But after all, the cause more probably lies in some imperfection in the original act of impregnation, causing the embryo to be imperfectly developed, rather than in the conditions to which it is subsequently exposed.
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.

In both cases the sterility is independent of general health, and is often accompanied by excess of size or great luxuriance.
In both cases the sterility occurs in various degrees; in both, the male element is the most liable to be affected; but sometimes the female more than the male.