299 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- 
