creased vigour or fertility
which is invariably produced by a judicious cross may be also effected
by a judicious change of climate and surroundings. We shall see in a
subsequent chapter, that this interchangeability of the beneficial
effects of crossing and of new conditions, serves to explain some very
puzzling phenomena in the forms and economy of flowers.

_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. Crosses between the two species of
zebra, or even between the zebra and the quagga, or the quagga and the
ass, might have led to a very different result. Again, in pre-Darwinian
times it was so universally the practice to argue in a circle, and
declare that the fertility of the offspring of a cross proved the
identity of species of the parents, that experiments in hybridity were
usually made between very remote species and even between species of
different genera, to avoid the possibility of the reply: "They are both
really the same species;" and the sterility of the hybrid offspring of
such remote crosses of course served to strengthen the popular belief.

Now that we have arrived at a different standpoint, and look upon a
species, not as a distinct entity due to special creation, but as an
assemblage of individuals which have become somewhat modified in
structure, form, and constitution so as to ada