ENVIRONMENT AND DISTRIBUTION 363 



may contain pools of water or streams, the cave population is 

 increased by a number of aquatic forms. 



Surface Animals. — Many animals having their habitat primar- 

 ily on the surface may at times enter the soil, and likewise some 

 burrowing forms may come to the surface. The surface habitat, 

 in sharp contrast to the subterranean, exposes animals to wide 

 variations in temperature, light, and humidity. Surface animals 

 may be divided into three general classes: (1) those living on the 

 ground, (2) those associated with plants, and (3) those able to 

 fly or glide through the air. 



Ecological Succession. — The physical environment, par- 

 ticularly of land and fresh-water forms, is constantly changing 

 and this change for the most part takes place in an orderly 

 manner. Lakes change to ponds, ponds to marshes, and marshes 

 to dry land, on which forests grow, or which becomes a treeless 

 prairie. Each change in physical environment is accompanied 

 by changes in the animal communities affected. Aquatic 

 communities in the usual course of events are succeeded by land 

 communities. Ecology is concerned with the study of all factors 

 involved in the determination of animal communities. It 

 consists largely in the study of the reactions of animals to environ- 

 mental conditions which result eventually in a balance or equilib- 

 rium between the individual members of a community and their 

 environment. Actually such balances or equilibria are unstable 

 and change with conditions. Then too, major geographical 

 changes, such as are known to have occurred in the past, may 

 completely destroy the balances established in the usual course 

 of ecological successions. 



Distribution. — Zoogeography is the study of the geophysical 

 distribution of animals. As might be expected, such studies 

 show that representatives of the same group of animals are found 

 living under similar conditions in different parts of the world, 

 ecological relations being what they are. However, a review of 

 the natural, geographical distribution of animals in modern 

 times shows areas devoid of animals that are capable of living in 

 those areas. Thus the introduction into Australia of the rabbit 

 by man has resulted in the rabbit becoming established com- 

 pletely as a member of the Australian fauna. Similarly the horse 

 introduced into the American continent by Spaniards has been 

 able to thrive in the plains of North and South America. Many 

 animals show a discontinuous distribution in that members of the 



