474 ESSENTIALS OF ZOOLOGY 



6. The Neotropical Region consists of Mexico, Central America, 

 and South America. 



The present distribution of animals on the earth gives us many 

 evidences concerning the past history of the earth. The North Ameri- 

 can fauna is notably different from the South American fauna. This 

 fact apparently indicates that North and South America were com- 

 pletely separated from each other by an extensive water barrier until 

 a relatively recent period. The great contrast between the faunas of 

 the islands of Bali and Lombok indicates that the Australian continent 

 and its neighboring islands have been separated from the Asiatic 

 mainland for a relatively long period of time. The study of distribu- 

 tion also gives many clues to the ancient changes of climate on the 

 earth. 



Bathjmaetric Distribution. — In discussing the vertical or altitudi- 

 nal distribution of animals two fundamental types of habitats must 

 be considered; namely, the water habitat or Hydrohios and the land 

 habitat or Geobios. The inhabitants of these two realms differ 

 greatly, but some few forms occupy both regions. 



The hydrobios iiicludes both fresh- and salt-water realms. The 

 variation in vertical distribution of fresh-water animals is relatively 

 insignificant when compared to that of marine animals. The seas 

 have been variously classified into vertical zones. The divisions 

 most commonly recognized are : 



1. The Littoral which is the area between tide marks. It is charac- 

 terized on rocky shores by growths of barnacles, mussels, and snails. 

 Life in sandy littoral regions, such as that along the Texas Gulf 

 Coast, is relatively scarce and consists chiefly of annelid worms and 

 other boring forms. 



2. The Suhlittoral includes the subtidal region to a depth of ap- 

 proximately 100 meters. This is about the lowest depth at which 

 plants can grow abundantly. It is the most productive region on 

 the earth as far as variety and numbers of animals are concerned. 



3. The Abyssal zone is marked by the absence of light and extends 

 to the greatest depths of the oceans. Life is not very abundant in 

 this zone. Plants cannot live at such depths, therefore, the animals 

 which live in the abyssal region are dependent upon other animals 

 as a source of food, or they subsist upon the floating plant life which 

 is continuously dying and sinking to lower levels. 



