As exemplifying the effects of climatical changes on distribution, I have
attempted 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.
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