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 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 independent creation. This view of the relation of
species in one region to those in another, does not differ much (by
substituting the word variety for species) from that lately advanced in
an ingenious paper by Mr. Wallace, in which he concludes, that every
species has come into existence coincident both in space and time with
a pre-existing closely allied species. And I now know from
correspondence, that this coincidence he attributes to generation with
modification.
The previous remarks on single and multiple centres of creation do
not directly bear on another allied question, namely whether all the
individuals of the same species have descended from
a single pair, or single hermaphrodite, or whether, as some authors
suppose, from many individuals simultaneously created. With those
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