It is universally
admitted, that in most cases the area inhabited by a species is continuous;
and that 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 have
been easily passed over by migration, the fact is given as something
remarkable and exceptional. The incapacity of migrating across a wide sea
is more clear in the case of terrestrial mammals than perhaps with any
other organic beings; and, accordingly, we find no inexplicable instances
of the same mammals inhabiting distant points of the world. No geologist
feels any difficulty in Great Britain possessing the same quadrupeds with
the rest of Europe, for they were no doubt once united. 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 wide and broken
interspaces. The great and striking influence of barriers of all kinds, is
intelligible only on the view that the great majority of species have been
produced on one side, and have not been able to migrate to the opposite
side.
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