138 THE ORIGIN OF SPECIES 



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 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 latitude 

 85° with those north of 25°, which consequently are sep- 

 arated 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 re- 

 gions. 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 temper- 



