ZOOLOGICAL REALMS. 63 
the Afrial. 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 resemblances 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 
Mammata. 
Zoological Realms— 
1. Arcroc@a = N, America, Eurasia and Africa. 
2. Neoc&a = S, America, W. Indies and part of 
Central America. 
3. Notocea = 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— 
Hotarcric = Europe, N. Asia and N. America. 
ORIENTAL = India and Further India. 
. Ersropian = Africa (South of the Sahara). 
. Mavacasy = Madagascar. 
SONORAN = United States. 
HOO wD 
