| A third great fact, partly included in the foregoing
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
in travelling, for instance, from north to
never fails to be struck by the manner in which successive groups of beings, specifically distinct,
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
the plains of La Plata by another species of the same genus; and not by a true ostrich or
like those
Africa and Australia under the same latitude. On these same plains of La
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
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
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
→though
they may be all peculiar
We may look
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