have proceeded from one spot, where their
parents were first produced: for, as explained in the last chapter, it
is incredible that individuals identically the same should ever have
been produced through natural selection from parents specifically
distinct.

We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are very many cases of
extreme difficulty, in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points, where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it, rejects the vera causa of ordinary generation
with subsequent migration, and calls in the agency of a miracle. It is
universally admitted, that in most cases the area inhabited by a species
is continuous; and when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space
could not be easily passed over by migration, the fact is given as
something remarkable and exceptional. The capacity of migrating across
the sea is more distinctly limited in terrestrial mammals, than perhaps
in any other organic beings; and, accordingly, we find no inexplicable
cases of the same mammal inhabiting distant points of the world. No
geologist will feel any difficulty in such cases as Great Britain having
been formerly united to Europe, and consequently possessing the same
quadrupeds. But if the same species can be produced at two separate
points, why do we not find a single mammal common to Europe and
Australia or South America? The conditions of life are nearly the
same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal plants
are identically the same at these distant points of the northern and
southern hemispheres? The answer, as I believe, is, that mammals have
not been able to migrate, whereas some plants, from their varied means
of dispersal, have migrated across the vast and broken interspace. The
great and striking influence which barriers of every kind have had on
distribution, is intelligible only on the view that the great majority
of species have been produced on one side alone, and have not been able
to migrate to the other s