ourse of a few centuries, certain plants and animals
from stations which they previously occupied. If, therefore, the Author
of nature had not been prodigal of those numerous contrivances, before
alluded to, for spreading all classes of organic beings over the
earth--if he had not ordained that the fluctuations of the animate and
inanimate creation should be in perfect harmony with each other, it is
evident that considerable spaces, now the most habitable on the globe,
would soon be as devoid of life as are the Alpine snows, or the dark
abysses of the ocean, or the moving sands of the Sahara.

The powers, then, of migration and diffusion conferred on animals and
plants are indispensable to enable them to maintain their ground, and
would be necessary, even though it were never intended that a species
should gradually extend its geographical range. But a facility of
shifting their quarters being once given, it cannot fail to happen that
the inhabitants of one province should occasionally penetrate into some
other; since the strongest of those barriers which I before described as
separating distinct regions are all liable to be thrown down, one after
the other, during the vicissitudes of the earth's surface.

_How changes in physical Geography affect the distribution of
species._--The numbers and distribution of particular species are
affected in two ways, by changes in the physical geography of the
earth:--First, these changes promote or retard the migrations of
species; secondly, they alter the physical conditions of the localities
which species inhabit. If the ocean should gradually wear its way
through an isthmus, like that of Suez, it would open a passage for the
intermixture of the aquatic tribes of two seas previously disjoined, and
would, at the same time, close a free communication which the
terrestrial plants and animals of two continents had before enjoyed.
These would be, perhaps, the most important consequences, in regard to
the distribution of species, which would result from the breach made by
the sea in such a spot; but there would be others of a distinct nature,
such as the conversion of a certain tract of land, which formed the
isthmus, into sea. This space, previously occupied by terrestrial plants
and animals, would be immediately delivered over to the aquatic; a local
revolution which might have happened in innumerable other parts of the
globe, without being attended by any alteration in the blending t