ond great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference of 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
{348} lofty and continuous mountain-ranges, and of great deserts, and
sometimes even of large rivers, we find different productions; though as
mountain-chains, deserts, &c., 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. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of Panama.
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 here three
marine faunas range far northward and southward, in parallel lines not far
from each other, under corresponding climates; but from being separated
from each other by impassable barriers, either of land or open sea, they
are 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 hardly one shell, crab or