We have already alluded to the case of the doves in India. In that country four species are widely distributed—namely, the Spotted Dove (_Turtur suratensis_), the Ring or Collared Dove (_Turtur risorius_), the Little Brown Dove (_Turtur cambayensis_), and the Red Turtle-dove (_Œnopopelia_ _tranqebarica_). The habits of all these four species appear to be identical, nevertheless in the first three the sexes show little or no dissimilarity in outward appearance, while in the last the sexual dimorphism is so great that the cock and hen were formerly thought to belong to different species. Another very curious case is that of the South American geese of the genus _Chloëphaga_, in which some species, as the familiar Upland or Magellan Goose of our parks (_C. magellanica_), have the sexes utterly unlike, while in others, as the Ruddy-headed Goose (_C. rubidiceps_), they are quite similar to each other. The ducks furnish us with another very good example of the apparently haphazard nature of sexual dimorphism. In the Common Mallard or Wild Duck (_Anas boscas_) the cock is far more showily coloured than the hen, but in all the species most nearly allied to it the males are as inconspicuous as the females, _e.g._ in the Indian Spotted-bill (_Anas pœcilorhyncha_), the Australian Grey Duck (_A. superciliosa_), the African Yellow Bill (_Anas undulata_), and the American Dusky Duck (_A. obscura_). As the dusky duck inhabits North America, where the mallard is also found, the case is particularly striking. Among mammals the lion and the tiger and the sable and roan antelopes (_Hippotragus niger_ and _H. equinus_) furnish familiar examples of nearly-related species, in one of which the sexes are alike and in the other dissimilar in appearance. Hormones Another important point to be borne in mind is the intimate correlation that exists between the reproductive organs and the general appearance of the organism, more especially of the secondary sexual characters. These last, in most cases, do not show themselves until the maturity of the sexual organs. The well-known effects of castration illustrate this connection. Again, females in which the reproductive organs have ceased to be functional often assume male characters.