THE STRUCTURE AND ACTIVITIES OF MAMMALS 417 



is met by northern animals in one of four ways: (1) by dying, 

 e.g. adult butterflies, (2) migrating, e.g. birds, (3) hibernating, 

 e.g. bears, (4) remaining active, e.g. rabbits. Animals living 

 in tropical regions pass the summer in many cases in a torpid 

 condition, and are said to be activating. 



A certain amount of water is necessary for life, as the bodies 

 of animals are made up of from 55 to 95 per cent water. Ani- 

 mals living in dry climates have thick skins, and thus evaporation 

 is prevented. 



Light plays a leading role in the lives of animals; many species 

 require it, but others shun it as much as possible, principally in 

 order to escape their enemies. 



And finally, food conditions are most effective, since carnivo- 

 rous animals, e.g. lions, must live where they may obtain flesh ; 

 herbivorous animals, e.g. deer, must live where suitable vegeta- 

 tion abounds ; and omnivorous animals, e.g. man, where both 

 flesh and vegetation of certain sorts exist. 



The Law or Dispersion. — Animals tend to migrate from 

 the region of their birth. It is supposed that every animal pro- 

 duces a greater number of offspring than can be supported in its 

 particular habitat, and since parents and offspring cannot oc- 

 cupy the same area, some individuals must either migrate or 

 die. 



The Law of Barriers and Highways. — Animals are con- 

 fined to certain habitats by barriers and are prevented from en- 

 tering a new region by mountains or lakes, by lack of food, and by 

 the interference of other animals. Common barriers are moun- 

 tains, bodies of water, open country for forest animals, and for- 

 ests for prairie-inhabiting species. The reverse of a barrier is a 

 highway. Apparently there are routes of migration which are 

 especially favored. 



Cosmopolitan Groups of Animals. — Some species of ani- 

 mals have wide ranges, e.g. some are found inhabiting practically 

 every large land area on the earth's surface. This is true of 

 many birds and of the bats among the mammals. 



