, which
has been arrived at by many naturalists under the designation of single
centres of creation, by some general considerations, more especially from
the importance of barriers and from the analogical distribution of
sub-genera, genera, and families.

With respect to the distinct species of the same genus, which on my theory
must have spread from one parent-source; if we make the same allowances as
before for our ignorance, and remember that some forms of life change most
slowly, enormous periods of time being thus granted for their migration, I
do not think that the difficulties are insuperable; though they often are
in this case, and in that of the individuals of the same species, extremely
great.

As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern
Glacial period, which I am fully convinced simultaneously affected the
whole world, or at least great meridional belts. As showing how diversified
are the means of occasional transport, I have discussed at some little
length the means of dispersal of fresh-water productions. {408}

If the difficulties be not insuperable in admitting that in the long course
of time the individuals of the same species, and likewise of allied
species, have proceeded from some one source; then I think all the grand
leading facts of geographical distribution are explicable on the theory of
migration (generally of the more dominant forms of life), together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water, which
separate our several zoological and botanical provinces. We can thus
understand the localisation of sub-genera, genera, and families; and how it
is that under different latitudes, for instance in South America, the
inhabitants of the plains and mountains, of the forests, marshes, and
deserts, are in so mysterious a manner linked together by affinity, and are
likewise linked to the extinct beings which formerly inhabited the same
continent. Bearing in mind that the mutual relation of organism to organism
is of the highest importance, we can see why two areas having nearly the
same physical conditions should often be inhabited by very different forms
of life; for according to the length of time which has elapsed since new
inhabitants entered one region; according to the nature of the
communication whi