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
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 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
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