We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude; for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts and even of
large rivers, we find different productions; though as mountain chains,
deserts, etc., are not as impassable, or likely to have endured so long, as
the oceans separating continents, the differences are very inferior in
degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are the
same on the opposite sides of the isthmus of Panama; and this fact has led
naturalists to believe that the isthmus was formerly open. Westward of the
shores of America, a wide space of open ocean extends, with not an island
as a halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the Pacific
with another and totally distinct fauna.
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