PERIODIC FLUCTUATIONS IN BODY TEMPERATURE 83 



ance of everyday life, so tliey relapse into somnolence 

 and inactivity. They pass into a state of imperfect 

 homoiothermism, which is very economical of energy and 

 during scarcity of food may permit them to withstand 

 unfavorable conditions in their environments. While 

 torpid they slowly burn away the energy sources of their 

 bodies to keep up a modicum of animal heat, thus com- 

 pensating for the inevitable loss. Hibernation is a con- 

 dition which apparently obtains in certain groups 

 throughout the course of mammalian evolution. It seems 

 to be a persistent, adaptive reaction of survival value. 



