322 PHYSIOLOGICAL REGULATIONS 



modifiabilities in diverse states of heat load, are described. Matur- 

 ing of the animal's heating and cooling system during early post- 

 natal life, and changes of the system after experimental lesions, 

 may be similarly characterized. 



Diversities among species are related to body size, supposed 

 phylogeny of temperature maintenance, the expenditure of water 

 for cooling, and the use of oxidations for warming. Quantitative 

 differences consist in the relative modifications elicited in heat 

 gains and heat losses by a given load of heat. 



The relations of heat in certain animals may be described in 

 the same terms as the relations of water. But whereas mature 

 mammals and birds manifest fixed patterns of compensation and 

 behavior toward heat, many other animals are indifferent to heat 

 until extremes are reached, whereupon they shun environments 

 that impose those extreme increments of heat content. Both kinds 

 are successful in preserving heat content, but those with the nar- 

 rowest deviations of content (homeotherms) also modify a larger 

 number of interrelated variables in accordance with content. 



