332 THE AMERICAN NATURALIST [Vol. XLIX 



average of 39.38° C. for 176 measurements), or slightly 

 less than the usual body temperature of birds (42° C.). 20 



The maintenance of a constant temperature is depend- 

 ent not upon one mechanism alone, but upon the coordi- 

 nated interaction of several mechanisms. The presence 

 of a coat of fur or feathers has long been recognized as a 

 factor in maintaining the constant temperatures of mam- 

 mals and birds. The development of such a protective 

 covering has many times been emphasized by evolution- 

 ists, and seems to be well accounted for on the theory of 

 natural selection. The fur or feathers tend to diminish 

 heat loss from the surface of the body, but have nothing 

 to do with the production or distribution of heat within the 

 body. A subcutaneous layer of fat may still further re- 

 duce the heat loss from the surface, as in the Cetacea. 



The production of heat is directly dependent upon oxi- 

 dation in the muscles and glands of the body. A fall of 

 general body temperature is attended by increased muscu- 

 lar activity, as shivering, when the temperature tends to 

 fall unduly low. The muscular activity is dependent in 

 its turn upon the nervous system, and upon the supply of 

 oxygen and oxidizable substances through the blood. 



The effect of the blood in maintaining a more nearly 

 constant temperature of the muscles, as well as the pro- 

 duction of heat by the muscles themselves, is shown by 

 Meade Smith's experiments on mammalian muscles. 21 

 Although more heat is produced in a muscle which is con- 

 tracting than in a resting muscle, there is still some heat 

 production while at rest. When the artery going to a 

 resting muscle was tied off, the difference in temperature 

 between muscle and blood due to heat production in the 

 muscle might be as much as 0.6° C. at the end of a five- 

 minute period. "When the circulation is intact, this dif- 

 ference in temperatures does not become so great. Te- 

 tanic stimulation of a muscle may lead to a considerable 

 increase in the temperature of the venous blood coming 



