ZOOLOGICAL REALMS. 63 



the ^rial. This general conclusion is not vitiated by 

 the equally certain fact that there have also been cross- 

 migrations and back-migrations of certain types. Certain 

 mammals (whales) have obviously reverted to pelagic 

 habitat and some neritic types (land-crabs) have passed 

 directly to the terrestrial. 



Topographical Distribution. — Just as the animal 

 kingdom is classified into phyla, classes and orders, so the 

 world's surface is divided by zoologists into realms, regions 

 and provinces, to emphasise degrees of difference in the 

 fauna. The same ideal of a natural classification is striven 

 after, and there is the same difficulty of distinguishing 

 between reserablances due to parallel evolution and those 

 due to genetic connection. 



The limits of the realms, regions and provinces are 

 mainly defined by the presence or absence of certain 

 Mammalia^ for, as will be seen later, they are specially 

 suitable for this purpose. Hence we need here merely 

 note the chief zoo-geographical realms and leave more 

 detailed consideration of them to the section dealing with 

 Mammalia. 



Zoological Realms — 



1. Arctogcea = N. America, Eurasia and Africa. 



2. Neogcea = S. America, W. Indies and part of 



Central America. 



3. NoTOGCEA = Australia, New Guinea, Polynesia, 



New Zealand and certain Malay 

 Islands. 



These three realms are divided into a number of import- 

 ant regions. 



The Regions of Arctogcea are — 



A. HoLARCTic = Europe, N. Asia and N. America. 



B. Oriental = India and Further India. 



C. Ethiopian = Africa (South of the Sahara). 



D. Malagasy = Madagascar. 



E. Sonoran = United States. 



