In the spring a livelier iris changes on the burnished dove,
In the spring the young man's fancy lightly turns to thoughts of love."
That birds are eminently capable of appreciating beauty is certain, and numerous illustrations are familiar to everyone. Suffice it here to notice the pretty Bower Birds of Australia, that adorn their love arbours with bright shells and flowers, and show as unmistakable a delight in them as the connoisseur among his art treasures.
From these and kindred facts Darwin draws the conclusion that the females are most charmed with, and select the most brilliant males, and that by continued selection of this character, the sexual hues have been gradually evolved.
To this theory Wallace takes exception. Admitting, as all must, the fact of sexually distinct ornamentation, he demurs to the conclusion that they have been produced by sexual selection.
In the first place, he insists upon the absence of all proof that the least attractive males fail to obtain partners, without which the theory must fail. Next he tells us that it was the case of the Argus pheasant, so admirably worked out by Darwin, that first shook his faith in sexual selection. Is it possible, he asks, that those exquisite eye-spots, shaded "like balls lying loose within sockets" (objects of which the birds could have had no possible experience) should have been produced ... "through thousands and tens of thousands of female birds, all preferring those males whose markings varied slightly in this one direction, this uniformity of choice continuing through thousands and tens of thousands of generations"?[1]
As an alternative explanation, he would advance no new theory, but simply apply the known laws of evolution. He points out, and dwells upon, the high importance of protection to the female while sitting on the nest. In this way he accounts for the more sombre hues of the female; and finds strong support in the fact that in those birds in which the male undertakes the household duties, he is of a domestic dun colour, and his gad-about-spouse is bedizened like a country-girl at fair time.
With regard to the brilliant hues themselves, he draws attention to the fact that depth and intensity of colour are a sign of vigour and health—that the pairing time is one of intense excitement, and that we should naturally expect to find the brightest hues then displayed. Moreover, he shows—and this is most important to us—that "the most highly-coloured and most richly varied markings occur on those parts which have undergone the greatest modification, or have acquired the most abnormal development."[2]
It is not our object to discuss these rival views; but they are here laid down in skeleton, that the nature of the problem of the principles of colouration may be easily understood.
Seeing, then, how infinitely varied is colouration, and how potently selection has modified it, the question may be asked, "Is it possible to find any general system or law which has determined the main plan of decoration, any system which underlies natural selection, and through which it works"? We venture to think there is; and the object of this work is to develop the laws we have arrived at after several years of study.