_Remarks on the Facts of Hybridity._
The facts that have now been adduced, though not very numerous, are
sufficiently conclusive to prove that the old belief, of the universal
sterility of hybrids and fertility of mongrels, is incorrect. 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.
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