ANIMALS 251 



1. Barriers of some sort, as mountains and oceans, 

 may prevent a given species of animal from reaching 

 certain regions. 



2. The species may reach another region, but when 

 there, it may be unable to maintain itself. 



3. It may maintain itself, but in so doing it may 

 become so changed that it will form a new species. 



Barriers to distribution. It is very evident that cer- 

 tain conditions will prevent the spread of animals. 

 Land forms, for example, can not live in water; and 

 salt water forms can not live in fresh water. Animals 

 are prevented from reaching certain parts of the earth 

 by mountains, "by rivers, by oceans, by deserts, and, 

 sometimes, by falls in rivers. There is no evident reason 

 why the lion and tiger could not live in South America 

 or coyotes in Europe, or certain birds, as the meadow 

 lark, in Europe, if it were not for the fact that they 

 have been prevented from reaching those places by 

 natural barriers. 



Animals can not maintain their ground. Polar bears 

 if introduced into the tropics could not adjust them- 

 selves to conditions so as to live. Likewise, tropical 

 animals could not survive the cold winters of temperate 

 regions. 



Sometimes animals have been introduced into regions 

 where the conditions were very little changed, and yet 

 they were not able to survive. This was probably 

 because of competition with animals already present 

 in the new region. 



