The Descent of Man

from their lessened fertility, may perhaps be
advanced as a proof of the specific distinctness
of the parent races. No doubt both animal and
vegetable hybrids, when produced from extremely
distinct species, are liable to premature
death; but the parents of mulattoes cannot be
put under the category of extremely distinct
species. The common Mule, so notorious for
long life and vigour, and yet so sterile, shews
how little necessary connection there is in hybrids
between lessened fertility and vitality;
other analogous cases could be cited.
Even if it should hereafter be proved that all the
races of men were perfectly fertile together, he
who was inclined from other reasons to rank
them as distinct species, might with justice argue
that fertility and sterility are not safe criterions
of specific distinctness. We know that
these qualities are easily affected by changed
conditions of life, or by close inter-breeding,
and that they are governed by highly complex