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, enormously 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
of time 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 of the full extent of the various climatal
and geographical changes which have affected the earth during modern
periods; and such changes will obviously have greatly facilitated
migration. As an example, I have attempted to show how potent has been the
influence of the Glacial period on the distribution both of the same and of
representative 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 inhabiting very distant and isolated
regions, as the process of modification has necessarily been slow, all the
means of migration will have been possible during a very long period; and
consequently the difficulty of the wide diffusion of species of the same
genus is in some degree lessened.

As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our present varieties, it may be asked,
Why do we not see these linking forms all around us? Why are not all
organic beings blended together in an inextricable chaos? With respect to
existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover _directly_ connecting {463} links
between them, but only between each and some extinct and supplanted form.
Even on a wide area, which has during a long period remained continuous,
and of which the climate and other conditions of life change insensibly in
going from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zone. For we have reason
to believe that only a few species are undergoing change at any one period;
and all changes are slowly effected. I have also shown that the
intermedi