THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS 371 



we shall find that the animals of the Atlantic region have 

 been replaced by forms quite different. Finally, after 

 climbing the Sierra Nevada, and descending into the Pacific 

 region, another assemblage of animals differing from either 

 of the above — but not so markedly from the second — 

 will be found. It must be borne in mind, however, that 

 there is no distinct dividing line between these faunal areas. 

 The first overlaps the second, the second the third, and vice 

 versa. 



The North American continent as a whole, exclusive of 

 the circumpolar region which is common to Asia and 

 Europe, may be divided into three primary faunal areas. 

 Beginning at the North Polar sea these are as follows : 

 the Boreal Region, the Austral Region, and the Tropical 

 Region. Here again the areas are not sharply defined, for 

 the adjacent ones overlap each other and animals common 

 to one are found in the other. 



Faunal areas of the earth. — The earth, as a whole, has 

 been divided into seven — some authorities make eight — 

 great areas. These are the Arctic, the North Temperate, 

 the South American, the Indo-African, the Madagascar, the 

 Patagonian, and the Australian. The differences between 

 the faun SB of these various areas are due to the cUmate and 

 the barriers between them. 



The manner in which the distribution of animals has 

 affected species. — It has already been shown that there 

 is a tremendous struggle going on between animals all the 

 time. By this struggle, many are killed, and many are 

 driven out of certain regions to seek other regions where 

 competition is not so great. Thus there is a constant tend- 

 ency among animals to distribute themselves over the earth 

 in search of conditions more favorable to their existence. 



