If we admit that certain antics (or cries) common to both sexes of certain birds, have had a like origin in the case of either, then, if by such common actions some common beauty is displayed, it is unreasonable to think that this has been acquired through the action of sexual selection in the case of the one sex (the male) and not in the case of the other (the female), for where the psychology and actions are the same, the laws governing them must be the same, and their effects the same.
The above considerations, enforced as they have been by much that I have myself observed, make me doubt whether the view that where any species of bird has come under the influence of sexual selection, it is the one sex only—almost always the male—that has been modified by its action, is a correct one. It seems to me more probable that where the sexes are alike, or where they differ markedly, and are both handsome, each of them has acquired such beauty as it possesses in accordance with the taste and choice of the opposite one. Darwin, though he did not consider this probable, yet recognised its possibility, as the following passage will show: "It may be suggested that in some cases a double process of selection has been carried on: that the males have selected the more attractive females, and the latter the more attractive males. This process, however, though it might lead to the modification of both sexes, would not make the one sex different from the other, unless, indeed, their tastes for the beautiful differed; but this is a supposition too improbable to be worth considering in the case of any animal excepting man. There are, however, many animals in which the sexes resemble each other, both being furnished with the same ornaments, which analogy would lead us to attribute to the agency of sexual selection. In such cases it may be suggested with more plausibility that there has been a double or mutual process of sexual selection, the more vigorous and precocious females selecting the more attractive and vigorous males, the latter rejecting all except the more attractive females. But from what we know of the habits of animals this view is hardly probable, for the male is generally eager to pair with any female. It is more probable that the ornaments common to both sexes were acquired by one sex, generally the male, and then transmitted to the offspring of both sexes."[15]
[15] Descent of Man, pp. 225-226.
I have given my reasons for doubting whether this last hypothesis really is more probable than the other one of a double process of sexual selection—at any rate as far as birds are concerned: and I suggest that, in their case, the whole question of the relations of the sexes to one another should be reconsidered after much more careful observation, especially in regard to those species where the male and female are alike, or where they differ markedly, and are both handsome. As to the possibility of the taste for the beautiful differing in the two sexes of any bird or animal, I cannot see why this should not sometimes be the case. One sex is attracted only by the beauty of the opposite one, so that if, owing to slight constitutional differences between them, the variations which occurred in the one were somewhat different to those which occurred in the other (which hardly seems very unlikely), these might be selected and "added up"—to use Darwin's expression—along two gradually diverging lines, and this would lead, insensibly and necessarily, to divergence of taste as between the male and the female. The law is for the one sex to admire what it gets in the other. Therefore, supposing individual differences in both, and a choice in regard to them on the opposite side, taste, in each case, must be guided by the variations offered for it to work upon; and though the final result of this, if such variations were affected by sex, might appear very surprising, there would be nothing remarkable in the process by which it had been arrived at. Must not, in fact, a difference of taste as between the two sexes—and that often a very decided one—in any case exist? For the male bird of paradise, let us say, is attracted by the dull hen, whilst she, presumably, admires only the resplendent cock. Beauty is only a relative term, and even the plainest bird possesses a good deal of it. We may, of course, say that it is only the hen bird, in such cases, which can be said to admire, but it would be difficult, I think, to defend this view. Both are sexually excited, and the eye is a channel for both.
These, then, are my arguments in favour of a process of intersexual selection in nature, and I think that those men, at any rate, who grant taste and choice to female animals, should be prepared to grant it, also, to their own sex, though the thinking woman, perhaps, may be expected to take another view. But, of course, I know that there are still numbers of people who do not accept the theory—or, as I would prefer to call it, the fact—of sexual selection at all, even in its narrower scope. I believe, however, that the chariness and hesitation which has been shown in adopting the latter of Darwin's two great principles, is a survival of that attitude of mind which caused such opposition to his whole teaching. Man's body is one thing, but his mind—especially all those supposed high things in it which we call, together, spirituality—is quite another. It offends our human pride to think that animals should woo and marry very much as we—when the better part of our nature is not in a strait-jacket—do ourselves. Therefore, there must be no preferences, no love-matches here, all must be in obedience to a blind sexual instinct—something very animal—about which we, of course, with our rings and our ceremonies, our novels, sonnets, spiritual affinities, and prudential considerations, know nothing. Unlike ourselves, the female brute must be ready to mate with any male brute that chance may throw in her way, and if it throw several, she must be absolutely impartial between them, there being neither looks, soul, nor money for her to found a choice on. Therefore she will go to the strongest, and ask no better, for love she knows not, nor can parental authority and filial obedience combine here to give the preference to riches or title, coupled with age or disease. Only by her complete passivity could the female brute be properly differentiated from the human female, and this she must be, or man (the worst brute that the world has yet seen or is ever likely to see) would lose his pre-eminence.
But do no difficulties attend this theory of entire impartiality on the part of the hen bird (for we will keep now to birds) in respect to the cock, during the pairing season? That she is sexually excited by him—as a male, at least, if not as an individual male—we would surely have to conclude, even in the absence of direct evidence, for how otherwise could the breeding be accomplished? Then what a most extraordinary thing it would be if she were excited in precisely the same degree—not one jot or tittle more or less—by any one male as by any other! Whatever the nature of that sexual appeal may be which every cock makes to every hen, and by virtue of which she feels that he is a cock, and not a hen like herself, why should we suppose that any two individuals should be more exactly alike in it than they are in anything else? But if there is not this absolute unity, then there is difference, and such difference in the degree of the sexual charm flung out by each male, must produce preference and choice in the female. The whole theory of evolution is based upon the undisputed and indisputable fact of individual variability; nor is there any one thing or quality, bodily or mental—amongst the higher animals at least—that does not vary largely in the different individuals possessing it. As it appears to me, therefore, choice in the one sex with regard to the other is what might have been, on a priori considerations, expected; though I can well understand that, as amongst ourselves, it would often be held in abeyance, or nullified, by the operation of higher—that is to say, more inexorable—laws, and also that its manifestations would often be too subtle and hidden for us to follow them. But we first, in deference to our human prejudices, assume something which is improbable in itself, and then obstinately resist a mass of the most striking evidence which shows our assumption to be wrong. In all intellectual and spiritual qualities, man, by the laws of evolution, may have greatly outdistanced his fellow animals; but it should never be forgotten that in judging of how far this has been the case, we—and there is no other court—are the most partial and prejudiced judges—dishonest, blinded, full of assumptions, delighting to deceive ourselves, and miserably vain.
If female birds are really so apathetic and male ones so equally satisfied with any partner they can get, it seems difficult to see on what principle the two, when paired, remain constant to one another during the nesting season, and still more, perhaps, why numbers of birds pair for life. Such a state of things ought, one would think, to lead to promiscuous intercourse. But if birds mate by preference and elective affinity, such constancy is what one might expect. What we want, however, to settle this and all other questions relating to the habits of animals is long, close, hard, exhaustive observation—real observation as distinct from mere writing, and even from good literature. There is wofully little of this, in my opinion, and none the less so because an impression exists that there is a great deal.