An important point, and one which does not seem to have been pointed out by any zoologist, is that eye-colour, comb, and spurs in birds and horns in mammals do not stand in the same relation to the sexual organs as do the other external characteristics. For example, the castrated Nilgai (_Boselaphus tragocamelus_) acquires horns, but not the characteristic male colour. In the common Indian Francolin Partridge (_Francolinus pondicerianius_), the cock differs from the hen only in the possession of spurs. The same applies to the various species of Snow Cock (_Tetraogallus_). There is a breed of game-cocks which display plumage like that of the hen, but such birds have the comb and spurs developed as in normally feathered cocks. The white eye of the white-eyed Pochard Drake (_Nyroca africana_), and the yellow eye of the cock Golden Pheasant (_Chrysolophus pictus_), which are purely male characters, show themselves earlier than the male plumage. Occasionally a hen golden pheasant assumes the plumage of the cock, but she never acquires the yellow eye. Many birds when kept in captivity lose some of the beauty of their plumage, and this is usually attributed to the sexual organs becoming impaired and reacting on the somatic tissue. But this explanation cannot in all cases be the correct one, because the linnet, although losing the male plumage in captivity, lives long and well in a cage and breeds readily with hen canaries. Another curious fact is that the male plumage sometimes appears pathologically in hen birds, more especially in those which have become sterile from age or disease. This phenomenon occurs comparatively frequently in the gold pheasant, and more rarely in the common pheasant, the fowl, and the duck. Phenomena such as these seem to suggest that in some cases the bright colours of the male may be pathological, that the hormones which the male sexual cells secrete may exercise an injurious effect on the somatic or body tissues. Decay is known to be accompanied by the production of brightly coloured pigment in the case of leaves. Finn suggests that the white plumage which the cock paradise fly-catcher assumes in the fourth year of his existence may be a livery of decay, a sign of senility. The Four Kinds of Mutations It is our belief that sexual dimorphism arises frequently, if not invariably, as a mutation. Mutations may be of four different kinds. Those which appear only, or especially, in conjunction with the male organs, for example, whiteness in domesticated geese allowed to breed indiscriminately. Those which appear only, or especially, in conjunction with the female organs; mutations of this description appear to be very rare, but it may be noted that in fowls allowed to breed indiscriminately, as in India, completely black hens are common, but completely black cocks are rarely, if ever, seen. This indicates an association between blackness and femininity.