Chap. XI. 
GEOGEAPHICAL DISTEIBUTION. 
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 con¬ 
ditions of the Old and New Worlds, how wddely differ¬ 
ent 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 com¬ 
pare the productions of South America south of lat. 
35° with those north of 25°, w^hich 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 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 di0*erence between the inhabitants of Aus¬ 
tralia, Africa, and South America under the same lati¬ 
tude : for these countries are almost as much isolated 
from each other as is possible. On each continent, 
also, we see the same fact; for on the opposite sides of 
