ns of longitude.

A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same 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, though nearly 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 emu, like those inhabiting 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 muskrat, but the
coypu and capybara, rodents of the South 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 are essentially American, though they may be all
peculiar species. We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on the American
continent and in the American seas. We see in these facts some deep
organic bond, throughout space and time, over the same areas of land and
water, independently of physical conditions. The naturalist must be dull
who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we
see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a subordinate
degree to the definite influence of different p