Chap. XL GEOGRAPHICAL DISTRIBUTION. 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 widely 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°, 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 l_ 

 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 tin 4 

 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 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 



