292 ANIMAL LIFE 



of climate to compete with those forms already adapted. 

 For this reason a marked change of climate like a marked 

 change of temperature forms a natural barrier to distribu- 

 tion and serves to circumscribe a natural fauna. 



Closely associated with climate is the nature of forest 

 growth, the growth of grass, and in general the development 

 of conditions which serve for food or shelter to animals. 

 These conditions depend in part on soil, partly on climate 

 and topography, and partly on the acts of man. The for- 

 est and forest soils, acting like a great sponge, retain the 

 waters of the rainy season, and thus regulate the size of 

 the streams. The stream that changes least in volume is 

 most favorable to the life of fishes, frogs, and water ani- 

 mals generally. The destruction of forests on the moun- 

 tain sides acts adversely to the life of these creatures as 

 well as to the interests of the farmer below whose lands 

 the streams should water. When the forests are destroyed, 

 the great host of wood creatures, the bears, squirrels, war- 

 blers, various birds, beasts, and insects of the woods can no 

 longer maintain themselves, and grow rare and disappear. 

 For reasons that are obvious the conditions that produce 

 forest, prairie, canebrake, sage -desert, cactus - desert, and 

 the like are potent in regulating the distribution of the 

 species. 



Still another set of conditions depends on the food sup- 

 ply. The planting of orchards tends to multiply greatly 

 the number of individuals of those species which prey upon 

 fruit. When food is abundant the severity of the struggle 

 for life is relaxed and individuals increase in number. A 

 species may be put to great stress by the disappearance of 

 the animal or plant on which it has depended. Each 

 change made by man among the wild animals or plants 

 may have far-reaching effects upon others. The coyote or 

 prairie-wolf destroys sheep in the ranges of the West. It 

 is thinned out by means of the bounty upon its scalp. 

 Then the jack-rabbit, on which it also feeds, greatly in- 



