Writing of the heron family, he says:—
“The young of the Ardea asha are white, the adults being slate-coloured; and not only the young, but the adults of the allied Buphus coromandus in their winter plumage are white, their colour changing into a rich golden buff during the breeding season. It is incredible that the young of these two species, as well as of some other members of the same family, should have been specially rendered pure white, and thus made conspicuous to their enemies; or that the adults of one of these two species should have been specially rendered white during the winter in a country which is never covered with snow. On the other hand, we have reason to believe that whiteness has been gained by many birds as a sexual ornament. We may therefore conclude that an early progenitor of the Ardea asha and the Buphus acquired a white plumage for nuptial purposes, and transmitted this colour to their young; so that the young and the old became white like certain existing egrets, the whiteness having afterwards been retained by the young whilst exchanged by the adults for more strongly pronounced tints. But if we could look still further backwards in time to the still earlier progenitors of these two species, we should probably see the adults dark-coloured. I infer that this would be the case, from the analogy of many other birds, which are dark whilst young, and when adult are white; and more especially from the adult of the Ardea gularis, the colours of which are the reverse of those of A. asha, for the young are dark-coloured and the adults white, the young having retained a former state of plumage. It appears, therefore, that the progenitors in their adult condition of the A. asha, the Buphus, and of some allies have undergone, during a long line of descent, the following changes of colour: firstly a dark shade, secondly pure white, and thirdly, owing to another change of fashion (if I may so express myself), their present slaty, reddish or golden-buff tints. These successive changes are intelligible only on the principle of novelty having been admired by the birds for the sake of novelty.”
This reasoning may appear far-fetched and unconvincing. It seems, however, quite likely that the hen may select as her mate the suitor who is conspicuously different from the others, not because she admires novelty, but because his conspicuousness attracts her attention and enables her to make up her mind quickly to take him and thus rid herself of the other troublesome admirers, who are all very much alike.
Sexual Dissimilarity
It is perhaps worthy of note that, after the most successful of her suitors has succeeded in securing the hen, it may happen that a disappointed rival makes love to her in the absence of her lord and master and thereby nullifies the effect of her previous selection.
It is to be observed that, even if we take it as proved, as Darwin believed, that the hens alone exercise a choice of mates, and that they select the most beautiful of their suitors, we are still far from arriving at an explanation of the fact that the males alone have acquired beauty. Admitting that the hens always mate with the most beautiful cocks, we should expect the offspring of each union to be all more or less alike in beauty—that is to say, more beautiful than the mother and less so than the cock. How are we to explain the one-sided inheritance of this beauty? Why is it confined to the cocks?
In order to meet this objection Darwin had to call to his aid unknown laws of inheritance. “The laws of inheritance,” he writes (Descent of Man, p. 759), “irrespectively of selection, appear to have determined whether the characters acquired by males for the sake of ornament, for producing various sounds, and for fighting together, have been transmitted to the males alone or to both sexes, either permanently or periodically, during certain seasons of the year. Why various characters should have been transmitted sometimes in one way and sometimes in another is not in most cases known; but the period of variability seems often to have been the determining cause. When the two sexes have inherited all characters in common, they necessarily resemble each other; but, as the successive variations may be differently transmitted, every possible gradation may be found, even within the same genus, from the closest similarity to the widest dissimilarity between the sexes.”
This statement, although it does not throw any light upon the problem, is somewhat damaging to the theory of sexual selection. If it be admitted that dissimilarity between the sexes is due to the fact that the males have varied in one way and the females in another way, there seems no necessity for invoking the aid of feminine preference.
Even greater is the difficulty presented by those species in which the males alone are provided with horns or antlers. “When,” writes Darwin (Descent of Man, p. 767), “the males are provided with weapons which in the females are absent, there can hardly be a doubt that these serve for fighting with other males; and that they were acquired through sexual selection, and were transmitted to the male sex alone. It is not probable, at least in most cases, that the females have been prevented from acquiring such weapons on account of their being useless, superfluous, or in some way injurious. On the contrary, as they are often used by the males for various purposes, more especially as a defence against their enemies, it is a surprising fact that they are so poorly developed, or quite absent, in the females of so many animals.”
We have, we believe, demonstrated that Darwin’s theory of sexual selection is unable to account satisfactorily for all the phenomena of sexual dimorphism. But, as we have seen, it is quite possible that sexual selection is a real factor of evolution.