iSO GEOGRAPHICAL DISTElBUTION. [Chap. XIL 



—at least as closely as the same species generally re- 

 quire. 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 con- 

 ditions are peculiar in only a slight degree. Notwith- 

 standing this general 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 35° 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 dissimikr. Or, again, we may com- 

 pare the productions of South America south of lat. 35° 

 with those north of 25°, which consequently are sepa- 

 rated by a space of ten degrees of latitude, and are ex- 

 posed 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 cli- 

 mate, there might have been free migration for the 



