Darwinism (Wallace)/Chapter X

by Alfred Russel Wallace

 

CHAPTER X

 

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and crustacea—In insects—In butterflies and moths—Probable causes of these colours—Sexual selection as a supposed cause—Sexual coloration of birds—Cause of dull colours of female birds—Relation of sex colour to nesting habits—Sexual colours of other vertebrates—Sexual selection by the struggles of males—Sexual characters due to natural selection—Decorative plumage of males and its effect on the females—Display of decorative plumage by the males—A theory of animal coloration—The origin of accessory plumes—Development of accessory plumes and their display—The effect of female preference will be neutralised by natural selection—General laws of animal coloration—Concluding remarks.

In the preceding chapters we have dealt chiefly with the coloration of animals as distinctive of the several species; and we have seen that, in an enormous number of cases, the colours can be shown to have a definite purpose, and to be useful either as a means of protection or concealment, of warning to enemies, or of recognition by their own kind. We have now to consider a subordinate but very widespread phenomenon—-the differences of colour or of ornamental appendages in the two sexes. These differences are found to have special relations with the three classes of coloration above referred to, in many cases confirming the explanation already given of their purport and use, and furnishing us with important aid in formulating a general theory of animal coloration.

In comparing the colours of the two sexes we find a perfect gradation, from absolute identity of colour up to such extreme difference that it is difficult to believe that the two forms can belong to the same species; and this diversity in the colours of the sexes does not bear any constant relation to affinity or systematic position. In both insects and birds we find examples of complete identity and extreme diversity of the sexes; and these differences occur sometimes in the same tribe or family, and sometimes even in the same genus.

It is only among the higher and more active animals that sexual differences of colour acquire any prominence. In the mollusca the two sexes, when separated, are always alike in colour, and only very rarely present slight differences in the form of the shell. In the extensive group of crustacea the two sexes as a rule are identical in colour, though there are often differences in the form of the prehensile organs; but in a very few cases there are differences of colour also. Thus, in a Brazilian species of shore-crab (Gelasimus) the female is grayish-brown, while in the male the posterior part of the cephalo-thorax is pure white, with the anterior part of a rich green. This colour is only acquired by the males when they become mature, and is liable to rapid change in a few minutes to dusky tints.^[1] In some of the freshwater fleas (Daphnoidae) the males are ornamented with red and blue spots, while in others similar colours occur in both sexes. In spiders also, though as a rule the two sexes are alike in colour, there are a few exceptions, the males being ornamented with brilliant colours on the abdomen, while the female is dull coloured.

 

Sexual Coloration in Insects.

It is only when we come to the winged insects that we find any large amount of peculiarity in sexual coloration, and even here it is only developed in certain orders. Flies (Diptera), field-bugs (Hemiptera), cicadas (Homoptera), and the grasshoppers, locusts, and crickets (Orthoptera) present very few and unimportant sexual differences of colour; but the last two groups have special musical organs very fully developed in the males of some of the species, and these no doubt enable the sexes to discover and recognise each other. In some cases, however, when the female is protectively coloured, as in the well-known leaf-insects already referred to (p. 207), the male is smaller and much less protectively formed and coloured. In the bees and wasps (Hymenoptera) it is also the rule that the sexes are alike in colour, though there are several cases among solitary bees where they differ; the female being black, and the male brown in Anthophora retusa, while in Andraena fulva the female is more brightly coloured than the male. Of the great order of beetles (Coleoptera) the same thing may be said. Though often so rich and varied in their colours the sexes are usually alike, and Mr. Darwin was only able to find about a dozen cases in which there was any conspicuous difference between them.^[2] They exhibit, however, numerous sexual characters, in the length of the antennae, and in horns, legs, or jaws remarkably enlarged or curiously modified in the male sex.

It is in the family of dragonflies (order Neuroptera) that we first meet with numerous cases of distinctive sexual coloration. In some of the Agrionidae the males have the bodies rich blue and the wings black, while the females have the bodies green and the wings transparent. In the North American genus Hetaerina the males alone have a carmine spot at the base of each wing; but in some other genera the sexes hardly differ at all.

The great order of Lepidoptera, including the butterflies and moths, affords us the most numerous and striking examples of diversity of sexual colouring. Among the moths the difference is usually but slight, being manifested in a greater intensity of the colour of the smaller winged male; but in a few cases there is a decided difference, as in the ghost-moth (Hepialus humuli), in which the male is pure white, while the female is yellow with darker markings. This may be a recognition colour, enabling the female more readily to discover her mate; and this view receives some support from the fact that in the Shetland Islands the male is almost as yellow as the female, since it has been suggested that at midsummer, when this moth appears, there is in that high latitude sufficient twilight all night to render any special coloration unnecessary.^[3]

Butterflies present us with a wonderful amount of sexual difference of colour, in many cases so remarkable that the two sexes of the same species remained for many years under different names and were thought to be quite distinct species. We find, however, every gradation from perfect identity to complete diversity, and in some cases we are able to see a reason for this difference. Beginning with the most extraordinary cases of diversity—as in Diadema misippus, where the male is black, ornamented with a large white spot on each wing margined with rich changeable blue, while the female is orange-brown with black spots and stripes—we find the explanation in the fact that the female mimics an uneatable Danais, and thus gains protection while laying its eggs on low plants in company with that insect. In the allied species, Diadema bolina, the females are also very different from the males, but are of dusky brown tints, evidently protective and very variable, some specimens having a general resemblance to the uneatable Euplaeas; so that we see here some of the earlier stages of both forms of protection. The remarkable differences in some South American Pieridae are similarly explained. The males of Pieris pyrrha, P. lorena, and several others, are white with a few black bands and marginal spots like so many of their allies, while the females are gaily coloured with yellow and brown, and exactly resemble some species of the uneatable Heliconidae of the same district. Similarly, in the Malay Archipelago, the female of Diadema anomala is glossy metallic blue, while the male is brown; the reason for this reversal of the usual rule being, that the female exactly mimics the brilliant colouring of the common and uneatable Euplaea midamus, and thus secures protection. In the fine Adolias dirtea, the male is black with a few specks of ochre-yellow and a broad marginal band of rich metallic greenish-blue, while the female is brownish-black entirely covered with rows of ochre-yellow spots. This latter coloration does not appear to be protective when the insect is seen in the cabinet, but it really is so. I have observed the female of this butterfly in Sumatra, where it settles on the ground in the forest, and its yellow spots so harmonise with the flickering gleams of sunlight on the dead leaves that it can only be detected with the greatest difficulty.

A hundred other cases might be quoted in which the female is either more obscurely coloured than the male, or gains protection by imitating some inedible species; and any one who has watched these female insects flying slowly along in search of the plants on which to deposit their eggs, will understand how important it must be to them not to attract the attention of insect-eating birds by too conspicuous colours. The number of birds which capture insects on the wing is much greater in tropical regions than in Europe; and this is perhaps the reason why many of our showy species are alike, or almost alike, in both sexes, while they are protectively coloured on the under side which is exposed to view when they are at rest. Such are our peacock, tortoise-shell, and red admiral butterflies; while in the tropics we more commonly find that the females are less conspicuous on the upper surface even when protectively coloured beneath.

We may here remark, that the cases already quoted prove clearly that either male or female may be modified in colour apart from the opposite sex. In Pieris pyrrha and its allies the male retains the usual type of coloration of the whole genus, while the female has acquired a distinct and peculiar style of colouring. In Adolias dirtea, on the other hand, the female appears to retain something like the primitive colour and markings of the two sexes, modified perhaps for more perfect protection; while the male has acquired more and more intense and brilliant colours, only showing his original markings by the few small yellow spots that remain near the base of the wings. In the more gaily coloured Pieridae, of which our orange-tip butterfly may be taken as a type, we see in the female the plain ancestral colours of the group, while the male has acquired the brilliant orange tip to its wings, probably as a recognition mark.

In those species in which the under surface is protectively coloured, we often find the upper surface alike in both sexes, the tint of colour being usually more intense in the male. But in some cases this leads to the female being more conspicuous, as in some of the Lycaenidae, where the female is bright blue and the male of a blue so much deeper and soberer in tint as to appear the less brilliantly coloured of the two.

Probable Causes of these Colours.

In the production of these varied results there have probably been several causes at work. There seems to be a constant tendency in the male of most animals—but especially of birds and insects—to develop more and more intensity of colour, often culminating in brilliant metallic blues or greens or the most splendid iridescent hues; while, at the same time, natural selection is constantly at work, preventing the female from acquiring these same tints, or modifying her colours in various directions to secure protection by assimilating her to her surroundings, or by producing mimicry of some protected form. At the same time, the need for recognition must be satisfied; and this seems to have led to diversities of colour in allied species, sometimes the female, sometimes the male undergoing the greatest change according as one or other could be modified with the greatest ease, and so as to interfere least with the welfare of the race. Hence it is that sometimes the males of allied species vary most, as in the different species of Epicalia; sometimes the females, as in the magnificent green species of Ornithoptera and the "Aeneas" group of Papilio.

The importance of the two principles—the need of protection and recognition—in modifying the comparative coloration of the sexes among butterflies, is beautifully illustrated in the case of the groups which are protected by their distastefulness, and whose females do not, therefore, need the protection afforded by sober colours.

In the great families, Heliconidae and Acraeidae, we find that the two sexes are almost always alike; and, in the very few exceptions, that the female, though differently, is not less gaily or less conspicuously coloured. In the Danaidae the same general rule prevails, but the cases in which the male exhibits greater intensity of colour than the female are perhaps more numerous than in the other two families. There is, however, a curious difference in this respect between the Oriental and the American groups of distasteful Papilios with warning colours, both of which are the subjects of mimicry. In the Eastern groups—of which P. hector and P. coon may be taken as types—the two sexes are nearly alike, the male being sometimes more intensely coloured and with fewer pale markings; but in the American groups—represented by P. aeneas, P. sesostris, and allies—there is a wonderful diversity, the males having a rich green or bluish patch on the fore wings, while the females have a band or spots of pure white, not always corresponding in position to the green spot of the males. There are, however, transitional forms, by which a complete series can be traced, from close similarity to great diversity of colouring between the sexes; and this may perhaps be only an extreme example of the intenser colour and more concentrated markings which are a very prevalent characteristic of male butterflies.

There are, in fact, many indications of a regular succession of tints in which colour development has occurred in the various groups of butterflies, from an original grayish or brownish neutral tint. Thus in the "Aeneas" group of Papilios we have the patch on the upper wings yellowish in P. triopas, olivaceous in P. bolivar, bronzy-gray with a white spot in P. erlaces, more greenish and buff in P. iphidamas, gradually changing to the fine blue of P. brissonius, and the magnificent green of P. sesostris. In like manner, the intense crimson spots of the lower wings can be traced step by step from a yellow or buff tint, which is one of the most widespread colours in the whole order. The greater purity and intensity of colour seem to be usually associated with more pointed wings, indicating greater vigour and more rapid flight.

Sexual Selection as a supposed Cause of Colour Development.

Mr. Darwin, as is well known, imputed most of the brilliant colours and varied patterns of butterflies' wings to sexual selection—that is, to a constant preference, by female butterflies, for the more brilliant males; the colours thus produced being sometimes transmitted to the males alone, sometimes to both sexes. This view has always seemed to me to be unsupported by evidence, while it is also quite inadequate to account for the facts. The only direct evidence, as set forth with his usual fairness by Mr. Darwin himself, is opposed to his views. Several entomologists assured him that, in moths, the females evince not the least choice of their partners; and Dr. Wallace of Colchester, who has largely bred the fine Bombyx cynthia, confirmed this statement. Among butterflies, several males often pursue one female, and Mr. Darwin says, that, unless the female exerts a choice the pairing must be left to chance. But, surely, it may be the most vigorous or most persevering male that is chosen, not necessarily one more brightly or differently coloured, and this will be true "natural selection." Butterflies have been noticed to prefer some coloured flowers to others; but that does not prove, or even render probable, any preference for the colour itself, but only for flowers of certain colours, on account of the more agreeable or more abundant nectar obtained from them. Dr. Schulte called Mr. Darwin's attention to the fact, that in the Diadema bolina the brilliant blue colour surrounding the white spots is only visible when we look towards the insect's head, and this is true of many of the iridescent colours of butterflies, and probably depends upon the direction of the striae on the scales. It is suggested, however, that this display of colour will be seen by the female as the male is approaching her, and that it has been developed by sexual selection.^[4] But in the majority of cases the males follow the female, hovering over her in a position which would render it almost impossible for her to see the particular colours or patterns on his upper surface; to do so the female should mount higher than the male, and fly towards him—being the seeker instead of the sought, and this is quite opposed to the actual facts. I cannot, therefore, think that this suggestion adds anything whatever to the evidence for sexual selection of colour by female butterflies. This question will, however, be again touched upon after we have considered the phenomena of sexual colour among the vertebrata.

Sexual Coloration of Birds.

The general rule among vertebrates, as regards colour, is, for the two sexes to be alike. This prevails, with only a few exceptions, in fishes, reptiles, and mammalia; but in birds diversity of sexual colouring is exceedingly frequent, and is, not improbably, present in a greater or less degree in more than half of the known species. It is this class, therefore, that will afford us the best materials for a discussion of the problem, and that may perhaps lead us to a satisfactory explanation of the causes to which sexual colour is due.

The most fundamental characteristic of birds, from our present point of view, is a greater intensity of colour in the male. This is the case in hawks and falcons; in many thrushes, warblers, and finches; in pigeons, partridges, rails, plovers, and many others. When the plumage is highly protective or of dull uniform tints, as in many of the thrushes and warblers, the sexes are almost or quite identical in colour; but when any rich markings or bright tints are acquired, they are almost always wanting or much fainter in the female, as we see in the black-cap among warblers, and the chaffinch among finches.

It is in tropical regions, where from a variety of causes colour has been, developed to its fullest extent, that we find the most remarkable examples of sexual divergence of colour. The most gorgeously coloured birds known are the birds of paradise, the chatterers, the tanagers, the humming-birds, and the pheasant-tribe, including the peacocks. In all these the females are much less brilliant, and, in the great majority of cases, exceptionally plain and dull coloured birds. Not only are the remarkable plumes, crests, and gorgets of the birds of paradise entirely wanting in the females, but these latter are usually without any bright colour at all, and rank no higher than our thrushes in ornamental plumage. Of the humming-birds the same may be said, except that the females are often green, and sometimes slightly metallic, but from their small size and uniform tints are never conspicuous. The glorious blues and purples, the pure whites and intense crimsons of the male chatterers are represented in the females by olive-greens or dull browns, as are the infinitely varied tints of the male tanagers. And in pheasants, the splendour of plumage which characterises the males is entirely absent in the females, which, though often ornamental, have always comparatively sober and protective tints. The same thing occurs with many other groups. In the Eastern tropics are many brilliant birds belonging to the families of the warblers, flycatchers, shrikes, etc., but the female is always much less brilliant than the male and often quite dull coloured.

Cause of Dull Colours of Female Birds.

The reason of this phenomenon is not difficult to find, if we consider the essential conditions of a bird's existence, and the most important function it has to fulfil. In order that the species may be continued, young birds must be produced, and the female birds have to sit assiduously on their eggs. While doing this they are exposed to observation and attack by the numerous devourers of eggs and birds, and it is of vital importance that they should be protectively coloured in all those parts of the body which are exposed during incubation. To secure this end all the bright colours and showy ornaments which decorate the male have not been acquired by the female, who often remains clothed in the sober hues which were probably once common to the whole order to which she belongs. The different amounts of colour acquired by the females have no doubt depended on peculiarities of habits and of environment, and on the powers of defence or of concealment possessed by the species. Mr. Darwin has taught us that natural selection cannot produce absolute, but only relative perfection; and as a protective colour is only one out of many means by which the female birds are able to provide for the safety of their young, those which are best endowed in other respects will have been allowed to acquire more colour than those with whom the struggle for existence is more severe.

Relation of Sex Colour to Nesting Habits.

This principle is strikingly illustrated by the existence of considerable numbers of birds in which both sexes are similarly and brilliantly coloured,—in some cases as brilliantly as the males of many of the groups above referred to. Such are the extensive families of the kingfishers, the woodpeckers, the toucans, the parrots, the turacos, the hangnests, the starlings, and many other smaller groups, all the species of which are conspicuously or brilliantly coloured, while in all of them the females are either coloured exactly like the males, or, when differently coloured, are equally conspicuous. When searching for some cause for this singular apparent exception to the rule of female protective colouring, I came upon a fact which beautifully explains it; for in all these cases, without exception, the species either nests in holes in the ground or in trees, or builds a domed or covered nest, so as completely to conceal the sitting-bird. We have here a case exactly parallel to that of the butterflies protected by distastefulness, whose females are either exactly like the males, or, if different, are equally conspicuous. We can hardly believe that so exact a parallel should exist between such remote classes of animals, except under the influence of a general law; and, in the need of protection by all defenceless animals, and especially by most female insects and birds, we have such a law, which has been proved to have influenced the colours of a considerable proportion of the animal kingdom.^[5]

The general relation which exists between the mode of nesting and the coloration of the sexes in those groups of birds which need protection from enemies, may be thus expressed: When both sexes are brilliant or conspicuous, the nest is such as to conceal the sitting-bird; but when the male is brightly coloured and the female sits exposed on the nest, she is always less brilliant and generally of quite sober and protective hues.

It must be understood that the mode of nesting has influenced the colour, not that the colour has determined the mode of nesting; and this, I believe, has been generally, though not perhaps universally, the case. For we know that colour varies more rapidly, and can be more easily modified and fixed by selection, than any other character; whereas habits, especially when connected with structure, and when they pervade a whole group, are much more persistent and more difficult to change, as shown by the habit of the dog turning round two or three times before lying down, believed to be that of the wild ancestral form which thus smoothed down the herbage so as to form a comfortable bed. We see, too, that the general mode of nesting is characteristic of whole families differing widely in size, form, and colours. Thus, all the kingfishers and their allies in every part of the world nest in holes, usually in banks, but sometimes in trees. The motmots and the puff-birds (Bucconidae) build in similar places; while the toucans, barbets, trogons, woodpeckers, and parrots all make their nests in hollow trees. This habit, pervading all the members of extensive families, must therefore be extremely ancient, more especially as it evidently depends in some degree on the structure of the birds, the bills, and especially the feet, of all these groups being unfitted for the construction of woven arboreal nests.^[6] But in all these families the colour varies greatly from species to species, being constant only in the one character of the similarity of the sexes, or, at all events, in their being equally conspicuous even though differently coloured.

When I first put forward this view of the connection between the mode of nesting and the coloration of female birds, I expressed the law in somewhat different terms, which gave rise to some misunderstanding, and led to numerous criticisms and objections. Several cases were brought forward in which the females were far less brilliant than the males, although the nest was covered. This is the case with the Maluridae, or superb warblers of Australia, in which the males are very brilliant during the pairing season and the females quite plain, yet they build domed nests. Here, there can be little doubt, the covered nest is a protection from rain or from some special enemies to the eggs; while the birds themselves are protectively coloured in both sexes, except for a short time during the breeding season when the male acquires brilliant colours; and this is probably connected with the fact of their inhabiting the open plains and thin scrub of Australia, where protective colours are as generally advantageous as they are in our north-temperate zones.

As I have now stated the law, I do not think there are any exceptions to it, while there are an overwhelming number of cases which give it a strong support. It has been objected that the domed nests of many birds are as conspicuous as the birds themselves would be, and would, therefore, be of no use as a protection to the birds and young. But, as a matter of fact, they do protect from attack, for hawks or crows do not pluck such nests to pieces, as in doing so they would be exposed to the attack of the whole colony; whereas a hawk or falcon could carry off a sitting-bird or the young at a swoop, and entirely avoid attack. Moreover, each kind of covered nest is doubtless directed against the attacks of the most dangerous enemies of the species, the purse-like nests, often a yard long, suspended from the extremity of thin twigs, being useful against the attacks of snakes, which, if they attempted to enter them, would be easily made to lose their hold and fall to the ground. Such birds as jays, crows, magpies, hawks, and other birds of prey, have also been urged as an exception; but these are all aggressive birds, able to protect themselves, and thus do not need any special protection for their females during nidification. Some birds which build in covered nests are comparatively dull coloured, like many of the weaver birds, but in others the colours are more showy, and in all the sexes are alike; so that none of these are in any way opposed to the rule. The golden orioles have, however, been adduced as a decided exception, since the females are showy and build in an open nest. But even here the females are less brilliant than the males, and are sometimes greenish or olivaceous on the upper surface; while they very carefully conceal their nests among dense foliage, and the male is sufficiently watchful and pugnacious to drive off most intruders.

On the other hand, how remarkable it is that the only small and brightly coloured birds of our own country in which the male and female are alike—the tits and starlings—either build in holes or construct covered nests; while the beautiful hangnests (Icteridae) of South America, which always build covered or purse-shaped nests, are equally showy in both sexes, in striking contrast with the chatterers and tanagers of the same country, whose females are invariably less conspicuous than the males. On a rough estimate, there are about 1200 species of birds in the class of showy males and females, with concealed nidification; while there are probably, from an equally rough estimate, about the same number in the contrasted class of showy males and dull females, with open nests. This will leave the great bulk of known birds in the classes of those which are more or less protectively coloured in both sexes; or which, from their organisation and habits, do not require special protective coloration, such as many of the birds of prey, the larger waders, and the oceanic birds.

There are a few very curious cases in which the female bird is actually more brilliant than the male, and which yet have open nests. Such are the dotterel (Eudromias morinellus), several species of phalarope, an Australian creeper (Climacteris erythropus), and a few others; but in every one of these cases the relation of the sexes in regard to nidification is reversed, the male performing the duties of incubation, while the female is the stronger and more pugnacious. This curious case, therefore, quite accords with the general law of coloration.^[7]

Sexual Colours of other Vertebrates.

We may consider a few of the cases of sexual colouring of other classes of vertebrates, as given by Mr. Darwin. In fishes, though the sexes are usually alike, there are several species in which the males are more brightly coloured, and have more elongated fins, spines, or other appendages, and in some few cases the colours are decidedly different. The males often fight together, and are altogether more vivacious and excitable than the females during the breeding season; and with this we may connect a greater intensity of coloration.

In frogs and toads the colours are usually alike, or a little more intense in the males, and the same may be said of most snakes. It is in lizards that we first meet with considerable sexual differences, many of the species having gular pouches, frills, dorsal crests, or horns, either confined to the males, or more developed in them than in the females, and these ornaments are often brightly coloured. In most cases, however, the tints of lizards are protective, the male being usually a little more intense in coloration; and the difference in extreme cases may be partly due to the need of protection for the female, which, when laden with eggs, must be less active and less able to escape from enemies than the male, and may, therefore, have retained more protective colours, as so many insects and birds have certainly done.^[8]

In mammalia there is often a somewhat greater intensity of colour in the male, but rarely a decided difference. The female of the great red kangaroo, however, is a delicate gray; while in the Lemur macaco of Madagascar the male is jet-black and the female brown. In many monkeys also there are some differences of colour, especially on the face. The sexual weapons and ornaments of male mammalia, as horns, crests, manes, and dewlaps, are well known, and are very numerous and remarkable. Having thus briefly reviewed the facts, we will now consider the theories to which they have given rise.

Sexual Selection by the Struggles of Males.

Among the higher animals it is a very general fact that the males fight together for the possession of the females. This leads, in polygamous animals especially, to the stronger or better armed males becoming the parents of the next generation, which inherits the peculiarities of the parents; and thus vigour and offensive weapons are continually increased in the males, resulting in the strength and horns of the bull, the tusks of the boar, the antlers of the stag, and the spurs and fighting instinct of the gamecock. But almost all male animals fight together, though not specially armed; even hares, moles, squirrels, and beavers fight to the death, and are often found to be scarred and wounded. The same rule applies to almost all male birds; and these battles have been observed in such different groups as humming-birds, finches, goatsuckers, woodpeckers, ducks, and waders. Among reptiles, battles of the males are known to occur in the cases of crocodiles, lizards, and tortoises; among fishes, in those of salmon and sticklebats. Even among insects the same law prevails; and male spiders, beetles of many groups, crickets, and butterflies often fight together.

From this very general phenomenon there necessarily results a form of natural selection which increases the vigour and fighting power of the male animal, since, in every case, the weaker are either killed, wounded, or driven away. This selection would be more powerful if males were always in excess of females, but after much research Mr. Darwin could not obtain any satisfactory evidence that this was the case. The same effect, however, is produced in some cases by constitution or habits; thus male insects usually emerge first from the pupa, and among migrating birds the males arrive first both in this country and in North America. The struggle is thus intensified, and the most vigorous males are the first to have offspring. This in all probability is a great advantage, as the early breeders have the start in securing food, and the young are strong enough to protect themselves while the later broods are being produced.

It is to this form of male rivalry that Mr. Darwin first applied the term "sexual selection." It is evidently a real power in nature; and to it we must impute the development of the exceptional strength, size, and activity of the male, together with the possession of special offensive and defensive weapons, and of all other characters which arise from the development of these or are correlated with them. But he has extended the principle into a totally different field of action, which has none of that character of constancy and of inevitable result that attaches to natural selection, including male rivalry; for by far the larger portion of the phenomena, which he endeavours to explain by the direct action of sexual selection, can only be so explained on the hypothesis that the immediate agency is female choice or preference. It is to this that he imputes the origin of all secondary sexual characters other than weapons of offence and defence, of all the ornamental crests and accessory plumes of birds, the stridulating sounds of insects, the crests and beards of monkeys and other mammals, and the brilliant colours and patterns of male birds and butterflies. He even goes further, and imputes to it a large portion of the brilliant colour that occurs in both sexes, on the principle that variations occurring in one sex are sometimes transmitted to the same sex only, sometimes to both, owing to peculiarities in the laws of inheritance. In this extension of sexual selection to include the action of female choice or preference, and in the attempt to give to that choice such wide-reaching effects, I am unable to follow him more than a very little way; and I will now state some of the reasons why I think his views are unsound.

Sexual Characters due to Natural Selection.

Besides the acquisition of weapons by the male for the purpose of fighting with other males, there are some other sexual characters which may have been produced by natural selection. Such are the various sounds and odours which are peculiar to the male, and which serve as a call to the female or as an indication of his presence. These are evidently a valuable addition to the means of recognition of the two sexes, and are a further indication that the pairing season has arrived; and the production, intensification, and differentiation of these sounds and odours are clearly within the power of natural selection. The same remark will apply to the peculiar calls of birds, and even to the singing of the males. These may well have originated merely as a means of recognition between the two sexes of a species, and as an invitation from the male to the female bird. When the individuals of a species are widely scattered, such a call must be of great importance in enabling pairing to take place as early as possible, and thus the clearness, loudness, and individuality of the song becomes a useful character, and therefore the subject of natural selection. Such is especially the case with the cuckoo, and with all solitary birds, and it may have been equally important at some period of the development of all birds. The act of singing is evidently a pleasurable one; and it probably serves as an outlet for superabundant nervous energy and excitement, just as dancing, singing, and field sports do with us. It is suggestive of this view that the exercise of the vocal power seems to be complementary to the development of accessory plumes and ornaments, all our finest singing birds being plainly coloured, and with no crests, neck or tail plumes to display; while the gorgeously ornamented birds of the tropics have no song, and those which expend much energy in display of plumage, as the turkey, peacocks, birds of paradise, and humming-birds, have comparatively an insignificant development of voice. Some birds have, in the wings or tail, peculiarly developed feathers which produce special sounds. In some of the little manakins of Brazil, two or three of the wing-feathers are curiously shaped and stiffened in the male, so that the bird is able to produce with them a peculiar snapping or cracking sound; and the tail-feathers of several species of snipe are so narrowed as to produce distinct drumming, whistling, or switching sounds when the birds descend rapidly from a great height. All these are probably recognition and call notes, useful to each species in relation to the most important function of their lives, and thus capable of being developed by the agency of natural selection.

Decorative Plumage of Birds and its Display.

Mr. Darwin has devoted four chapters of his Descent of Man to the colours of birds, their decorative plumage, and its display at the pairing season; and it is on this latter circumstance that he founds his theory, that both the plumage and the colours have been developed by the preference of the females, the more ornamented males becoming the parents of each successive generation. Any one who reads these most interesting chapters will admit, that the fact of the display is demonstrated; and it may also be admitted, as highly probable, that the female is pleased or excited by the display. But it by no means follows that slight differences in the shape, pattern, or colours of the ornamental plumes are what lead a female to give the preference to one male over another; still less that all the females of a species, or the great majority of them, over a wide area of country, and for many successive generations, prefer exactly the same modification of the colour or ornament.

The evidence on this matter is very scanty, and in most cases not at all to the point. Some peahens preferred an old pied peacock; albino birds in a state of nature have never been seen paired with other birds; a Canada goose paired with a Bernicle gander; a male widgeon preferred a pintail duck to its own species; a hen canary preferred a male greenfinch to either linnet, goldfinch, siskin, or chaffinch. These cases are evidently exceptional, and are not such as generally occur in nature; and they only prove that the female does exert some choice between very different males, and some observations on birds in a state of nature prove the same thing; but there is no evidence that slight variations in the colour or plumes, in the way of increased intensity or complexity, are what determines the choice. On the other hand, Mr. Darwin gives much evidence that it is not so determined. He tells us that Messrs. Hewitt, Tegetmeier, and Brent, three of the highest authorities and best observers, "do not believe that the females prefer certain males on account of the beauty of their plumage." Mr. Hewitt was convinced "that the female almost invariably prefers the most vigorous, defiant, and mettlesome male;" and Mr. Tegetmeier, "that a gamecock, though disfigured by being dubbed, and with his hackles trimmed, would be accepted as readily as a male retaining all his natural ornaments."^[9] Evidence is adduced that a female pigeon will sometimes take an antipathy to a particular male without any assignable cause; or, in other cases, will take a strong fancy to some one bird, and will desert her own mate for him; but it is not stated that superiority or inferiority of plumage has anything to do with these fancies. Two instances are indeed given, of male birds being rejected, which had lost their ornamental plumage; but in both cases (a widow-finch and a silver pheasant) the long tail-plumes are the indication of sexual maturity. Such cases do not support the idea that males with the tail-feathers a trifle longer, or the colours a trifle brighter, are generally preferred, and that those which are only a little inferior are as generally rejected,—and this is what is absolutely needed to establish the theory of the development of these plumes by means of the choice of the female.

It will be seen, that female birds have unaccountable likes and dislikes in the matter of their partners, just as we have ourselves, and this may afford us an illustration. A young man, when courting, brushes or curls his hair, and has his moustache, beard, or whiskers in perfect order, and no doubt his sweetheart admires them; but this does not prove that she marries him on account of these ornaments, still less that hair, beard, whiskers, and moustache were developed by the continued preferences of the female sex. So, a girl likes to see her lover well and fashionably dressed, and he always dresses as well as he can when he visits her; but we cannot conclude from this that the whole series of male costumes, from the brilliantly coloured, puffed, and slashed doublet and hose of the Elizabethan period, through the gorgeous coats, long waistcoats, and pigtails of the early Georgian era, down to the funereal dress-suit of the present day, are the direct result of female preference. In like manner, female birds may be charmed or excited by the fine display of plumage by the males; but there is no proof whatever that slight differences in that display have any effect in determining their choice of a partner.

Display of Decorative Plumage.

The extraordinary manner in which most birds display their plumage at the time of courtship, apparently with the full knowledge that it is beautiful, constitutes one of Mr. Darwin's strongest arguments. It is, no doubt, a very curious and interesting phenomenon, and indicates a connection between the exertion of particular muscles and the development of colour and ornament; but, for the reasons just given, it does not prove that the ornament has been developed by female choice. During excitement, and when the organism develops superabundant energy, many animals find it pleasurable to exercise their various muscles, often in fantastic ways, as seen in the gambols of kittens, lambs, and other young animals. But at the time of pairing, male birds are in a state of the most perfect development, and possess an enormous store of vitality; and under the excitement of the sexual passion they perform strange antics or rapid flights, as much probably from an internal impulse to motion and exertion as with any desire to please their mates. Such are the rapid descent of the snipe, the soaring and singing of the lark, and the dances of the cock-of-the-rock and of many other birds.

It is very suggestive that similar strange movements are performed by many birds which have no ornamental plumage to display. Goatsuckers, geese, carrion vultures, and many other birds of plain plumage have been observed to dance, spread their wings or tails, and perform strange love-antics. The courtship of the great albatross, a most unwieldy and dull coloured bird, has been thus described by Professor Moseley: "The male, standing by the female on the nest, raises his wings, spreads his tail and elevates it, throws up his head with the bill in the air, or stretches it straight out, or forwards, as far as he can, and then utters a curious cry."^[10] Mr. Jenner Weir informs me that "the male blackbird is full of action, spreads out his glossy wing and tail, turns his rich golden beak towards the female, and chuckles with delight," while he has never seen the more plain coloured thrush demonstrative to the female. The linnet distends his rosy breast, and slightly expands his brown wings and tail; while the various gay coloured Australian finches adopt such attitudes and postures as, in every case, to show off their variously coloured plumage to the best advantage.^[11]

A Theory of Animal Coloration.

Having rejected Mr. Darwin's theory of female choice as incompetent to account for the brilliant colours and markings of the higher animals, the preponderance of these colours and markings in the male sex, and their display during periods of activity or excitement, I may be asked what explanation I have to offer as a preferable substitute. In my Tropical Nature I have already indicated such a theory, which I will now briefly explain, supporting it by some additional facts and arguments, which appear to me to have great weight, and for which I am mainly indebted to a most interesting and suggestive posthumous work by Mr. Alfred Tylor.^[12]

The fundamental or ground colours of animals are, as has been shown in preceding chapters, very largely protective, and it is not improbable that the primitive colours of all animals were so. During the long course of animal development other modes of protection than concealment by harmony of colour arose, and thenceforth the normal development of colour due to the complex chemical and structural changes ever going on in the organism, had full play; and the colours thus produced were again and again modified by natural selection for purposes of warning, recognition, mimicry, or special protection, as has been already fully explained in the preceding chapters.

Mr. Taylor has, however, called attention to an important principle which underlies the various patterns or ornamental markings of animals—namely, that diversified coloration follows the chief lines of structure, and changes at points, such as the joints, where function changes. He says, "If we take highly decorated species—that is, animals marked by alternate dark or light bands or spots, such as the zebra, some deer, or the carnivora, we find, first, that the region of the spinal column is marked by a dark stripe; secondly, that the regions of the appendages, or limbs, are differently marked; thirdly, that the flanks are striped or spotted, along or between the regions of the lines of the ribs; fourthly, that the shoulder and hip regions are marked by curved lines; fifthly, that the pattern changes, and the direction of the lines, or spots, at the head, neck, and every joint of the limbs; and lastly, that the tips of the ears, nose, tail, and feet, and the eye are emphasised in colour. In spotted animals the greatest length of the spot is generally in the direction of the largest development of the skeleton."

This structural decoration is well seen in many insects. In caterpillars, similar spots and markings are repeated in each segment, except where modified for some form of protection. In butterflies, the spots and bands usually have reference to the form of the wing and the arrangement of the nervures; and there is much evidence to show that the primitive markings are always spots in the cells, or between the nervures, or at the junctions of nervures, the extension and coalescence of these spots forming borders, bands, or blotches, which have become modified in infinitely varied ways for protection, warning, or recognition. Even in birds, the distribution of colours and markings follows generally the same law. The crown of the head, the throat, the ear-coverts, and the eyes have usually distinct tints in all highly coloured birds; the region of the furcula has often a distinct patch of colour, as have the pectoral muscles, the uropygium or root of the tail, and the under tail-coverts.^[13]

Mr. Tylor was of opinion the primitive form of ornamentation consisted of spots, the confluence of these in certain directions forming lines or bands; and, these again, sometimes coalescing into blotches, or into more or less uniform tints covering a large portion of the surface of the body. The young lion and tiger are both spotted; and in the Java hog (Sus vittatus) very young animals are banded, but have spots over the shoulders and thighs. These spots run into stripes as the animal grows older; then the stripes expand, and at last, meeting together, the adult animal becomes of a uniform dark brown colour. So many of the species of deer are spotted when young, that Darwin concludes the ancestral form, from which all deer are derived, must have been spotted. Pigs and tapirs are banded or spotted when young; an imported young specimen of Tapirus Bairdi was covered with white spots in longitudinal rows, here and there forming short stripes.^[14] Even the horse, which Darwin supposes to be descended from a striped animal, is often spotted, as in dappled horses; and great numbers show a tendency to spottiness, especially on the haunches.

Ocelli may also be developed from spots, or from bars, as pointed out by Mr. Darwin. Spots are an ordinary form of marking in disease, and these spots sometimes run together, forming blotches. There is evidence that colour markings are in some way dependent on nerve distribution. In the disease known as frontal herpes, an eruption occurs which corresponds exactly to the distribution of the ophthalmic division of the fifth cranial nerve, mapping out all its little branches even to the one which goes to the tip of the nose. In a Hindoo suffering from herpes the pigment was destroyed in the arm along the course of the ulnar nerve, with its branches along both sides of one finger and the half of another. In the leg the sciatic and scaphenous nerves were partly mapped out, giving to the patient the appearance of an anatomical diagram.^[15]

These facts are very interesting, because they help to explain the general dependence of marking on structure which has been already pointed out. For, as the nerves everywhere follow the muscles, and these are attached to the various bones, we see how it happens, that the tracts in which distinct developments of colour appear, should so often be marked out by the chief divisions of the bony structure in vertebrates, and by the segments in the annulosa. There is, however, another correspondence of even greater interest and importance. Brilliant colours usually appear just in proportion to the development of tegumentary appendages. Among birds the most brilliant colours are possessed by those which have developed frills, crests, and elongated tails like the humming-birds; immense tail-coverts like the peacock; enormously expanded wing-feathers, as in the argus-pheasant; or magnificent plumes from the region of the coracoids in many of the birds of paradise. It is to be noted, also, that all these accessory plumes spring from parts of the body which, in other species, are distinguished by patches of colour; so that we may probably impute the development of colour and of accessory plumage to the same fundamental cause.

Among insects, the most brilliant and varied coloration occurs in the butterflies and moths, groups in which the wing-membranes have received their greatest expansion, and whose specialisation has been carried furthest in the marvellous scaly covering which is the seat of the colour. It is suggestive, that the only other group in which functional wings are much coloured is that of the dragonflies, where the membrane is exceedingly expanded. In like manner, the colours of beetles, though greatly inferior to those of the lepidoptera, occur in a group in which the anterior pair of wings has been thickened and modified in order to protect the vital parts, and in which these wing-covers (elytra), in the course of development in the different groups, must have undergone great changes, and have been the seat of very active growth.

The Origin of Accessory Plumes.

Mr. Darwin supposes, that these have in almost every case been developed by the preference of female birds for such males as possessed them in a higher degree than others; but this theory does not account for the fact that these plumes usually appear in a few definite parts of the body. We require some cause to initiate the development in one part rather than in another. Now, the view that colour has arisen over surfaces where muscular and nervous development is considerable, and the fact that it appears especially upon the accessory or highly developed plumes, leads us to inquire whether the same cause has not primarily determined the development of these plumes. The immense tuft of golden plumage in the best known birds of paradise (Paradisea apoda and P. minor) springs from a very small area on the side of the breast. Mr. Frank E. Beddard, who has kindly examined a specimen for me, says that "this area lies upon the pectoral muscles, and near to the point where the fibres of the muscle converge towards their attachment to the humerus. The plumes arise, therefore, close to the most powerful muscle of the body, and near to where the activities of that muscle would be at a maximum. Furthermore, the area of attachment of the plumes is just above the point where the arteries and nerves for the supply of the pectoral muscles, and neighbouring regions, leave the interior of the body. The area of attachment of the plume is, also, as you say in your letter, just above the junction of the coracoid and sternum." Ornamental plumes of considerable size rise from the same part in many other species of paradise birds, sometimes extending laterally in front, so as to form breast shields. They also occur in many humming-birds, and in some sun-birds and honey-suckers; and in all these cases there is a wonderful amount of activity and rapid movement, indicating a surplus of vitality, which is able to manifest itself in the development of these accessory plumes.^[16]

In a quite distinct set of birds, the gallinaceae, we find the ornamental plumage usually arising from very different parts, in the form of elongated tail-feathers or tail-coverts, and of ruffs or hackles from the neck. Here the wings are comparatively little used, the most constant activities depending on the legs, since the gallinaceae are pre-eminently walking, running, and scratching birds. Now the magnificent train of the peacock—the grandest development of accessory plumes in this order—springs from an oval or circular area, about three inches in diameter, just above the base of the tail, and, therefore, situated over the lower part of the spinal column near the insertion of the powerful muscles which move the hind limbs and elevate the tail. The very frequent presence of neck-ruffs or breast-shields in the males of birds with accessory plumes may be partly due to selection, because they must serve as a protection in their mutual combats, just as does the lion's or the horse's mane. The enormously lengthened plumes of the bird of paradise and of the peacock can, however, have no such use, but must be rather injurious than beneficial in the bird's ordinary life. The fact that they have been developed to so great an extent in a few species is an indication of such perfect adaptation to the conditions of existence, such complete success in the battle for life, that there is, in the adult male at all events, a surplus of strength, vitality, and growth-power which is able to expend itself in this way without injury. That such is the case is shown by the great abundance of most of the species which possess these wonderful superfluities of plumage. Birds of paradise are among the commonest birds in New Guinea, and their loud voices can be often heard when the birds themselves are invisible in the depths of the forest; while Indian sportsmen have described the peafowl as being so abundant, that from twelve to fifteen hundred have been seen within an hour at one spot; and they range over the whole country from the Himalayas to Ceylon. Why, in allied species, the development of accessory plumes has taken different forms, we are unable to say, except that it may be due to that individual variability which has served as the starting-point for so much of what seems to us strange in form, or fantastic in colour, both in the animal and vegetable world.

Development of Accessory Plumes and their Display.

If we have found a vera causa for the origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of the integument where muscular and nervous action are greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to the very strongest and most energetic the parentage of the next generation. And, as all the evidence goes to show that, so far as female birds exercise any choice, it is of "the most vigorous, defiant, and mettlesome male," this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culmination. That culmination will be reached when the excessive length or abundance of the plumes begins to be injurious to the bearer of them; and it may be this check to the further lengthening of the peacock's train that has led to the broadening of the feathers at the ends, and the consequent production of the magnificent eye-spots which now form its crowning ornament.

The display of these plumes will result from the same causes which led to their production. Just in proportion as the feathers themselves increased in length and abundance, the skin-muscles which serve to elevate them would increase also; and the nervous development as well as the supply of blood to these parts being at a maximum, the erection of the plumes would become a habit at all periods of nervous or sexual excitement. The display of the plumes, like the existence of the plumes themselves, would be the chief external indication of the maturity and vigour of the male, and would, therefore, be necessarily attractive to the female. We have, thus, no reason for imputing to her any of those aesthetic emotions which are excited in us, by the beauty of form, colour, and pattern of these plumes; or the still more improbable aesthetic tastes, which would cause her to choose her mate on account of minute differences in their forms, colours, or patterns.

As co-operating causes in the production of accessory ornamental plumes, I have elsewhere suggested^[17] that crests and other erectile feathers may have been useful in making the bird more formidable in appearance, and thus serving to frighten away enemies; while long tail or wing feathers might serve to distract the aim of a bird of prey. But though this might be of some use in the earlier stages of their development, it is probably of little importance compared with the vigour and pugnacity of which the plumes are the indication, and which enable most of their possessors to defend themselves against the enemies which are dangerous to weaker and more timid birds. Even the tiny humming-birds are said to attack birds of prey that approach too near to their nests.

The Effect of Female Preference will be Neutralised by Natural Selection.

The various facts and arguments now briefly set forth, afford an explanation of the phenomena of male ornament, as being due to the general laws of growth and development, and make it unnecessary to call to our aid so hypothetical a cause as the cumulative action of female preference. There remains, however, a general argument, arising from the action of natural selection itself, which renders it almost inconceivable that female preference could have been effective in the way suggested; while the same argument strongly supports the view here set forth. Natural selection, as we have seen in our earlier chapters, acts perpetually and on an enormous scale in weeding out the "unfit" at every stage of existence, and preserving only those which are in all respects the very best. Each year, only a small percentage of young birds survive to take the place of the old birds which die; and the survivors will be those which are best able to maintain existence from the egg onwards, an important factor being that their parents should be well able to feed and protect them, while they themselves must in turn be equally able to feed and protect their own offspring. Now this extremely rigid action of natural selection must render any attempt to select mere ornament utterly nugatory, unless the most ornamented always coincide with "the fittest" in every other respect; while, if they do so coincide, then any selection of ornament is altogether superfluous. If the most brightly coloured and fullest plumaged males are not the most healthy and vigorous, have not the best instincts for the proper construction and concealment of the nest, and for the care and protection of the young, they are certainly not the fittest, and will not survive, or be the parents of survivors. If, on the other hand, there is generally this correlation—if, as has been here argued, ornament is the natural product and direct outcome of superabundant health and vigour, then no other mode of selection is needed to account for the presence of such ornament. The action of natural selection does not indeed disprove the existence of female selection of ornament as ornament, but it renders it entirely ineffective; and as the direct evidence for any such female selection is almost nil, while the objections to it are certainly weighty, there can be no longer any reason for upholding a theory which was provisionally useful in calling attention to a most curious and suggestive body of facts, but which is now no longer tenable.

The term "sexual selection" must, therefore, be restricted to the direct results of male struggle and combat. This is really a form of natural selection, and is a matter of direct observation; while its results are as clearly deducible as those of any of the other modes in which selection acts. And if this restriction of the term is needful in the case of the higher animals it is much more so with the lower. In butterflies the weeding out by natural selection takes place to an enormous extent in the egg, larva, and pupa states; and perhaps not more than one in a hundred of the eggs laid produces a perfect insect which lives to breed. Here, then, the impotence of female selection, if it exist, must be complete; for, unless the most brilliantly coloured males are those which produce the best protected eggs, larvae, and pupae, and unless the particular eggs, larvae, and pupae, which are able to survive, are those which produce the most brilliantly coloured butterflies, any choice the female might make must be completely swamped. If, on the other hand, there is this correlation between colour development and perfect adaptation to conditions in all stages, then this development will necessarily proceed by the agency of natural selection and the general laws which determine the production of colour and of ornamental appendages.^[18]

General Laws of Animal Coloration.

The condensed account which has now been given of the phenomena of colour in the animal world will sufficiently show the wonderful complexity and extreme interest of the subject; while it affords an admirable illustration of the importance of the great principle of utility, and of the effect of the theories of natural selection and development in giving a new interest to the most familiar facts of nature. Much yet remains to be done, both in the observation of new facts as to the relations between the colours of animals and their habits or economy, and, more especially, in the elucidation of the laws of growth which determine changes of colour in the various groups; but so much is already known that we are able, with some confidence, to formulate the general principles which have brought about all the beauty and variety of colour which everywhere delight us in our contemplation of animated nature. A brief statement of these principles will fitly conclude our exposition of the subject.

1. Colour may be looked upon as a necessary result of the highly complex chemical constitution of animal tissues and fluids. The blood, the bile, the bones, the fat, and other tissues have characteristic, and often brilliant colours, which we cannot suppose to have been determined for any special purpose, as colours, since they are usually concealed. The external organs, with their various appendages and integuments, would, by the same general laws, naturally give rise to a greater variety of colour.

2. We find it to be the fact that colour increases in variety and intensity as external structures and dermal appendages become more differentiated and developed. It is on scales, hair, and especially on the more highly specialised feathers, that colour is most varied and beautiful; while among insects colour is most fully developed in those whose wing membranes are most expanded, and, as in the lepidoptera, are clothed with highly specialised scales. Here, too, we find an additional mode of colour production in transparent lamellae or in fine surface striae which, by the laws of interference, produce the wonderful metallic hues of so many birds and insects.

3. There are indications of a progressive change of colour, perhaps in some definite order, accompanying the development of tissues or appendages. Thus spots spread and fuse into bands, and when a lateral or centrifugal expansion has occurred—as in the termination of the peacocks' train feathers, the outer web of the secondary quills of the Argus pheasant, or the broad and rounded wings of many butterflies—into variously shaded or coloured ocelli. The fact that we find gradations of colour in many of the more extensive groups, from comparatively dull or simple to brilliant and varied hues, is an indication of some such law of development, due probably to progressive local segregation in the tissues of identical chemical or organic molecules, and dependent on laws of growth yet to be investigated.

4. The colours thus produced, and subject to much individual variation, have been modified in innumerable ways for the benefit of each species. The most general modification has been in such directions as to favour concealment when at rest in the usual surroundings of the species, sometimes carried on by successive steps till it has resulted in the most minute imitation of some inanimate object or exact mimicry of some other animal. In other cases bright colours or striking contrasts have been preserved, to serve as a warning of inedibility or of dangerous powers of attack. Most frequent of all has been the specialisation of each distinct form by some tint or marking for purposes of easy recognition, especially in the case of gregarious animals whose safety largely depends upon association and mutual defence.

5. As a general rule the colours of the two sexes are alike; but in the higher animals there appears a tendency to deeper or more intense colouring in the male, due probably to his greater vigour and excitability. In many groups in which this superabundant vitality is at a maximum, the development of dermal appendages and brilliant colours has gone on increasing till it has resulted in a great diversity between the sexes; and in most of these cases there is evidence to show that natural selection has caused the female to retain the primitive and more sober colours of the group for purposes of protection.

Concluding Remarks.

The general principles of colour development now sketched out enable us to give some rational explanation of the wonderful amount of brilliant colour which occurs among tropical animals. Looking on colour as a normal product of organisation, which has either been allowed free play, or has been checked and modified for the benefit of the species, we can see at once that the luxuriant and perennial vegetation of the tropics, by affording much more constant means of concealment, has rendered brilliant colour less hurtful there than in the temperate and colder regions. Again, this perennial vegetation supplies abundance of both vegetable and insect food throughout the year, and thus a greater abundance and greater variety of the forms of life are rendered possible, than where recurrent seasons of cold and scarcity reduce the possibilities of life to a minimum. Geology furnishes us with another reason, in the fact, that throughout the tertiary period tropical conditions prevailed far into the temperate regions, so that the possibilities of colour development were still greater than they are at the present time. The tropics, therefore, present to us the results of animal development in a much larger area and under more favourable conditions than prevail to-day. We see in them samples of the productions of an earlier and a better world, from an animal point of view; and this probably gives a greater variety and a finer display of colour than would have been produced, had conditions always been what they are now. The temperate zones, on the other hand, have recently suffered the effects of a glacial period of extreme severity, with the result that almost the only gay coloured birds they now possess are summer visitors from tropical or sub-tropical lands. It is to the unbroken and almost unchecked course of development from remote geological times that has prevailed in the tropics, favoured by abundant food and perennial shelter, that we owe such superb developments as the frills and crests and jewelled shields of the humming-birds, the golden plumes of the birds of paradise, and the resplendent train of the peacock. This last exhibits to us the culmination of that marvel and mystery of animal colour which is so well expressed by a poet-artist in the following lines. The marvel will ever remain to the sympathetic student of nature, but I venture to hope that in the preceding chapters I have succeeded in lifting—if only by one of its corners—the veil of mystery which has for long shrouded this department of nature.

On a Peacock's Feather.

In Nature's workshop but a shaving,
Of her poem but a word,
But a tint brushed from her palette,
This feather of a bird!
Yet set it in the sun glance,
Display it in the shine,
Take graver's lens, explore it,
Note filament and line,
Mark amethyst to sapphire,
And sapphire to gold,
And gold to emerald changing
The archetype unfold!
Tone, tint, thread, tissue, texture,
Through every atom scan,
Conforming still, developing,
Obedient to plan.
This but to form a pattern
On the garment of a bird!
What then must be the poem,
This but its lightest word!
Sit before it; ponder o'er it,
'Twill thy mind advantage more,
Than a treatise, than a sermon,
Than a library of lore.

1. Darwin's Descent of Man, p. 271.
2. Darwin's Descent of Man, p. 294, and footnote.
3. Nature, 1871, p. 489.
4. Darwin in Nature, 1880, p. 237.
5. See the author's Contributions to Natural Selection, chap. vii. in which these facts were first brought forward.
6. On this point see the author's Contributions to Natural Selection, chap. v. i.
7. Seebohm's History of British Birds, vol. ii., introduction, p. xiii.
8. For details see Darwin's Descent of Man, chap. xii.
9. Descent of Man, pp. 417, 418, 420.
10. Notes of a Naturalist on the Challenger.
11. Descent of Man, pp. 401, 402.
12. Coloration in Animals and Plants, London, 1886.
13. Coloration of Animals, Pl. X, p. 90; and Pls. II, III, and IV, pp. 30, 40, 42.
14. See coloured Fig. in Proc. Zool. Soc., 1871, p. 626.
15. A. Tylor's Coloration, p. 40; and Photograph in Hutchinson's Illustrations of Clinical Surgery, quoted by Tylor.
16. For activity and pugnacity of humming-birds, see Tropical Nature, pp. 130, 213.
17. Tropical Nature, p. 209. In Chapter V of this work the views here advocated were first set forth, and the reader is referred there for further details.
18. The Rev. O. Pickard-Cambridge, who has devoted himself to the study of spiders, has kindly sent me the following extract from a letter, written in 1869, in which he states his views on this question:—
    "I myself doubt that particular application of the Darwinian theory which attributes male peculiarities of form, structure, colour, and ornament to female appetency or predilection. There is, it seems to me, undoubtedly something in the male organisation of a special, and sexual nature, which, of its own vital force, develops the remarkable male peculiarities so commonly seen, and of no imaginable use to that sex. In as far as these peculiarities show a great vital power, they point out to us the finest and strongest individuals of the sex, and show us which of them would most certainly appropriate to themselves the best and greatest number of females, and leave behind them the strongest and greatest number of progeny. And here would come in, as it appears to me, the proper application of Darwin's theory of Natural Selection; for the possessors of greatest vital power being those most frequently produced and reproduced, the external signs of it would go on developing in an ever-increasing exaggeration, only to be checked where it became really detrimental in some respect or other to the individual."
    This passage, giving the independent views of a close observer—one, moreover, who has studied the species of an extensive group of animals both in the field and in the laboratory—very nearly accords with my own conclusions above given; and, so far as the matured opinions of a competent naturalist have any weight, afford them an important support.