One of the most interesting facts, as showing how susceptible to changed conditions or to slight constitutional changes are the reproductive powers of animals, is the very general difficulty of getting those which are kept in confinement to breed; and this is frequently the only bar to domesticating wild species. Thus, elephants, bears, foxes, and numbers of species of rodents, very rarely breed in confinement; while other species do so more or less freely. Hawks, vultures, and owls hardly ever breed in confinement; neither did the falcons kept for hawking ever breed. Of the numerous small seed-eating birds kept in aviaries, hardly any breed, neither do parrots. Gallinaceous birds usually breed freely in confinement, but some do not; and even the guans and curassows, kept tame by the South American Indians, never breed. This shows that change of climate has nothing to do with the phenomenon; and, in fact, the same species that refuse to breed in Europe do so, in almost every case, when tamed or confined in their native countries. This inability to reproduce is not due to ill-health, since many of these creatures are perfectly vigorous and live very long. With our true domestic animals, on the other hand, fertility is perfect, and is very little affected by changed conditions. Thus, we see the common fowl, a native of tropical India, living and multiplying in almost every part of the world; and the same is the case with our cattle, sheep, and goats, our dogs and horses, and especially with domestic pigeons. It therefore seems probable, that this facility for breeding under changed conditions was an original property of the species which man has domesticated--a property which, more than any other, enabled him to domesticate them. Yet, even with these, there is evidence that great changes of conditions affect the fertility. In the hot valleys of the Andes sheep are less fertile; while geese taken to the high plateau of Bogota were at first almost sterile, but after some generations recovered their fertility. These and many other facts seem to show that, with the majority of animals, even a slight change of conditions may produce infertility or sterility; and also that after a time, when the animal has become thoroughly acclimatised, as it were, to the new conditions, the infertility is in some cases diminished or altogether ceases. It is stated by Bechstein that the canary was long infertile, and it is only of late years that good breeding birds have become common; but in this case no doubt selection has aided the change.