GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 470 



the monkeys are found in both, but the tailless monkeys 

 are peculiar to the Old World, while the prehensile 

 tailed monkeys are peculiar to America. The great 

 pachyderms are peculiar to the Old, while the sloths are 

 peculiar to the New. Among birds the great family of 

 humming birds, containing over four hundred species, 

 are peculiar to America, while the true ostriches are con- 

 fined to Africa. Among plants, the great family of cac- 

 tuses, with its innumerable species, is peculiar to 

 America. 



Nos. 2 and 3, in the southern hemisphere, continue 

 quite distinct in South America and Africa, because these 

 are still widely separated. There are some evidences, 

 however, derived entirely from their faunas and floras, 

 that they were once more approximated than now. 



SUBDIVISIONS OF CONTINENTAL FAUNAS. 



We have already spoken of subdivisions of these de- 

 termined by temperature, and in a more marked manner 

 by east and west barriers, such as mountain chains, etc. 

 But they are divided also by north and south barriers. 

 Thus the United States fauna is divided in a very marked 

 way by north and south mountain chains into distinct 

 faunal regions. By the Appalachian range the division 

 is not very marked, because the chain is not very high 

 nor very long, but the Rocky Mountain chain, running 

 the length of the continent, and, together with the inter- 

 mountain deserts, more than one thousand miles wide, 

 form an insuperable barrier to most species of animals 

 and plants. So that California is a very distinct faunal 

 and floral region. Similarly, but in less degree, the 

 Ural separates a European from an Asiatic fauna and 

 flora. 



Special Cases.— i. Australia.— -We have thus far 

 spoken only of the two great continental masses, east- 

 32 



