Infertile horses sent from their native home to a different climate (as, for instance, from Scotland to Newmarket) become fertile. A judicious crossing of varieties or races threatened with infertility will often lead to increased vigour and fertility in the new generation, just as change of locality will produce such a result. Physiological processes which are not obvious and cannot be exactly estimated or measured are then, we must conclude, largely connected with the question of sterility and fertility. Mr. Darwin has collected facts which go far to prove that colour (as in the case of the black pigs of Virginia, which I cited in Chapter X.), instead of being a trifling and unimportant character, as was supposed by the older naturalists, is really one of great significance, often correlated with important constitutional differences. It is pointed out by Alfred Wallace that in all the recorded cases in which a decided infertility occurs between varieties (or races) of the same species of plants (such as those just cited), those varieties are distinguished by a difference of colour. He gives reasons for thinking that the correlation of colour with infertility which has been detected in several cases in plants may also extend to animals in a state of nature. The constant preference of animals–even mere varieties of dog, sheep, horses, and pigeons–for their like, has been well established by observation. Colour is one of the readiest appeals to the eye in guiding animals in such selection and association, and is connected with deep-seated constitutional qualities. "Birds of a feather flock together" is a popular statement confirmed by the careful observation of naturalists. Thus we arrive at some indication of features which may determine the inter-breeding, or the abstention from inter-breeding, of diverse races sprung from one original stock. The "colour bar" is not merely the invention of human prejudice, but already exists in wild plants and animals.