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 organic beings which
never intercross (if such exist), the species, on my theory, must have
descended from a succession of improved varieties, which will never have
blended with other individuals or varieties, but will have supplanted
each other; so that, at e