So that three marine faunas range
northward and southward in parallel lines not far from each other, under
corresponding climate; but from being separated from each other by
impassable barriers, either of land or open sea, they are almost wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places, or
continuous coasts, until, after travelling over a hemisphere, we come to
the shores of Africa; and over this vast space we meet with no well-defined
and distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa on almost exactly opposite
meridians 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.
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