GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



America, about to the 49th parallel of latitude, belongs to a 

 vast region (what is called the Holarctic region), which 

 includes Europe, northern Africa, and extratropical Asia. 

 Most of the remainder of North America, including the 

 Mexican plateau, makes up a separate region — the Sonoran. 

 The hot lowlands of Mexico, Central America, and the 

 southern tips of Florida and Lower California are included 

 in the same region as South America (the Neotropical 

 region), as are also the West Indian islands. The regions 

 thus designated are determined primarily by the distribu- 

 tion of mammals, the warm-blooded quadrupeds, for these 

 animals are better known than almost any other group except 

 the birds, and their geological history has been much more 

 fully and minutely deciphered than that of any other of the 

 higher groups. These zoological regions are really an inev- 

 itable result of the many changes, climatic and geographic, 

 through which the earth has passed while mammals were 

 abundant and diversified. 



It is practically certain that no group of mammals arose 

 twice independently in unconnected areas, and it is this fact 

 that enables us to trace, by the aid of fossils, the migration 

 of mammals from continent to continent. If a group of 

 mammals could arise independently and more than once the 

 presence of a given group in North America and in Asia 

 would be no indication that those continents were once con- 

 nected, but if each group arose but once and spread as far 

 as geographic and climatic conditions permitted, then its 

 presence in two areas now disconnected indicates the former 

 connection, direct or indirect, of those areas. The outlines 

 of the zoological regions and their geographic relations 

 afford a key to their history and to the manner in which 

 they received their faunas. The complete zoological dif- 

 ference of Australia, for example, from any other continent 



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