FITNESS OF TEMPERATURE CONSTANCY 93 



of high temperatures. Thus the higher mammal main- 

 tains its body temperature without direct reference to 

 muscular activity, and metabolism may always proceed 

 under optimum conditions. 



In homoiotherms is seen the culmination of changes 

 which began in the early communal life of Metazoa for 

 the apparent advantage to be obtained from a relative 

 independence from a fluctuating environment. Perhaps 

 the word ** advantage" implies a teleological explana- 

 tion, but another view is equally plausible — that, through 

 the agency of selection, evolution has been directed along 

 the lines of modifications in the internal physiological 

 environment of the organism as much as in morphologi- 

 cal changes. Animals apparently become more special- 

 ized, or morphologically adapted, to particular environ- 

 ments when conditions are most stable. When conditions 

 lack stability or are rapidly changing, evolution of mor- 

 phological characters acquires no momentum. On the 

 other hand the variations which lead to successive lib- 

 erations from the influences of a varying environment 

 would be of the greatest survival value in the environ- 

 ment of greatest instability. Such variations have pro- 

 ceeded along several lines, one of which has terminated 

 in regulation at a constant temperature level. 



For the organism as a whole the most favorable tem- 

 perature is that which is conducive to the harmonious 

 interaction of all its manifold activities. The attainment 

 of a constant temperature by the homoiotherms has 

 given them two advantages. (1) A temperature which 

 is usually above that of the environment in which an 

 animal lives permits a greater expenditure of energy. 

 The velocities of chemical reactions going on in an 

 animal's body are proportional to the ratio existing 

 between the chemical force and the chemical resistance. 



