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.
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