint, whilst the females
remained unchanged? I will here only say, that this, though perhaps not
impossible, would be extremely difficult; for the natural result of
breeding from the pale-blue males would be to change the whole stock of
both sexes to this tint. If, however, variations of the desired tint
appeared, which were from the first limited in their development to the
male sex, there would not be the least difficulty in making a breed with
the two sexes of a different colour, as indeed has been effected with a
Belgian breed, in which the males alone are streaked with black. In a
similar manner, if any variation appeared in a female pigeon, which was
from the first sexually limited in its development to the females, it would
be easy to make a breed with the females alone thus characterised; but if
the variation was not thus originally limited, the process would be
extremely difficult, perhaps impossible. (37. Since the publication of
the first edition of this work, it has been highly satisfactory to me to
find the following remarks (the 'Field,' Sept. 1872) from so experienced a
breeder as Mr. Tegetmeier. After describing some curious cases in pigeons,
of the transmission of colour by one sex alone, and the formation of a sub-
breed with this character, he says: "It is a singular circumstance that
Mr. Darwin should have suggested the possibility of modifying the sexual
colours of birds by a course of artificial selection. When he did so, he
was in ignorance of these facts that I have related; but it is remarkable
how very closely he suggested the right method of procedure.")
ON THE RELATION BETWEEN THE PERIOD OF DEVELOPMENT OF A CHARACTER AND ITS
TRANSMISSION TO ONE SEX OR TO BOTH SEXES.
Why certain characters should be inherited by both sexes, and other
characters by one sex alone, namely by that sex in which the character
first appeared, is in most cases quite unknown. We cannot even conjecture
why with certain sub-breeds of the pigeon, black striae, though transmitted
through the female, should be developed in the male alone, whilst every
other character is equally transferred to both sexes. Why, again, with
cats, the tortoise-shell colour should, with rare exceptions, be developed
in the female alone. The very same character, such as deficient or
supernumerary digits, colour-blindness, etc., may with mankind be inherited
by the males alone of one family, and in another family by the females
alone, though in both cases transmitted through the opposite as well as
through the same sex. (38. References are given in my 'Variation of
Animals and Plants under Domestication,' vol. ii. p. 72.) Although we are
thus ignorant, the two following rules seem often to hold good--that
variations which first appear in either sex at a late period of life, tend
to be developed in the same sex alone; whilst variations which first appear
early in life in either sex tend to be developed in both sexes. I am,
however, far from supposing that this is the sole determining cause. As I
have not elsewhere discussed this subject, and it has an important bearing
on sexual selection, I must here enter into lengthy and somewhat intricate
details.
It is in itself probable that any character appearing at an early age would
tend to be inherited equally by both sexes, for the sexes do not differ
much in constitution before the power of reproduction is gained. On the
other hand, after this power has been gained and the sexes have come to
differ in constitution, the gemmules (if I may again use the language of
pangenesis) which are cast off from each varying part in the one sex would
be much more likely to possess the proper affinities for uniting with the
tissues of the same sex, and thus becoming developed, than with those of
the opposite sex.
I was first led to infer that a relation of this kind exists, from the fact
that whenever and in whatever manner the adult male differs from the adult
female, he differs in the same manner from the young of both sexes. The
generality of this fact is quite remarkable: it holds good with almost all
mammals, birds, amphibians, and fishes; also with many crustaceans,
spiders, and some few insects, such as certain orthoptera and libellulae.
In all these cases the variations, through the accumulation of which the
male acquired his proper masculine characters, must have occurred at a
somewhat late period of life; otherwise the young males would have been
similarly characterised; and conformably with our rule, the variations are
transmitted to and developed in the adult males alone. When, on the other
hand, the adult male closely resembles the young of both sexes (these, with
rare exceptions, being alike), he generally resembles the adult female; and
in most of these cases the variations through which the young and old
acquired their present characters, probably occurred, according to our
rule, during youth. But there is here room for doubt, for characters are
sometimes transferred to the offspring at an earlier age than that at which
they first appeared in the parents, so that the parents may have varied
when adult, and have transferred their characters to their offspring whilst
young. There are, moreover, many animals, in which the two sexes closely
resemble each other, and yet both differ from their young: and here the
characters of the adults must have been acquired late in life;
nevertheless, these characters, in apparent contradiction to our rule, are
transferred to both sexes. We must not however, overlook the possibility
or even probability of successive variations of the same nature occurring,
under exposure to similar conditions, simultaneously in both sexes at a
rather late period of life; and in this case the variations would be
transferred to the offspring of both sexes at a corresponding late age; and
there would then be no real contradiction to the rule that variations
occurring late in life are transferred exclusively to the sex in which they
first appeared. This latter rule seems to hold true more generally than
the second one, namely, that variations which occur in either sex early in
life tend to be transferred to both sexes. As it was obviously impossible
even to estimate in how large a number of cases throughout the animal
kingdom these two propositions held good, it occurred to me to investigate
some striking or crucial instances, and to rely on the result.
An excellent case for investigation is afforded by the Deer family. In all
the species, but one, the horns are developed only in the males, though
certainly transmitted through the females, and capable of abnormal
development in them. In the reindeer, on the other hand, the female is
provided with horns; so that in this species, the horns ought, according to
our rule, to appear early in life, long before the two sexes are mature and
have come to differ much in constitution. In all the other species the
horns ought to appear later in life, which would lead to their development
in that sex alone, in which they first appeared in the progenitor of the
whole Family. Now in seven species, belonging to distinct sections of the
family and inhabiting different regions, in which the stags alone bear
horns, I find that the horns first appear at periods, varying from nine
months after birth in the roebuck, to ten, twelve or even more months in
the stags of the six other and larger species. (39. I am much obliged to
Mr. Cupples for having made enquiries for me in regard to the Roebuck and
Red Deer of Scotland from Mr. Robertson, the experienced head-forester to
the Marquis of Breadalbane. In regard to Fallow-deer, I have to thank Mr.
Eyton and others for information. For the Cervus alces of N. America, see
'Land and Water,' 1868, pp. 221 and 254; and for the C. Virginianus and
strongyloceros of the same continent, see J.D. Caton, in 'Ottawa Acad. of
Nat. Sc.' 1868, p. 13. For Cervus Eldi of Pegu, see Lieut. Beaven,
'Proccedings of the Zoological Society,' 1867, p. 762.) But with the
reindeer the case is widely different; for, as I hear from Prof. Nilsson,
who kindly made special enquiries for me in Lapland, the horns appear in
the young animals within four or five weeks after birth, and at the same
time in both sexes. So that here we have a structure, developed at a most
unusually early age in one species of the family, and likewise common to
both sexes in this one species alone.
In several kinds of antelopes, only the males are provided with horns,
whilst in the greater number both sexes bear horns. With respect to the
period of development, Mr. Blyth informs me that there was at one time in
the Zoological Gardens a young koodoo (Ant. strepsiceros), of which the
males alone are horned, and also the young of a closely-allied species, the
eland (Ant. oreas), in which both sexes are horned. Now it is in strict
conformity with our rule, that in the young male koodoo, although ten
months old, the horns were remarkably small, considering the size
ultimately attained by them; whilst in the young male eland, although only
three months old, the horns were already very much larger than in the
koodoo. It is also a noticeable fact that in the prong-horned antelope
(40. Antilocapra Americana. I have to thank Dr. Canfield for information
with respect to the horns of the female: see also his paper in
'Proceedings of the Zoological Society,' 1866, p. 109. Also Owen, 'Anatomy
of Vertebrates,' vol. iii. p. 627), only a few of the females, about one in
five, have horns, and these are in a rudimentary state, though sometimes
above four inches long: so that as far as concerns the possession of horns
by the males alone, this species is in an intermediate condition, and the
horns do not appear until about five or six months after birth. Therefore
in comparison with what little we know of the development of the horns in
other antelopes, and from what we do know with respect to the horns of
deer, cattle, etc., those of the prong-horned antelope appear at an
intermediate period of life,--that is, not very early, as in cattle and
sheep, nor very late, as in the larger deer and antelopes. The horns of
sheep, goats, and cattle, which are well developed in both sexes, though
not quite equal in size, can be felt, or even seen, at birth or soon
afterwards. (41. I have been assured that the horns of the sheep in North
Wales can always be felt, and are sometimes even an inch in length, at
birth. Youatt says ('Cattle,' 1834, p. 277), that the prominence of the
frontal bone in cattle penetrates the cutis at birth, and that the horny
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