that each species has been produced
within one area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a moment
pretend that any explanation could be offered of many such cases. But after
some preliminary remarks, I will discuss a few of the most striking classes
of facts; namely, the existence of the same species on the summits of
distant mountain-ranges, and at distant points in the arctic and antarctic
regions; and secondly (in the following chapter), the wide distribution of
freshwater productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the mainland, though separated by hundreds of
miles of open sea. If the existence of the same species at distant and
isolated points of the earth's surface, can in many instances be explained
on the view of each species having migrated from a single birthplace; then,
considering our ignorance with respect to former climatal and geographical
changes and various occasional means of transport, the belief that this has
been the universal law, seems to me incomparably the safest.

In discussing this subject, we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
distinct species of a genus, which on my theory have all descended from a
common progenitor, can have migrated (undergoing modification during some
part of their migration) from the area inhabited by their progenitor. If it
can be shown to be almost invariably the case, that a region, of which
{355} most of its inhabitants are closely related to, or belong to the same
genera with the species of a second region, has probably received at some
former period immigrants from this other region, my theory will be
strengthened; for we can clearly understand, on the principle of
modification, why the inhabitants of a region should be related to those of
another region, whence it has been stocked. A volcanic island, for
instance, upheaved and formed at the distance of a few hundreds of miles
from a continent, would probably receive from it in the course of time a
few colonists, and their descendants, though modified, would still be
plainly related by inheritance to the inhabitants of the continent. Cases
of this nature are common, and are, as we shall hereafter more fully see,
inexplicable on the theory of inde