130 GEOGRAPHICAL DISTRIBUTION. [Chap. XIL 



least as closely as tke same species generally require. 

 No doubt small areas can be pointed out in the Old 

 World hotter tban 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 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 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, tliat 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 



