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 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
305