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On the Origin of Species

Geographical Distribution
Present distribution cannot be accounted for by differences in physical conditions --
Importance of barriers -- Affinity of the productions of the same continent -- 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 -- Alternate Glacial periods in the
North and South.
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.
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 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
 
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