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.