xxi. 4 GEOGRAPHICAL REGIONS 573 



ture gradients and desert areas. Zoogeographers divide the land into 

 six regions (Fig. 353), Palaearctic (Central and N. Asia, Europe), 

 Nearctic (N. America), Neotropical (Central and S. America), 

 Ethiopian (Africa), Oriental (S. Asia and E. Indies), and Australasian. 

 The Palaearctic and Nearctic have similar climates and have been 



Fig. 353. Polar projection, showing the zoogeographical realms. (From Lull, 



Organic Evolution, copyright 191 7, 1929 by The Macmillan Company, and 



used with their permission.) 



connected several times during the Tertiary by an Alaska-Siberian 

 bridge; they are therefore often grouped together as Holarctic. The 

 Ethiopian region is now separated from the Palaearctic by the sudden 

 change of temperature and desert conditions of North Africa, but in 

 earlier times the animals of the two regions mixed freely. The same 

 is true of the Palaearctic and Oriental regions. South America was 

 connected with the Nearctic region in the Eocene, but the bridge was 

 then broken until the Pleistocene; the Neotropical land faunas there- 

 fore differ considerably from the others. Australasia east of Wallace's 

 line has been separated since the late Cretaceous. 



The land masses have therefore been connected, in the main, in the 

 north and separated in the south. In other words, America, North 



