The doctrine that such a universal law existed was never more than a plausible generalisation, founded on a few inconclusive facts derived from domesticated animals and cultivated plants.

The facts were, and still are, inconclusive for several reasons.

They are founded, primarily, on what occurs among animals in domestication; and it has been shown that domestication both tends to increase fertility, and was itself rendered possible by the fertility of those particular species being little affected by changed conditions.

The exceptional fertility of all the varieties of domesticated animals does not prove that a similar fertility exists among natural varieties.

In the next place, the generalisation is founded on too remote crosses, as in the case of the horse and the ass, the two most distinct and widely separated species of the genus Equus, so distinct indeed that they have been held by some naturalists to form distinct genera.