l 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 {353} 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 side. Some few families, many sub-families,
very many genera, and a still greater number of sections of genera are
confined to a single region; and it has been observed by several
naturalists, that the most natural genera, or those genera in which the
species are most closely related to each other, are generally local, or
confined to one area. What a strange anomaly it would be, if, when coming
one step lower in the series, to the individuals of the same species, a
directly opposite rule prevailed; and species were not local, but had been
produced in two or more distinct areas!

Hence it seems to me, as it has to many other naturalists, that the view of
each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur, in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatal changes, which have certainly occurred within
recent geological times, must have interrupted or rendered discontinuous
the {354} formerly continuous range of many species. So that we are reduced
to consider whether the exceptions to continuity of range are so numerous
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations,