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. Such cases as the presence of peculiar species of bats on
oceanic islands and the absence of all other terrestrial mammals, are
facts utterly inexplicable on the theory of independent acts of
creation.

The existence of closely allied representative species in any two areas,
implies on the theory of descent with modification, that the same
parent-forms formerly inhabited both areas: and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species are still common to both. Wherever many closely allied
yet distant species occur, doubtful forms and varieties belonging to the
same groups likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in the
striking relation of nearly all the plants and animals of the Galapagos
Archipelago, of Juan Fernandez, and of the other American islands, to
the plants and animals of the neighbouring American mainland; and of
those of the Cape Verde Archipelago, and of the other African islands to
the African mainland. It must be admitted that these facts receive no
explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings can
be arranged within a few great classes, in groups subordinate to groups,
and with the extinct groups often falling in between the recent groups,
is intelligible on the theory of natural selection with its
contingencies of extinction and divergence of character. On these same
principles we see how it is that the mutual affinities of the forms
within each class are so complex and