312 ON THE ORIGIN OF SPECIES 



a most rare case to find a group of organisms confined 

 to any small spot, having conditions peculiar in only a 

 slight 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 

 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 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 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 

 lofty and continuous mountain-ranges, and of great 

 deserts, and sometimes even of large rivers, we find 

 different productions ; though as mountain -chains, 



