The sterility of first crosses and of their hybrid progeny has not been acquired through natural selection.

In the case of first crosses it seems to depend on several circumstances; in some instances in chief part on the early death of the embryo.

In the case of hybrids, it apparently depends on their whole organisation having been disturbed by being compounded from two distinct forms; the sterility being closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions of life.

He who will explain these latter cases will be able to explain the sterility of hybrids.