THE HOLARCTIC REALM 457 



Ethiopian Realm, including Africa south of the Sahara, 

 Madagascar, and part of Arabia. It would be neither 

 possible nor desirable to set forth in a book of this char- 

 acter the details of the floras or faunas of these great 

 realms. The purpose is rather to exhibit general biologi- 

 cal principles and merely to illustrate them with sufficient 

 detail to make them understandable. This end may be 

 accomplished by consideration, in the remaining para- 

 graphs of this chapter, of the chief vegetation types 

 found in a limited region such as the United States and 

 Canada, and in the following chapter a brief presenta- 

 tion of the characteristic animal species of the various 

 realms. 



The Holarctic Realm constitutes a nearly continu- 

 ous land mass, being broken only by the very narrow 

 Bering Strait. Moreover there is abundant evidence 

 that in recent geologic time the connections have been 

 far more extensive both in that region and via Green- 

 land, so that communication and distribution were much 

 easier than now. This was facilitated also by the warmer 

 climate in the far north at that time. However, a con- 

 siderable geologic time has elapsed since the present im- 

 perfect communication was established and the climate 

 became colder, with the result that the species which 

 have evolved since that time have been more largely 

 confined to the continent of their origin. The conse- 

 quence is that North America and Eurasia resemble 

 each other far more than either of them resembles any 

 other realm, but still have characteristic differences. 

 North America may therefore be thought of as one of 

 the vast regions of this enormous realm. The same prin- 

 ciples, however, apply everywhere and may be illus- 

 trated satisfactorily by considering a still more limited 

 region. 



Vegetation of the United States and Canada (see 

 accompanying map). — Within this area are to be found 

 the principal vegetation types of the Temperate and 



