long endurance
of allied forms on the same continent--of marsupials in Australia, of
edentata in America, and other such cases--is intelligible, for within
the same country the existing and the extinct will be closely allied by
descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatical and geographical changes and
to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so
striking a parallelism in the distribution of organic beings throughout
space, and in their geological succession throughout time; for in both
cases the beings have been connected by the bond of ordinary generation,
and the means of modification have been the same. We see the full
meaning of the wonderful fact, which has 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
are the 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, 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 pe