GENERAL ZOOLOGY 



vironments in which the prevaiHng conditions make it possible for them to 

 survive and to reproduce. The maintenance of this harmony between 

 organism and environment, as well as the clearly adaptive nature of the 

 differences between related organisms living in different environments, 

 indicates that adaptation to environmental conditions must have played a 

 major role in the evolution of animals. The evaluation of this role of the 

 environment as a conditioning or guiding factor in evolution has been of great 

 significance in the development of theories of evolution, to be discussed in 

 Chapter 20. In the present chapter we shall consider some of the general 

 principles that have emerged from studies of the environmental relationships 

 of existing animals — principles illustrating the significance of environmental 

 factors in determining the survival and distribution of animals. 



The Physicochemical Environment 



The physicochemical environment is a composite of a great many physical 

 and chemical factors, any one of which may be of primary importance in 

 determining the suitability or unsuitability of a particular environment 

 for a specific type of organism. Although any organism is always exposed 

 to many of these factors simultaneously, it is instructive, and indeed often 

 necessary, to isolate the effects of a few of them individually in order to 

 understand how they limit the activities and distribution of living things. 



Temperature. It is probable that temperature affects animals more 

 conspicuously, and in more different ways, than any other environmental 

 factor. Temperatures vary widely, in different geographical locations, at 

 different depths or altitudes, and even in the same localities at different 

 times of the day or seasons of the year. Changes in temperature exert 

 marked effects on the metabolic rates and activities of animals, as well as 

 on their processes of growth and reproduction. 



The metabolic processes of animals are fundamentally chemical reactions, 

 and the rates at which these reactions proceed are determined by the tempera- 

 ture of the living system. The majority of animals are dependent for the 

 maintenance of their body temperatures on heat from the external environ- 

 ment (i.e., are ectothermous or "cold-blooded"); therefore, the rates of 

 their metabolic reactions are determined by the temperature of the environ- 

 ment. With lowered temperature there is a progressive decrease in metabolic 

 rate, to a point at which dormancy ensues and metabolism is barely detect- 

 able. At still lower temperatures, below 0°C., the fluids of the body 

 eventually freeze and the animal usually dies. There is great variability 

 between different species of ectothermous animals in the degree of resistance 

 to low temperatures. Some insects, for example, living usually at more 

 normal temperatures, can remain in a cold-induced dormancy for long 

 periods and can even withstand freezing. Others die after a few hours or 

 days of exposure to intense cold. On the other hand, there are many species 



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