y the same manner. The
plains near the Straits of Magellan are inhabited by one species of
Rhea (American ostrich), and northward the plains of La Plata by another
species of the same genus; and not by a true ostrich or emeu, like those
found in Africa and Australia under the same latitude. On these same
plains of La Plata, we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits and belonging to the
same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera and we find an
alpine species of bizcacha; we look to the waters, and we do not find
the beaver or musk-rat, but the coypu and capybara, rodents of the
American type. Innumerable other instances could be given. If we look
to the islands off the American shore, however much they may differ in
geological structure, the inhabitants, though they may be all peculiar
species, are essentially American. We may look back to past ages, as
shown in the last chapter, and we find American types then prevalent on
the American continent and in the American seas. We see in these facts
some deep organic bond, prevailing throughout space and time, over
the same areas of land and water, and independent of their physical
conditions. The naturalist must feel little curiosity, who is not led to
inquire what this bond is.

This bond, on my theory, is simply inheritance, that cause which alone,
as far as we positively know, produces organisms quite like, or, as we
see in the case of varieties nearly like each other. The dissimilarity
of the inhabitants of different regions may be attributed to
modification through natural selection, and in a quite subordinate
degree to the direct influence of different physical conditions.
The degree of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been effected
with more or less ease, at periods more or less remote;--on the nature
and number of the former immigrants;--and on their action and reaction,
in their mutual struggles for life;--the relation of organism to
organism being, as I have already often remarked, the most important of
all relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many