276 AN INTRODUCTION TO ZOOLOGY 



continent of North America has been divided by scientists into 

 definite regions, according to the sum total of the temperature 

 during the season of growth ; and regions of a certain temperature, 

 though widely separated, are liable to support similar faunas 

 (255). Winter is met by northern animals in one of four ways : 

 (i) 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 case? 

 in a torpid condition, and are said to be aestivating. 



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. In dry 

 climates animals have thick skins, and thus evaporation is pre- 

 vented. Aquatic animals obtain air from water, as do also 

 some terrestrial species, e.g. the earthworm. The density of 

 the water and its salinity determine the distribution of many 

 aquatic organisms. 



Light, as we have seen, plays a leading role in the lives of ani- 

 mals; many species require it (Euglena), but others shun it as 

 much as possible (crayfish), principally in order to escape their 

 enemies. Molar agents, such as currents, limit the number of 

 species and individuals, especially where they act with much 

 force. And finally, food conditions are most effective, since 

 carnivorous animals, e.g. lions, must live where they may obtain 

 flesh, herbivorous animals, e.g. deer, must live where suitable 

 vegetation abounds, and omnivorous animals, e.g. crayfishes, 

 where both flesh and vegetation of certain sorts exist. Table 

 XIV presents roughly the four principal kinds of fauna, and their 

 modes of existence. 



The general statement may be made that the major habitats 

 are broken up into minor habitats by variations in the conditions, 

 and the constitution of the organism. There are great differ-- 

 ences in the exactness with which the different forms are confined 

 to different sets of environmental conditions. This will be 

 better understood after the other laws of distribution have been 

 discussed. 



