--Centres of creation--Means of dispersal, by changes of
climate and of the level of the land, and by occasional
means--Dispersal during the Glacial period co-extensive with the world.

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 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 {347} 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 sec