A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the
species themselves are distinct at different points and stations. It is
a law of the widest generality, and every continent offers innumerable
instances. Nevertheless the naturalist in travelling, for instance,
from north to south never fails to be struck by the manner in which
successive groups of beings, specifically distinct, yet clearly
related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly
constructed, but not quite alike, with eggs coloured in nearly 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
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