NEOTROPICAL EEALM. 73 



Red River, in Arkansas. The bison, which was at one time very 

 abundant on the western prairies, has now almost wholly disap- 

 peared, and impending destruction likewise threatens the prong- 

 horn (Antilocapra Americana), which in the furcation and shedding 

 of its horns may be considered to be the most divergent type of all 

 antelopes. Characteristic rodent forms of the prairies are the prairie- 

 dog (Cynomys Ludovicianus) and the gopher (Geomys bursarius). 

 More properly belonging to the eastern wooded sections are the 

 beaver (in the north), identical with the European species, the 

 skunk, and Canada porcupine. 



6. The Rocky Mountain Sub-Region comprises the mountainous 

 tracts of the West-Central United States, and a similar region in 

 Canada extending to about the fifty-fifth parallel of latitude. Its 

 southern boundary is determined by the as yet not very exactly 

 defined Sonoran transition region. Its general and distinctive faunal 

 features have already been indicated. 



7. The Californian Sub-Region is comprised within the border- 

 land which stretches along the Pacific west of the main mountain 

 axis, and extends northward to about the fiftieth parallel of latitude. 

 Many of its faunal features are borrowed from the Neotropical 

 realm, but in its entirety the fauna is distinctively North American. 

 Among its characteristic mammalian forms are the sewellel (Ha- 

 ploodon) and the grizzly bear (Ursus horribilis). Other distinctive 

 types are the California condor (Sarcorhamphus Californianus), the 

 ground cuckoos (Geococcyx), and the singular bird known as 

 Chamaea. The number of humming-birds is greater than in the 

 Eastern United States. 



THE NEOTROPICAL REALM. 



This region, as usually recognised, comprises the continent of 

 South America, the West India Islands, Central America, and the 

 lowlands tierras calientes on either side of the Mexican plateau. 

 While, therefore, it is in the main clearly circumscribed by its 

 water-boundary, the northern portion, or that which borders on the 

 Holarctic, is much less sharply defined, as must invariably be the 

 case where two faunal regions overlap. The principal features of 

 the greater portion of this vast tract are singularly uniform. An 

 enormous expanse of forest, unequalled for its continuity and luxu- 

 riance of growth, occupies fully one-half of the surface area, cover- 



