o 

 O 



40 GEOGRAPHICAL DISTKIBUTION OF SPECIES. [Ch. XXXVIII. 



modenij so tliat there lias not been time for tlie cai 

 extinction to make gaps in the series of new varieties. 



Nem 



■We 



Nearctic 

 f Mexico 



t 



North 



compar 



this great province with the nearest lands on the east and 

 w^est, the north of Africa on the one side and China on the 

 other, we find a complete dissimilarity between the fauna of 

 the American and that of the African and Asiatic continents; 

 but, the farther we go north and enter those latitudes where 

 the three continents approach each other, the more the dis- 

 cordance in genera and species diminishes. It has often, in- 

 deed, been said that the whole circumpolar region forms one 

 province ; but some of the American species formerly iden- 

 tified with the European^ — the badger, for exam^Dle— have 

 been found to differ on closer examination, and the musk-ox 

 {Ovihos moschahis) is peculiar to America, although the same 

 animal formerly ranged, as we know from its fossil remains, 

 over Germany, France, and England. 



The predominant influence of climate over all the other 

 causes which limit the range of species in the mammalia is 

 perhaps nowhere so conspicuously displayed as in the region 

 now under consideration. It will be observed that on this 

 continent between the Rocky Mountains and the Atlantic 

 there are no great geographical barriers running east and 

 west, such as high snow-clad mountains, barren deserts, or 

 wide arms of the sea, capable of checking the free migration 

 of species from north to south. Yet the arctic fauna, so ad- 

 mirably described by Sir John Richardson, has scarcely any 

 species in common with the fauna of the state of New York, 

 which is 600 miles farther south, and comprises about forty 

 distinct mammifers. 



4 



600 miles, and enter another zone, running east and west, in 

 South Carolina, Georgia, Alabama, and the contiguous states, 

 Ave again meet with a noAV assemblage of land quadrupeds, 

 and this again differs from the fauna of Texas farther to the 

 south, where frosts are unknown. But notwithstanding the 

 distinctness of those zones of indigenous mammalia, there 

 are some species, such as the buffalo (Biso7i Americamis), the 



If airain we travel farther south about 



