444 COMPARATIVE ZOOLOGY 



is diversified, though comparatively sparse. Examples 

 of all the five invertebrate divisions were found in the 

 Bay of Biscay, at the depth of 2435 fathoms. 179 



Distribution in the sea is influenced by the tempera- 

 ture and composition of the. water and the character of 

 the bottom. The depth acts indirectly by modifying 

 the temperature. Northern animals approach nearer 

 to the equator in the sea than on the land, on account 

 of cold currents. The heavy aquatic mammals, as 

 whales, walruses, seals, and porpoises, are mainly 

 polar. 



The land consists of the following somewhat distinct 

 areas : the Neotropic, comprising South America, the 

 West Indies, and most of Mexico ; the Nearctic, includ- 

 ing the rest of America; the Palearctic, composed of 

 the eastern continent north of the Tropic of Cancer, 

 and the Himalayas ; the Ethiopian, or Africa south of 

 the Tropic of Cancer ; the Oriental, or India, the south- 

 ern part of China, the Malay Peninsula, and the islands 

 as far east as Java, Borneo, and the Philippine Islands ; 

 and the Australian, or the eastern half of the Malay 

 Islands and Australia. These are the regions of Sclater 

 and Wallace. Other writers unite the northern parts of 

 both hemispheres into one region, and the Oriental with 

 the Ethiopian regions. 



Life in the polar regions is characterized by great 

 uniformity, the species being few in number, though 

 the number of individuals is immense. The same ani- 

 mals inhabit the arctic portions of the three continents ; 

 while the antarctic ends of the continents, Australia, 

 Cape of Good Hope, and Cape Horn, exhibit strong 

 contrasts. Those three continental peninsulas are, zoo- 

 logically, separate worlds. In fact, the whole southern 

 hemisphere is peculiar. Its fauna is antique. Aus- 

 tralia possesses a strange mixture of the old and new. 



