36 HOMOIOTHERMISM 



anism is dependent upon the development of the nenro- 

 muscular apparatus which goes on at the same time. 

 Kendeigh and Baldwin (1928) found that the resistance 

 of young house wrens against cold follows the sigmoid 

 growth curve. Such development of temperature resist- 

 ance, according to these authors, is due largely (1) to 

 the mass of body increasing faster proportionately than 

 the external dissipating surface; (2) to the development 

 of a feathery covering; (3) to the development of an 

 internal dissipating surface, which probably is under 

 nervous and respiratory control; and (4) to the produc- 

 tion of heat by the metabolism. They believe the third 

 factor is the most important. 



