THE BASIS OF ADAPTATION 255 



Zoiigeographical Realms. The earth is now divided into several 

 zoogeographical regions, indicative of the surficial distribution 

 of animals (Fig. 152). Lydekker proposed three chief divisions, 

 each including territory which has been sufficiently isolated in 

 the past to produce a characteristic fauna. These arc the Arcto- 

 gaeic realm, including America north of the Mexican plateau, 

 Europe, Asia, Africa, and all adjacent islands; the Neogaeic realm, 

 including South America and adjacent islands, Central America 

 north to the Arctogaeic, and the West Indies; the Notogaeic 

 includes Australia and the islands of the Pacific. The Notogaeic 

 realm has been isolated since the beginning of the Tertiary, and 

 has consequently developed a peculiar fauna including a number 

 of primitive forms. The Neogaeic also enjoyed a long period of 

 isolation during the Tertiary, and has a characteristic fauna as a 

 result, but the greater area of the Arctogaeic has been open more 

 or less constantly to intermigrations, and is populated by a 

 remarkably homogeneous fauna. 



Within these realms various subdivisions are recognized in 

 which the faunae are distinguished by minor characteristics. The 

 subdivisions commonly used are: 



1. Palaearctic, including Europe, Asia north of Persia, the 

 Himalayan Mountains and the Nan-ling mountains of China, the 

 adjacent islands, and Africa north of the Sahara desert. 



2. The Nearctic, including the American portion of the Arcto- 

 gaeic realm. 



3. The Neotropical region is the same as the Neogaeic realm. 



4. The Ethiopian region includes Africa, Arabia and adjacent 

 islands, south of the Palaearctic region. 



5. The Oriental region includes India, the Malay peninsula and 

 other parts of Asia south of the Palaearctic boundary, and in 

 addition the Malayan archipelago. 



6. The Australian region is the same as the Notogaeic realm. 

 There is some transition between it and the last in the Malayan 

 archipelago. 



Few places on the earth are completely devoid of fife, although 

 such limited areas as the Great Salt Lake desert appear to be so. 

 In this region the earth is a glistening mass of salt for miles. Not 

 a plant is visible within the outer borders of the desert, where a 

 few stunted shrubs are able to live. Excepting such relatively 

 small areas, every region has its living forms. No matter how 



