uced 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.
Some few families, many subfamilies, very many genera, 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 confined to the same country, or if they have a
wide range that their range is continuous. What a strange anomaly it
would be if a directly opposite rule were to prevail when we go down one
step lower in the series, namely to the individuals of the same species,
and these had not been, at least at first, confined to some one region!

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 climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the 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, that each species has been produced within
one area, and has migrated thence as far as it could. It would be
hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points;