hat hybrids
produced by a cross between two distinct species are few in number,
owing to their perishing soon after conception or at a very early age,
or if surviving that they are rendered more or less sterile, it seems
highly probable that this result is due to their having been in fact
subjected to a great change in their conditions of life, from being
compounded of two distinct organisations. He who will explain in a
definite manner why, for instance, an elephant or a fox will not breed
under confinement in its native country, whilst the domestic pig or dog
will breed freely under the most diversified conditions, will at the
same time be able to give a definite answer to the question why two
distinct species, when crossed, as well as their hybrid offspring,
are generally rendered more or less sterile, while two domesticated
varieties when crossed and their mongrel offspring are perfectly
fertile.

Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, are descended from common parents; and therefore,
in however distant and isolated parts of the world they may now be
found, they must in the course of successive generations have travelled
from some one point to all the others. We are often wholly unable even
to conjecture how this could have been effected. Yet, as we have reason
to believe that some species have retained the same specific form for
very long periods of time, immensely long as measured by years, too much
stress ought not to be laid on the occasional wide diffusion of the same
species; for during very long periods there will always have been a good
chance for wide migration by many means. A broken or interrupted range
may often be accounted for by the extinction of the species in the
intermediate regions. It cannot be denied that we are as yet
very ignorant as to the full extent of the various climatical and
geographical changes which have affected the earth during modern
periods; and such changes will often have facilitated migration. As an
example, I have attempted to show how potent has been the influence of
the Glacial period on the distribution of the same and of allied species
throughout the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct species of the
same genus