to show how important a part the last Glacial period has
played, which affected even the equatorial regions, and which, during
the alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. 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 all the individuals of the same species, and likewise
of the several species belonging to the same genus, have proceeded
from some one source; then all the grand leading facts of geographical
distribution are explicable on the theory of migration, together with
subsequent modification and the multiplication of new forms. We can thus
understand the high importance of barriers, whether of land or water, in
not only separating but in apparently forming the several zoological and
botanical provinces. We can thus understand the concentration of related
species within the same areas; 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 linked together
in so mysterious a manner, 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 the colonists entered one
of the regions, or both; according to the nature of the communication
which allowed certain forms and not others to enter, either in greater
or lesser numbers; according or not as those which entered happened to
come into more or less direct competition with each other and with the
aborigines; and according as the immigrants were capable of varying
more or less rapidly, there would ensue in the to or more regions,
independently of their physical conditions, infinitely diversified
conditions of life; there would be an almost endless amount of organic
action and reaction, and we should find some groups of beings greatly,
and some only slightly modified; some developed in great force, some
existing