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.
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
meridia