The Descent of Man and Selection in Relation to Sex Volume II Part 10

The female Kemas goat is paler, and the female _Capra aegagrus_ is said to be more uniformly tinted than their respective males. Deer rarely present any s.e.xual differences in colour. Judge Caton, however, informs me that with the males of the Wapiti deer (_Cervus Canadensis_) the neck, belly, and legs are much darker than the same parts in the female; but during the winter the darker tints gradually fade away and disappear. I may here mention that Judge Caton has in his park three races of the Virginian deer, which differ slightly in colour, but the differences are almost exclusively confined to the blue winter or breeding coat; so that this case may be compared with those given in a previous chapter of closely-allied or representative species of birds which differ from each other only in their nuptial plumage.[358] The females of _Cervus paludosus_ of S. America, as well as the young of both s.e.xes, do not possess the black stripes on the nose, and the blackish-brown line on the breast which characterise the adult males.[359] Lastly, the mature male of the beautifully coloured and spotted Axis deer is considerably darker, as I am informed by Mr. Blyth, than the female; and this hue the castrated male never acquires.

The last Order which we have to consider-for I am not aware that s.e.xual differences in colour occur in the other mammalian groups-is that of the Primates. The male of the _Lemur macaco_ is coal-black, whilst the female is reddish-yellow, but highly variable in colour.[360] Of the Quadrumana of the New World, the females and young of _Mycetes caraya_ are greyish-yellow and alike; in the second year the young male becomes reddish-brown, in the third year black, excepting the stomach, which, however, becomes quite black in the fourth or fifth year. There is also a strongly-marked difference in colour between the s.e.xes in _Mycetes seniculus_ and _Cebus capucinus_; the young of the former and I believe of the latter species resembling the females. With _Pithecia leucocephala_ the young likewise resemble the females, which are brownish-black above and light rusty-red beneath, the adult males being black. The ruff of hair round the face of _Ateles marginatus_ is tinted yellow in the male and white in the female. Turning to the Old World, the males of _Hylobates hoolock_ are always black, with the exception of a white band over the brows; the females vary from whity-brown to a dark tint mixed with black, but are never wholly black.[361] In the beautiful _Cercopithecus diana_ the head of the adult male is of an intense black, whilst that of the female is dark grey; in the former the fur between the thighs is of an elegant fawn-colour, in the latter it is paler. In the equally beautiful and curious moustache monkey (_Cercopithecus cephus_) the only difference between the s.e.xes is that the tail of the male is chesnut and that of the female grey; but Mr. Bartlett informs me that all the hues become more strongly p.r.o.nounced in the male when adult, whilst in the female they remain as they were during youth.

According to the coloured figures given by Solomon Muller, the male of _Semnopithecus chrysomelas_ is nearly black, the female being pale brown. In the _Cercopithecus cynosurus_ and _griseo-viridis_ one part of the body which is confined to the male s.e.x is of the most brilliant blue or green, and contrasts strikingly with the naked skin on the hinder part of the body, which is vivid red.

[Ill.u.s.tration: Fig. 67. Head of male Mandrill (from Gervais, 'Hist. Nat des Mammiferes').]

Lastly, in the Baboon family, the adult male of _Cynocephalus hamadryas_ differs from the female not only by his immense mane, but slightly in the colour of the hair and of the naked callosities. In the drill (_Cynocephalus leucophus_) the females and young are much paler-coloured, with less green, than the adult males. No other member of the whole cla.s.s of mammals is coloured in so extraordinary a manner as the adult male mandrill (_Cynocephalus mormon_). The face at this age becomes of a fine blue, with the ridge and tip of the nose of the most brilliant red. According to some authors the face is also marked with whitish stripes, and is shaded in parts with black, but the colours appear to be variable. On the forehead there is a crest of hair, and on the chin a yellow beard. "Toutes les parties superieures de leurs cuisses et le grand es.p.a.ce nu de leurs fesses sont egalement colores du rouge le plus vif, avec un melange de bleu qui ne manque reellement pas d'elegance."[362] When the animal is excited all the naked parts become much more vividly tinted. Several authors have used the strongest expressions in describing these resplendent colours, which they compare with those of the most brilliant birds. Another most remarkable peculiarity is that when the great canine teeth are fully developed, immense protuberances of bone are formed on each cheek, which are deeply furrowed longitudinally, and the naked skin over them is brilliantly-coloured, as just described. (Fig. 67.) In the adult females and in the young of both s.e.xes these protuberances are scarcely perceptible; and the naked parts are much less brightly coloured, the face being almost black, tinged with blue. In the adult female, however, the nose at certain regular intervals of time becomes tinted with red.

In all the cases. .h.i.therto given the male is more strongly or brightly coloured than the female, and differs in a greater degree from the young of both s.e.xes. But as a reversed style of colouring is characteristic of the two s.e.xes with some few birds, so with the Rhesus monkey (_Macacus rhesus_) the female has a large surface of naked skin round the tail, of a brilliant carmine red, which periodically becomes, as I was a.s.sured by the keepers in the Zoological Gardens, even more vivid, and her face is also pale red. On the other hand with the adult male and with the young of both s.e.xes, as I saw in the Gardens, neither the naked skin at the posterior end of the body, nor the face, shew a trace of red. It appears, however, from some published accounts, that the male does occasionally, or during certain seasons, exhibit some traces of the red.

Although he is thus less ornamented than the female, yet in the larger size of his body, larger canine teeth, more developed whiskers, more prominent superciliary ridges, he follows the common rule of the male excelling the female.

I have now given all the cases known to me of a difference in colour between the s.e.xes of mammals. The colours of the female either do not differ in a sufficient degree from those of the male, or are not of a suitable nature, to afford her protection, and therefore cannot be explained on this principle. In some, perhaps in many cases, the differences may be the result of variations confined to one s.e.x and transmitted to the same s.e.x, without any good having been thus gained, and therefore without the aid of selection. We have instances of this kind with our domesticated animals, as in the males of certain cats being rusty-red, whilst the females are tortoise-sh.e.l.l coloured.

a.n.a.logous cases occur under nature; Mr. Bartlett has seen many black varieties of the jaguar, leopard, vulpine phalanger and wombat; and he is certain that all, or nearly all, were males. On the other hand, both s.e.xes of wolves, foxes, and apparently of American squirrels, are occasionally born black. Hence it is quite possible that with some mammals the blackness of the males, especially when this colour is congenital, may simply be the result, without the aid of selection, of one or more variations having occurred, which from the first were s.e.xually limited in their transmission. Nevertheless it can hardly be admitted that the diversified, vivid, and contrasted colours of certain quadrupeds, for instance of the above-mentioned monkeys and antelopes, can thus be accounted for. We should bear in mind that these colours do not appear in the male at birth, as in the case of most ordinary variations, but only at or near maturity; and that unlike ordinary variations, if the male be emasculated, they never appear or subsequently disappear. It is on the whole a much more probable conclusion that the strongly-marked colours and other ornamental characters of male quadrupeds are beneficial to them in their rivalry with other males, and have consequently been acquired through s.e.xual selection. The probability of this view is strengthened by the differences in colour between the s.e.xes occurring almost exclusively, as may be observed by going through the previous details, in those groups and subgroups of mammals, which present other and distinct secondary s.e.xual characters; these being likewise due to the action of s.e.xual selection.

Quadrupeds manifestly take notice of colour. Sir S. Baker repeatedly observed that the African elephant and rhinoceros attacked with special fury white or grey horses. I have elsewhere shewn[363] that half-wild horses apparently prefer pairing with those of the same colour, and that herds of fallow-deer of a different colour, though living together, have long kept distinct. It is a more significant fact that a female zebra would not admit the addresses of a male a.s.s until he was painted so as to resemble a zebra, and then, as John Hunter remarks, "she received him very readily. In this curious fact, we have instinct excited by mere colour, which had so strong an effect as to get the better of everything else. But the male did not require this, the female being an animal somewhat similar to himself, was sufficient to rouse him."[364]

In an early chapter we have seen that the mental powers of the higher animals do not differ in kind, though so greatly in degree, from the corresponding powers of man, especially of the lower and barbarous races; and it would appear that even their taste for the beautiful is not widely different from that of the Quadrumana. As the negro of Africa raises the flesh on his face into parallel ridges "or cicatrices, high above the natural surface, which unsightly deformities, are considered great personal attractions;"[365]-as negroes, as well as savages in many parts of the world, paint their faces with red, blue, white, or black bars,-so the male mandrill of Africa appears to have acquired his deeply-furrowed and gaudily-coloured face from having been thus rendered attractive to the female. No doubt it is to us a most grotesque notion that the posterior end of the body should have been coloured for the sake of ornament even more brilliantly than the face; but this is really not more strange than that the tails of many birds should have been especially decorated.

With mammals we do not at present possess any evidence that the males take pains to display their charms before the female; and the elaborate manner in which this is performed by male birds, is the strongest argument in favour of the belief that the females admire, or are excited by, the ornaments and colours displayed before them. There is, however, a striking parallelism between mammals and birds in all their secondary s.e.xual characters, namely in their weapons for fighting with rival males, in their ornamental appendages, and in their colours. In both cla.s.ses, when the male differs from the female, the young of both s.e.xes almost always resemble each other, and in a large majority of cases resemble the adult female. In both cla.s.ses the male a.s.sumes the characters proper to his s.e.x shortly before the age for reproduction; if emasculated he either never acquires such characters or subsequently loses them. In both cla.s.ses the change of colour is sometimes seasonal, and the tints of the naked parts sometimes become more vivid during the act of courts.h.i.+p. In both cla.s.ses the male is almost always more vividly or strongly coloured than the female, and is ornamented with larger crests either of hair or feathers, or other appendages. In a few exceptional cases the female in both cla.s.ses is more highly ornamented than the male. With many mammals, and at least in the case of one bird, the male is more odoriferous than the female. In both cla.s.ses the voice of the male is more powerful than that of the female. Considering this parallelism there can be little doubt that the same cause, whatever it may be, has acted on mammals and birds; and the result, as far as ornamental characters are concerned, may safely be attributed, as it appears to me, to the long-continued preference of the individuals of one s.e.x for certain individuals of the opposite s.e.x, combined with their success in leaving a larger number of offspring to inherit their superior attractions.

_Equal transmission of ornamental characters to both s.e.xes._-With many birds, ornaments, which a.n.a.logy leads us to believe were primarily acquired by the males, have been transmitted equally, or almost equally, to both s.e.xes; and we may now enquire how far this view may be extended to mammals. With a considerable number of species, especially the smaller kinds, both s.e.xes have been coloured, independently of s.e.xual selection, for the sake of protection; but not, as far as I can judge, in so many cases, nor in nearly so striking a manner as in most of the lower cla.s.ses. Audubon remarks that he often mistook the musk-rat,[366]

whilst sitting on the banks of a muddy stream, for a clod of earth, so complete was the resemblance. The hare on her form is a familiar instance of concealment through colour; yet this principle partly fails in a closely-allied species, namely the rabbit, for as this animal runs to its burrow, it is made conspicuous to the sportsman and no doubt to all beasts of prey, by its upturned pure-white tail. No one has ever doubted that the quadrupeds which inhabit snow-clad regions, have been rendered white to protect them from their enemies, or to favour their stealing on their prey. In regions where snow never lies long on the ground a white coat would be injurious; consequently species thus coloured are extremely rare in the hotter parts of the world. It deserves notice that many quadrupeds, inhabiting moderately cold regions, although they do not a.s.sume a white winter dress, become paler during this season; and this apparently is the direct result of the conditions to which they have long been exposed. Pallas[367] states that in Siberia a change of this nature occurs with the wolf, two species of Mustela, the domestic horse, the _Equus hemionus_, the domestic cow, two species of antelopes, the musk-deer, the roe, the elk, and reindeer.

The roe, for instance, has a red summer and a greyish-white winter coat; and the latter may perhaps serve as a protection to the animal whilst wandering through the leafless thickets, sprinkled with snow and h.o.a.r-frost. If the above named animals were gradually to extend their range into regions perpetually covered with snow, their pale winter-coats would probably be rendered, through natural selection, whiter and whiter by degrees, until they became as white as snow.

[Ill.u.s.tration: Fig. 68. Tragelaphus scriptus, male (from the Knowsley Menagerie).]

[Ill.u.s.tration: Fig. 69. Damalis pygarga, male (from the Knowsley Menagerie).]

Although we must admit that many quadrupeds have received their present tints as a protection, yet with a host of species, the colours are far too conspicuous and too singularly arranged to allow us to suppose that they serve for this purpose. We may take as an ill.u.s.tration certain antelopes: when we see that the square white patch on the throat, the white marks on the fetlocks, and the round black spots on the ears, are all more distinct in the male of the _Portax picta_, than in the female;-when we see that the colours are more vivid, that the narrow white lines on the flank and the broad white bar on the shoulder are more distinct in the male _Oreas derbya.n.u.s_ than in the female;-when we see a similar difference between the s.e.xes of the curiously-ornamented _Tragelaphus scriptus_ (fig. 68),-we may conclude that these colours and various marks have been at least intensified through s.e.xual selection. It is inconceivable that such colours and marks can be of any direct or ordinary service to these animals; and as they have almost certainly been intensified through s.e.xual selection, it is probable that they were originally gained through this same process, and then partially transferred to the females. If this view be admitted, there can be little doubt that the equally singular colours and marks of many other antelopes, though common to both s.e.xes, have been gained and transmitted in a like manner. Both s.e.xes, for instance, of the Koodoo (_Strepsiceros Kudu_, fig. 62) have narrow white vertical lines on their hinder flanks, and an elegant angular white mark on their foreheads.

Both s.e.xes in the genus Damalis are very oddly coloured; in _D. pygarga_ the back and neck are purplish-red, shading on the flanks into black, and abruptly separated from the white belly and a large white s.p.a.ce on the b.u.t.tocks; the head is still more oddly coloured, a large oblong white mask, narrowly-edged with black, covers the face up to the eyes (fig. 69); there are three white stripes on the forehead, and the ears are marked with white. The fawns of this species are of a uniform pale yellowish-brown. In _Damalis albifrons_ the colouring of the head differs from that in the last species in a single white stripe replacing the three stripes, and in the ears being almost wholly white.[368] After having studied to the best of my ability the s.e.xual differences of animals belonging to all cla.s.ses, I cannot avoid the conclusion that the curiously-arranged colours of many antelopes, though common to both s.e.xes, are the result of s.e.xual selection primarily applied to the male.

The same conclusion may perhaps be extended to the tiger, one of the most beautiful animals in the world, the s.e.xes of which cannot be distinguished by colour, even by the dealers in wild beasts. Mr. Wallace believes[369] that the striped coat of the tiger "so a.s.similates with the vertical stems of the bamboo, as to a.s.sist greatly in concealing him from his approaching prey." But this view does not appear to me satisfactory. We have some slight evidence that his beauty may be due to s.e.xual selection, for in two species of Felis a.n.a.logous marks and colours are rather brighter in the male than in the female. The zebra is conspicuously striped, and stripes on the open plains of South Africa cannot afford any protection. Burch.e.l.l[370] in describing a herd says, "their sleek ribs glistened in the sun, and the brightness and regularity of their striped coats presented a picture of extraordinary beauty, in which probably they are not surpa.s.sed by any other quadruped." Here we have no evidence of s.e.xual selection, as throughout the whole group of the Equidae the s.e.xes are identical in colour.

Nevertheless he who attributes the white and dark vertical stripes on the flanks of various antelopes to s.e.xual selection, will probably extend the same view to the Royal Tiger and beautiful Zebra.

We have seen in a former chapter that when young animals belonging to any cla.s.s follow nearly the same habits of life with their parents, and yet are coloured in a different manner, it may be inferred that they have retained the colouring of some ancient and extinct progenitor. In the family of pigs, and in the genus Tapir, the young are marked with longitudinal stripes, and thus differ from every existing adult species in these two groups. With many kinds of deer the young are marked with elegant white spots, of which their parents exhibit not a trace. A graduated series can be followed from the Axis deer, both s.e.xes of which at all ages and during all seasons are beautifully spotted (the male being rather more strongly coloured than the female)-to species in which neither the old nor the young are spotted. I will specify some of the steps in this series. The Mantchurian deer (_Cervus Mantchuricus_) is spotted during the whole year, but the spots are much plainer, as I have seen in the Zoological Gardens, during the summer, when the general colour of the coat is lighter, than during the winter, when the general colour is darker and the horns are fully developed. In the hog-deer (_Hyelaphus porcinus_) the spots are extremely conspicuous during the summer when the coat is reddish-brown, but quite disappear during the winter when the coat is brown.[371] In both these species the young are spotted. In the Virginian deer the young are likewise spotted, and about five per cent. of the adult animals living in Judge Caton's park, as I am informed by him, temporarily exhibit at the period when the red summer coat is being replaced by the bluish winter coat, a row of spots on each flank, which are always the same in number, though very variable in distinctness. From this condition there is but a very small step to the complete absence of spots at all seasons in the adults; and lastly, to their absence at all ages, as occurs with certain species.

From the existence of this perfect series, and more especially from the fawns of so many species being spotted, we may conclude that the now living members of the deer family are the descendants of some ancient species which, like the Axis deer, was spotted at all ages and seasons.

A still more ancient progenitor probably resembled to a certain extent the _Hyomoschus aquaticus_-for this animal is spotted, and the hornless males have large exserted canine teeth, of which some few true deer still retain rudiments. It offers, also, one of those interesting cases of a form linking together two groups, as it is intermediate in certain osteological characters between the pachyderms and ruminants, which were formerly thought to be quite distinct.[372]

A curious difficulty here arises. If we admit that coloured spots and stripes have been acquired as ornaments, how comes it that so many existing deer, the descendants of an aboriginally spotted animal, and all the species of pigs and tapirs, the descendants of an aboriginally striped animal, have lost in their adult state their former ornaments? I cannot satisfactorily answer this question. We may feel nearly sure that the spots and stripes disappeared in the progenitors of our existing species at or near maturity, so that they were retained by the young and, owing to the law of inheritance at corresponding ages, by the young of all succeeding generations. It may have been a great advantage to the lion and puma from the open nature of the localities which they commonly haunt, to have lost their stripes, and to have been thus rendered less conspicuous to their prey; and if the successive variations, by which this end was gained, occurred rather late in life, the young would have retained their stripes, as we know to be the case.

In regard to deer, pigs, and tapirs, Fritz Muller has suggested to me that these animals by the removal through natural selection of their spots or stripes would have been less easily seen by their enemies; and they would have especially required this protection, as soon as the carnivora increased in size and number during the Tertiary periods. This may be the true explanation, but it is rather strange that the young should not have been equally well protected, and still more strange that with some species the adults should have retained their spots, either partially or completely, during part of the year. We know, though we cannot explain the cause, that when the domestic a.s.s varies and becomes reddish-brown, grey or black, the stripes on the shoulders and even on the spine frequently disappear. Very few horses, except dun-coloured kinds, exhibit stripes on any part of their bodies, yet we have good reason to believe that the aboriginal horse was striped on the legs and spine, and probably on the shoulders.[373] Hence the disappearance of the spots and stripes in our adult existing deer, pigs, and tapirs, may be due to a change in the general colour of their coats; but whether this change was effected through s.e.xual or natural selection, or was due to the direct action of the conditions of life, or some other unknown cause, it is impossible to decide. An observation made by Mr. Sclater well ill.u.s.trates our ignorance of the laws which regulate the appearance and disappearance of stripes; the species of Asinus which inhabit the Asiatic continent are dest.i.tute of stripes, not having even the cross shoulder-stripe, whilst those which inhabit Africa are conspicuously striped, with the partial exception of _A. taeniopus_, which has only the cross shoulder-stripe and generally some faint bars on the legs; and this species inhabits the almost intermediate region of Upper Egypt and Abyssinia.[374]

[Ill.u.s.tration: Fig. 70. Head of Semnopithecus rubicundus. This and the following figures (from Prof. Gervais) are given to shew the odd arrangement and development of the hair on the head.]

[Ill.u.s.tration: Fig. 71. Head of Semnopithecus comatus.]

[Ill.u.s.tration: Fig. 72. Head of Cebus capucinus.]

[Ill.u.s.tration: Fig. 73. Head of Ateles marginatus.]

[Ill.u.s.tration: Fig. 74. Head of Cebus vellerosus.]

_Quadrumana._-Before we conclude, it will be advisable to add a few remarks to those already given on the ornamental characters of monkeys.

In most of the species the s.e.xes resemble each other in colour, but in some, as we have seen, the males differ from the females, especially in the colour of the naked parts of the skin, in the development of the beard, whiskers, and mane. Many species are coloured either in so extraordinary or beautiful a manner, and are furnished with such curious and elegant crests of hair, that we can hardly avoid looking at these characters as having been gained for the sake of ornament. The accompanying figures (figs. 70 to 74) serve to shew the arrangement of the hair on the face and head in several species. It is scarcely conceivable that these crests of hair and the strongly-contrasted colours of the fur and skin can be the result of mere variability without the aid of selection; and it is inconceivable that they can be of any ordinary use to these animals. If so, they have probably been gained through s.e.xual selection, though transmitted equally, or almost equally, to both s.e.xes. With many of the Quadrumana, we have additional evidence of the action of s.e.xual selection in the greater size and strength of the males, and in the greater development of their canine teeth, in comparison with the females.

With respect to the strange manner in which both s.e.xes of some species are coloured, and of the beauty of others, a few instances will suffice.

The face of the _Cercopithecus petaurista_ (fig. 75) is black, the whiskers and beard being white, with a defined, round, white spot on the nose, covered with short white hair, which gives to the animal an almost ludicrous aspect. The _Semnopithecus frontatus_ likewise, has a blackish face with a long black beard, and a large naked spot on the forehead of a bluish-white colour. The face of _Macacus lasiotus_ is dirty flesh-coloured, with a defined red spot on each cheek. The appearance of _Cercocebus aethiops_ is grotesque, with its black face, white whiskers and collar, chesnut head, and a large naked white spot over each eyelid.

In very many species, the beard, whiskers, and crests of hair round the face are of a different colour from the rest of the head, and when different, are always of a lighter tint,[375] being often pure white, sometimes bright yellow, or reddish. The whole face of the South American _Brachyurus calvus_ is of a "glowing scarlet hue;" but this colour does not appear until the animal is nearly mature.[376] The naked skin of the face differs wonderfully in colour in the various species. It is often brown or flesh-colour, with parts perfectly white, and often as black as that of the most sooty negro. In the Brachyurus the scarlet tint is brighter than that of the most blus.h.i.+ng Caucasian damsel. It is sometimes more distinctly orange than in any Mongolian, and in several species it is blue, pa.s.sing into violet or grey. In all the species known to Mr. Bartlett, in which the adults of both s.e.xes have strongly-coloured faces, the colours are dull or absent during early youth. This likewise holds good with the Mandrill and Rhesus, in which the face and the posterior parts of the body are brilliantly coloured in one s.e.x alone. In these latter cases we have every reason to believe that the colours were acquired through s.e.xual selection; and we are naturally led to extend the same view to the foregoing species, though both s.e.xes when adult have their faces coloured in the same manner.

[Ill.u.s.tration: Fig 75. Cercopithecus petaurista (from Brehm).]

Although, according to our taste, many kinds of monkeys are far from beautiful, other species are universally admired for their elegant appearance and bright colours. The _Semnopithecus nemaeus_, though peculiarly coloured, is described as extremely pretty; the orange-tinted face is surrounded by long whiskers of glossy whiteness, with a line of chesnut-red over the eyebrows; the fur on the back is of a delicate grey, with a square patch on the loins, the tail and the fore-arms all of a pure white; a gorget of chesnut surmounts the chest; the hind thighs are black, with the legs chesnut-red. I will mention only two other monkeys on account of their beauty; and I have selected these as they present slight s.e.xual differences in colour, which renders it in some degree probable that both s.e.xes owe their elegant appearance to s.e.xual selection. In the moustache-monkey (_Cercopithecus cephus_) the general colour of the fur is mottled-greenish, with the throat white; in the male the end of the tail is chesnut; but the face is the most ornamented part, the skin being chiefly bluish-grey, shading into a blackish tint beneath the eyes, with the upper lip of a delicate blue, clothed on the lower edge with a thin black moustache; the whiskers are orange-coloured, with the upper part black, forming a band which extends backwards to the ears, the latter being clothed with whitish hairs. In the Zoological Society's Gardens I have often overheard visitors admiring the beauty of another monkey, deservedly called _Cercopithecus Diana_ (fig. 76); the general colour of the fur is grey; the chest and inner surface of the forelegs are white; a large triangular defined s.p.a.ce on the hinder part of the back is rich chesnut; in the male the inner sides of the thighs and the abdomen are delicate fawn-coloured, and the top of the head is black; the face and ears are intensely black, finely contrasted with a white transverse crest over the eyebrows and with a long white peaked beard, of which the basal portion is black.[377]

[Ill.u.s.tration: Fig. 76. Cercopithecus Diana (from Brehm).]

In these and many other monkeys, the beauty and singular arrangement of their colours, and still more the diversified and elegant arrangement of the crests and tufts of hair on their heads, force the conviction on my mind that these characters have been acquired through s.e.xual selection exclusively as ornaments.

_Summary._-The law of battle for the possession of the female appears to prevail throughout the whole great cla.s.s of mammals. Most naturalists will admit that the greater size, strength, courage, and pugnacity of the male, his special weapons of offence, as well as his special means of defence, have all been acquired or modified through that form of selection which I have called s.e.xual selection. This does not depend on any superiority in the general struggle for life, but on certain individuals of one s.e.x, generally the male s.e.x, having been successful in conquering other males, and on their having left a larger number of offspring to inherit their superiority, than the less successful males.

There is another and more peaceful kind of contest, in which the males endeavour to excite or allure the females by various charms. This may be effected by the powerful odours emitted by the males during the breeding-season; the odoriferous glands having been acquired through s.e.xual selection. Whether the same view can be extended to the voice is doubtful, for the vocal organs of the males may have been strengthened by use during maturity, under the powerful excitements of love, jealousy, or rage, and transmitted to the same s.e.x. Various crests, tufts, and mantles of hair, which are either confined to the male, or are more developed in this s.e.x than in the females, seem in most cases to be merely ornamental, though they sometimes serve as a defence against rival males. There is even reason to suspect that the branching horns of stags, and the elegant horns of certain antelopes, though properly serving as weapons of offence or of defence, have been partly modified for the sake of ornament.

When the male differs in colour from the female he generally exhibits darker and more strongly-contrasted tints. We do not in this cla.s.s meet with the splendid red, blue, yellow, and green colours, so common with male birds and many other animals. The naked parts, however, of certain Quadrumana must be excepted; for such parts, often oddly situated, are coloured in some species in the most brilliant manner. The colours of the male in other cases may be due to simple variation, without the aid of selection. But when the colours are diversified and strongly p.r.o.nounced, when they are not developed until near maturity, and when they are lost after emasculation, we can hardly avoid the conclusion that they have been acquired through s.e.xual selection for the sake of ornament, and have been transmitted exclusively, or almost exclusively, to the same s.e.x. When both s.e.xes are coloured in the same manner, and the colours are conspicuous or curiously arranged, without being of the least apparent use as a protection, and especially when they are a.s.sociated with various other ornamental appendages, we are led by a.n.a.logy to the same conclusion, namely, that they have been acquired through s.e.xual selection, although transmitted to both s.e.xes. That conspicuous and diversified colours, whether confined to the males or common to both s.e.xes, are as a general rule a.s.sociated in the same groups and subgroups with other secondary s.e.xual characters, serving for war or for ornament, will be found to hold good if we look back to the various cases given in this and the last chapter.

The law of the equal transmission of characters to both s.e.xes, as far as colour and other ornaments are concerned, has prevailed far more extensively with mammals than with birds; but in regard to weapons, such as horns and tusks, these have often been transmitted either exclusively, or in a much higher degree to the males than to the females. This is a surprising circ.u.mstance, for as the males generally use their weapons as a defence against enemies of all kinds, these weapons would have been of service to the female. Their absence in this s.e.x can be accounted for, as far as we can see, only by the form of inheritance which has prevailed. Finally with quadrupeds the contest between the individuals of the same s.e.x, whether peaceful or b.l.o.o.d.y, has with the rarest exceptions been confined to the males; so that these have been modified through s.e.xual selection, either for fighting with each other or for alluring the opposite s.e.x, far more commonly than the females.

CHAPTER XIX.

SECONDARY s.e.xUAL CHARACTERS OF MAN.

Differences between man and woman-Causes of such differences and of certain characters common to both s.e.xes-Law of battle-Differences in mental powers-and voice-On the influence of beauty in determining the marriages of mankind-Attention paid by savages to ornaments-Their ideas of beauty in woman-The tendency to exaggerate each natural peculiarity.

With mankind the differences between the s.e.xes are greater than in most species of Quadrumana, but not so great as in some, for instance, the mandrill. Man on an average is considerably taller, heavier, and stronger than woman, with squarer shoulders and more plainly-p.r.o.nounced muscles. Owing to the relation which exists between muscular development and the projection of the brows,[378] the superciliary ridge is generally more strongly marked in man than in woman. His body, and especially his face, is more hairy, and his voice has a different and more powerful tone. In certain tribes the women are said, whether truly I know not, to differ slightly in tint from the men; and with Europeans, the women are perhaps the more brightly coloured of the two, as may be seen when both s.e.xes have been equally exposed to the weather.

Man is more courageous, pugnacious, and energetic than woman, and has a more inventive genius. His brain is absolutely larger, but whether relatively to the larger size of his body, in comparison with that of woman, has not, I believe been fully ascertained. In woman the face is rounder; the jaws and the base of the skull smaller; the outlines of her body rounder, in parts more prominent; and her pelvis is broader than in man;[379] but this latter character may perhaps be considered rather as a primary than a secondary s.e.xual character. She comes to maturity at an earlier age than man.

As with animals of all cla.s.ses, so with man, the distinctive characters of the male s.e.x are not fully developed until he is nearly mature; and if emasculated they never appear. The beard, for instance, is a secondary s.e.xual character, and male children are beardless, though at an early age they have abundant hair on their heads. It is probably due to the rather late appearance in life of the successive variations, by which man acquired his masculine characters, that they are transmitted to the male s.e.x alone. Male and female children resemble each other closely, like the young of so many other animals in which the adult s.e.xes differ; they likewise resemble the mature female much more closely, than the mature male. The female, however, ultimately a.s.sumes certain distinctive characters, and in the formation of her skull, is said to be intermediate between the child and the man.[380] Again, as the young of closely allied though distinct species do not differ nearly so much from each other as do the adults, so it is with the children of the different races of man. Some have even maintained that race-differences cannot be detected in the infantile skull.[381] In regard to colour, the newborn negro child is reddish nut-brown, which soon becomes slaty-grey; the black colour being fully developed within a year in the Sudan, but not until three years in Egypt. The eyes of the negro are at first blue, and the hair chesnut-brown rather than black, being curled only at the ends. The children of the Australians immediately after birth are yellowish-brown, and become dark at a later age. Those of the Guaranys of Paraguay are whitish-yellow, but they acquire in the course of a few weeks the yellowish-brown tint of their parents. Similar observations have been made in other parts of America.[382]

I have specified the foregoing familiar differences between the male and female s.e.x in mankind, because they are curiously the same as in the Quadrumana. With these animals the female is mature at an earlier age than the male; at least this is certainly the case with the _Cebus azarae_.[383] With most of the species the males are larger and much stronger than the females, of which fact the gorilla offers a well-known instance. Even in so trifling a character as the greater prominence of the superciliary ridge, the males of certain monkeys differ from the females,[384] and agree in this respect with mankind. In the gorilla and certain other monkeys, the cranium of the adult male presents a strongly-marked sagittal crest, which is absent in the female; and Ecker found a trace of a similar difference between the two s.e.xes in the Australians.[385] With monkeys when there is any difference in the voice, that of the male is the more powerful. We have seen that certain male monkeys, have a well-developed beard, which is quite deficient, or much less developed in the female. No instance is known of the beard, whiskers, or moustache being larger in a female than in the male monkey.

Even in the colour of the beard there is a curious parallelism between man and the Quadrumana, for when in man the beard differs in colour from the hair of the head, as is often the case, it is, I believe, invariably of a lighter tint, being often reddish. I have observed this fact in England, and Dr. Hooker, who attended to this little point for me in Russia, found no exception to the rule. In Calcutta, Mr. J. Scott, of the Botanic Gardens, was so kind as to observe with care the many races of men to be seen there, as well as in some other parts of India, namely, two races in Sikhim, the Bhoteas, Hindoos, Burmese, and Chinese.

Although most of these races have very little hair on the face, yet he always found that when there was any difference in colour between the hair of the head and the beard, the latter was invariably of a lighter tint. Now with monkeys, as has already been stated, the beard frequently differs in a striking manner in colour from the hair of the head, and in such cases it is invariably of a lighter hue, being often pure white, sometimes yellow or reddish.[386]