CHAPTER 



19 



ECOLOGY: 



Environmental Relationships 



The characteristics of an animal, and the range of activities of which it is 

 capable, are largely determined by its genes; these represent the material of 

 inheritance, or heredity. It is clear, however, that although an animal comes 

 into being with a specific hereditary endowment, the expression of its inherent 

 characteristics is conditioned at all levels by intrinsic and extrinsic factors 

 which collectively may be termed its environment. What an animal is, there- 

 fore, depends on the complex interaction of hereditary and environmental 

 factors. The study of heredity, discussed at length in Chapter 6, forms the 

 subject matter of the field of genetics; the environmental relationships of 

 organisms are the concern of the field of ecology, using the term in its 

 broadest sense. At the molecular, cellular, and organismal levels, various 

 aspects of the internal environment of the cells and tissues of the individual 

 are studied by the biochemist, the cytologist, and the physiologist; the ecol- 

 ogist deals with the interrelationship between the organism and its external 

 environment. 



The external environment consists of every factor in the habitat which in 

 any way, directly or indirectly, affects the organism. Animals are dependent 

 on the external environment in a great many ways. Among other things, the 

 environment furnishes a substrate for support and locomotion, a medium for 

 gaseous exchange, and food materials for the growth, maintenance, and 

 repair of the living organism. A great many environments, tremendously 

 varied, have proved themselves capable of supplying these needs to 

 various groups of animals: oceans, lakes, ponds, and streams; hot and 

 cold springs; arctic, temperate, and tropical land masses; dung heaps, fallen 

 logs, the bodies of plants and other animals; and so on. Each of these 

 environments may be thought of as a composite of what we call physico- 

 chemical factors, such as temperature, oxygen, light, and others; and biotic 



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