globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions
can be accounted for by their climatal and other physical conditions.
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 northern parts where the
circumpolar land is almost continuous, 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; 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 and 35 , 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 with those north of 25 , 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
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