period; if we remember how profoundly ignorant
we are with respect to the many and curious means of occasional
transport,--a subject which has hardly ever been properly experimentised
on; if we bear in mind how often a species may have ranged continuously
over a wide area, and then have become extinct in the intermediate
tracts, I think the difficulties in believing that all the individuals
of the same species, wherever located, have descended from the same
parents, are not insuperable. And we are led to this conclusion, 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 grave.

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.

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 desert