Darwinism (1889) Part 21

b.u.t.terflies present us with a wonderful amount of s.e.xual difference of colour, in many cases so remarkable that the two s.e.xes 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 ident.i.ty 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 b.u.t.terfly 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 s.e.xes, while they are protectively coloured on the under side which is exposed to view when they are at rest. Such are our peac.o.c.k, tortoise-sh.e.l.l, and red admiral b.u.t.terflies; 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 s.e.x. 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 s.e.xes, 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 b.u.t.terfly 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 s.e.xes, 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 a.s.similating 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 s.e.xes among b.u.t.terflies, is beautifully ill.u.s.trated 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 s.e.xes 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. c.o.o.n may be taken as types--the two s.e.xes 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 s.e.xes; 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 b.u.t.terflies.

There are, in fact, many indications of a regular succession of tints in which colour development has occurred in the various groups of b.u.t.terflies, 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 a.s.sociated with more pointed wings, indicating greater vigour and more rapid flight.

_s.e.xual Selection as a supposed Cause of Colour Development._

Mr. Darwin, as is well known, imputed most of the brilliant colours and varied patterns of b.u.t.terflies' wings to s.e.xual selection--that is, to a constant preference, by female b.u.t.terflies, for the more brilliant males; the colours thus produced being sometimes transmitted to the males alone, sometimes to both s.e.xes. 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 a.s.sured 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 b.u.t.terflies, 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." b.u.t.terflies 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 b.u.t.terflies, 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 s.e.xual selection.[121] 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 s.e.xual selection of colour by female b.u.t.terflies. This question will, however, be again touched upon after we have considered the phenomena of s.e.xual colour among the vertebrata.

_s.e.xual Coloration of Birds._

The general rule among vertebrates, as regards colour, is, for the two s.e.xes to be alike. This prevails, with only a few exceptions, in fishes, reptiles, and mammalia; but in birds diversity of s.e.xual 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 cla.s.s, 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 s.e.xual 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 s.e.xes 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 s.e.xual 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 peac.o.c.ks. 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 a.s.siduously 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 s.e.x Colour to Nesting Habits._

This principle is strikingly ill.u.s.trated by the existence of considerable numbers of birds in which both s.e.xes 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 woodp.e.c.k.e.rs, 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 b.u.t.terflies 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 cla.s.ses 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.[122]

The general relation which exists between the mode of nesting and the coloration of the s.e.xes in those groups of birds which need protection from enemies, may be thus expressed: When both s.e.xes 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, woodp.e.c.k.e.rs, 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.[123] 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 s.e.xes, 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 s.e.xes, 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 s.e.xes 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 t.i.ts 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 s.e.xes, 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 cla.s.s of showy males and females, with concealed nidification; while there are probably, from an equally rough estimate, about the same number in the contrasted cla.s.s of showy males and dull females, with open nests. This will leave the great bulk of known birds in the cla.s.ses of those which are more or less protectively coloured in both s.e.xes; 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 s.e.xes 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.[124]

_s.e.xual Colours of other Vertebrates._

We may consider a few of the cases of s.e.xual colouring of other cla.s.ses of vertebrates, as given by Mr. Darwin. In fishes, though the s.e.xes 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 s.e.xual 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.[125]

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 s.e.xual 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.

_s.e.xual 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 gamec.o.c.k. 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, woodp.e.c.k.e.rs, 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 b.u.t.terflies 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 const.i.tution 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 "s.e.xual 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 s.e.xual 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 s.e.xual 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 b.u.t.terflies. He even goes further, and imputes to it a large portion of the brilliant colour that occurs in both s.e.xes, on the principle that variations occurring in one s.e.x are sometimes transmitted to the same s.e.x only, sometimes to both, owing to peculiarities in the laws of inheritance. In this extension of s.e.xual 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.

_s.e.xual 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 s.e.xual 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 s.e.xes, 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 s.e.xes 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, peac.o.c.ks, 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 circ.u.mstance 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 peac.o.c.k; 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 gamec.o.c.k, though disfigured by being dubbed, and with his hackles trimmed, would be accepted as readily as a male retaining all his natural ornaments."[126] Evidence is adduced that a female pigeon will sometimes turn antipathy to a particular male without any a.s.signable 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 s.e.xual 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 ill.u.s.tration. 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 s.e.x. So, a girl likes to see her lover well and fas.h.i.+onably 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 courts.h.i.+p, apparently with the full knowledge that it is beautiful, const.i.tutes 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.