ct, which must have struck every traveller, namely, that on
the same continent, under the most diverse conditions, under heat and
cold, on mountain and lowland, on deserts and marshes, most of the
inhabitants within each great class are plainly related; for they will
generally be descendants of the same progenitors and early colonists.
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, under the most different climates; and
likewise the close alliance of some of the inhabitants of the sea in
the northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants
being widely different, if they have been for a long period completely
separated from each other; for as the relation of organism to organism
is the most important of all relations, and as the two areas will have
received colonists from some third source or from each other, at various
periods and in different proportions, the course of modification in the
two areas will inevitably be different.

On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that
many should be peculiar. We can clearly see why those animals which
cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
should not inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, which can traverse the ocean, should so often
be found on islands far distant from any continent. Such facts as the
presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation.

The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and varieties of
the same species likewise occur. It is a rule of high generality that
the inhabitants