it that, relying on the clearness of the evidence
obtained respecting the larger quadrupeds, he ventured to call in
question the identifications announced by some contemporary naturalists
of species of animals said to be common to the southern extremities of
America and Africa.[871]

The migration of quadrupeds from one part of the globe to another,
observes Dr. Prichard, is prevented by uncongenial climates and the
branches of the ocean which intersect continents. "Hence, by a
reference to the geographical site of countries, we may divide the
earth into a certain number of regions fitted to become the abodes of
particular groups of animals, and we shall find, on inquiry, that each
of these provinces, thus conjecturally marked out, is actually inhabited
by a distinct nation of quadrupeds."[872] It will be observed that the
language of Buffon respecting "natural barriers," which has since been
so popular, would be wholly without meaning if the geographical
distribution of organic beings had not led naturalists to adopt very
generally _the doctrine of specific centres_, or, in other words, to
believe that each species, whether of plant or animal, originated in a
single birth-place. Reject this view, and the fact that not a single
native quadruped is common to Australia, the Cape of Good Hope, and
South America, can in no ways be explained by adverting to the wide
extent of intervening ocean, or to the sterile deserts, or the great
heat or cold of the climates, through which each species must have
passed, before it could migrate from one of those distant regions to
another. It might fairly be asked of one who talked of impassable
barriers, why the same kangaroos, rhinoceroses, or lamas, should not
have been created simultaneously in Australia, Africa, and South
America! The horse, the ox, and the dog, although foreign to these
countries until introduced by man, are now able to support themselves
there in a wild state, and we can scarcely doubt that many of the
quadrupeds at present peculiar to Australia, Africa, and South America,
might have continued in like manner to inhabit each of the three
continents had they been indigenous or could they once have got a
footing there as new colonists.

At the same time every zoologist will be willing to concede, that even
if the departure of each species from a single centre had not appeared
to be part of the plan of Nature, the range of species in general must
have become limited,