itions.
Of late, almost every author who has studied the subject has come to
this conclusion. The case of America alone would almost suffice to prove
its truth; for if we exclude the arctic and northern temperate parts,
all authors agree that one of the most fundamental divisions in
geographical distribution is that between the New and Old Worlds; yet
if we travel over the vast American continent, from the central parts of
the United States to its extreme southern point, we meet with the most
diversified conditions; 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 so closely as the
same species generally require. No doubt small areas can be pointed out
in the Old World hotter than any in the New World; but these are not
inhabited by a fauna different from that of the surrounding districts;
for it is rare to find a group of organisms confined to a small area,
of which the conditions are peculiar in only a slight degree.
Notwithstanding this general parallelism in the conditions of 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 and 35 degrees, 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 latitude 35 degrees with those
north of 25 degrees, which consequently are separated by a space of
ten degrees of latitude, and are exposed to considerably different
conditions; yet they are 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 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