On this same principle of former
migration, combined in most cases with modification, we can understand, by
the aid of the Glacial period, the identity of some few plants, and the
close alliance of many others, on the most distant mountains, and in the
northern and southern temperate zones; and likewise the close alliance of
some of the inhabitants of the sea in the northern and southern temperate
latitudes, though separated by the whole intertropical ocean. Although two
countries may present physical conditions as closely similar as the same
species ever require, we need feel no surprise at their inhabitants being
widely different, if they have been for a long period completely sundered
from each other; for as the relation of organism to organism is the most
important of all relations, and as the two countries will have received
colonists at various periods and in different proportions, from some other
country or from each other, the course of modification in the two areas
will inevitably have been different.
On this view of migration, with subsequent modification, we see why oceanic
islands are inhabited by only few species, but of these, why many are
peculiar or endemic forms. We clearly see why species belonging to those
groups of animals which cannot cross wide spaces of the ocean, as frogs and
terrestrial mammals, do not inhabit oceanic islands; and why, on the other
hand, new and peculiar species of bats, animals which can traverse the
ocean, are often found on islands far distant from any continent.
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