In regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is somewhat different.

I have more than once alluded to a large body of facts showing that, when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected.

This, in fact, is the great bar to the domestication of animals.

Between the sterility thus superinduced and that of hybrids, there are many points of similarity.