y the same character from
an enormously remote geological period, so certain species have migrated
over vast spaces, and have not become greatly modified.

On these views, it is obvious, that the several species of the same genus,
though inhabiting the most distant quarters of the world, must originally
have proceeded from the same source, as they have descended from the same
progenitor. In the case of those species, which have undergone during whole
geological periods but little modification, there is not much difficulty in
believing that they may have migrated from the same region; for during the
vast geographical and climatal changes which will have supervened since
ancient times, almost any amount of migration is possible. But in many
other cases, in which we have reason to believe that the species of a genus
have been produced within comparatively recent times, there is great
difficulty on this head. It {352} is also obvious that the individuals of
the same species, though now inhabiting distant and isolated regions, must
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
mamma