CHAPTER XXXIV 



REGULATORY MECHANISMS 



All animals below the birds and mammals are what is usually 

 called " cold-blooded," or poikilothermous. Actually, these 

 animals are not so much cold as dependent on the environ- 

 mental temperature, which may be hot. It is a mistake to 

 regard " cold-blooded " animals as necessarily cold, lethargic 

 and sluggish, for in a tropical climate their temperature is high 

 and they may be very active. Nevertheless, since the processes 

 of life can only go on within a certain limited range of tempera- 

 ture, the fact that an animal is dependent on its environment 

 for its temperature necessarily restricts the kinds of environ- 

 ments in which it is capable of living. Further, within the 

 suitable habitat, the degree of activity of the animal will depend 

 on the temperature. This inconstancy of thermal conditions 

 is a serious bar to the further evolutionary progress of the 

 poikilothermous animals. 



The advantage which the birds and mammals have in being 

 " warm-blooded " (homothermous) is not only the fact that 

 the temperature at which their biological processes go on is 

 high, but still more the fact that this temperature is constantly 

 maintained, regardless of the temperature of the environment. 



The processes of metabolism, and especially muscular 

 activity, entail the production of heat. Some warm-blooded 

 animals shiver when they are cold, and their muscles are then 

 thrown into series of contractions. There is therefore a source 

 of heat within the organism which tends to make the tempera- 

 ture rise. At the same time, heat is continually being lost by 

 radiation from the surface of the animal. The maintenance 

 of a constant temperature within the animal therefore depends 



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