CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.
1. GEOGRAPHICAL DISTRIBUTION.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be wholly accounted for by 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 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.