ibition being enforced against introducing two original
stocks of the same species.

Now it is easy to show that the result of such a mode of colonizing
would correspond exactly, so far as regards the grouping of animals and
plants, with that now observed throughout the globe. In the first place,
it would be necessary for naturalists, before they imported species into
particular localities, to study attentively the climate and other
physical conditions of each spot. It would be no less requisite to
introduce the different species in succession, so that each plant and
animal might have time and opportunity to multiply before the species
destined to prey upon it was admitted. Many herbs and shrubs, for
example, must spread far and wide before the sheep, the deer, and the
goat could be allowed to enter, lest they should devour and annihilate
the original stocks of many plants, and then perish themselves for want
of food. The above-mentioned herbivorous animals in their turn must be
permitted to make considerable progress before the entrance of the first
pair of wolves or lions. Insects must be allowed to swarm before the
swallow could be permitted to skim through the air, and feast on
thousands at one repast.

It is evident that, however equally in this case our original stocks
were distributed over the whole surface of land and water, there would
nevertheless arise distinct botanical and zoological provinces, for
there are a great many natural barriers which oppose common obstacles to
the advance of a variety of species. Thus, for example, almost all the
animals and plants naturalized by us, towards the extremity of South
America, would be unable to spread beyond a certain limit, towards the
east, west, and south; because they would be stopped by the ocean, and a
few of them only would succeed in reaching the cooler latitudes of the
northern hemisphere, because they would be incapable of bearing the heat
of the tropics, through which they must pass. In the course of ages,
undoubtedly, exceptions would arise, and some species might become
common to the temperate and polar regions, or both sides of the equator;
for I have before shown that the powers of diffusion conferred on some
classes are very great. But we might confidently predict that these
exceptions would never become so numerous as to invalidate the general
rule.

Some of the plants and animals transplanted by us to the coast of Chili
and Peru would never be ab