146 LAND MAMMALS IN THE WESTERN HEMISPHERE 



of the Old World form a vast, connected land-mass, and the 

 final separation of North America from this great complex 

 is an event of geologically recent date. For reasons that will 

 be made clear in the course of the history, the junction of the 

 two Americas has had comparatively little effect upon the 

 zoology of the northern continent, except in its tropical portion. 

 It is obvious from a glance at the map, that the great zoological 

 divisions are of very unequal size, but the arrangement is made 

 on the basis of degrees of difference in the mammalian faunas. 

 These degrees of difference are, in turn, an expression of length 

 of separation or of the difficulty of communication between 

 connected lands. 



The following table gives the major divisions of the earth 

 apart from Australia : 



I. Neog^ic Realm. Neotropical Region.— 8ont\i and Central America, 

 lowlands of Mexico, the West Indies. 



' 1. Malagasy Region. — Madagascar. 



2. Ethiopian Region. — Africa south of the Sahara 

 Desert. 



3. Oriental Region. — Southern peninsulas of Asia, 

 Malay Archipelago. 



! 4. Holarctic Region. — N. Africa, Europe, Asia, 

 (except southern part), boreal N. America. 

 5. Sonoran Region. — Remainder of N. America 

 [ (except lowlands of Mexico) . 



North America, as is expressed by this scheme, is zoolog- 

 ically composite ; the northern half, including nearly all of 

 Canada, belongs to the vast Holarctic Region, which also 

 comprises Europe, Africa north of the Sahara and Asia north 

 of the Himalaya Mountains. The remainder of the continent, 

 exclusive of the Mexican coastal lowlands, is set off as the 

 Sonoran Region. Inasmuch as we have here to do with 

 broadly continuous land-areas, not demarcated by great physi- 

 cal features, and as the genera and species of mammals differ 

 greatly in regard to their ability to withstand a wide range 

 of climatic variations, it is not to be expected that the boun- 



II. Arctog.eic Realm. 



