t diversified conditions; the most humid districts,
arid deserts, lofty mountains, grassy plains, forests, marshes, lakes,
and great rivers, under almost every temperature. There is hardly a
climate or condition in the Old World which cannot be paralleled in the
New--at least as closely as the same species generally require; for it
is a most rare case to find a group of organisms confined to any small
spot, having conditions peculiar in only a slight degree; for instance,
small areas in the Old World could be pointed out hotter than any in
the New World, yet these are not inhabited by a peculiar fauna or flora.
Notwithstanding this parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes
25 deg and 35 deg, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or again we may compare the productions
of South America south of lat. 35 deg with those north of 25 deg, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to
the productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the
sea.

A second 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 lofty and continuous mountain-ranges, and of great deserts, and
sometimes even of large rivers, we find different productions; though as
mountain