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 be 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
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 intermediate varieties
which will at first probably exist in the intermediate zones, will be
liable to be supplanted by the allied forms on either hand; and the
latter, from existing in greater numbers, will generally be modified
and improved at a quicker rate than the intermediate varieties, which
exist in lesser numbers; so that the intermediate varieties will, in
the long run, be supplanted and exterminated.
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