of allied
forms on the same continent,--of marsupials in Australia, of edentata in
America, and other such cases,--is intelligible, for within a confined
country, the recent and the extinct will naturally be 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 climatal 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 {477}
modification have been the same. We see the full meaning of the wonderful
fact, 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 whic