nor do I for a
moment pretend that any explanation could be offered of many instances.
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 fresh water productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the nearest
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 climatical and geographical changes,
and to the various occasional means of transport, the belief that a
single birthplace is the 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
species of a genus which must on our theory all be descended from a
common progenitor, can have migrated, undergoing modification during
their migration from some one area. If, when most of the species
inhabiting one region are different from those of another region, though
closely allied to them, it can be shown that migration from the one
region to the other has probably occurred at some former period, our
general view will be much strengthened; for the explanation is obvious
on the principle of descent with modification. 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
related by inheritance to the inhabitants of that continent. Cases of
this nature are common, and are, as we shall hereafter see, inexplicable
on the theory of independent creation. This view of the relation of the
species of one region to those of another, does not differ much from
that advanced by Mr. Wallace, who concludes that "every species has come
into existence coincident both in space and time with a pre-existing
closely allied species." And it is now well known that he attributes
this coincidence to descent with modification.

The question of si