EVOLUTIONARY ASPECTS IN MAMMALS 



like locomotion, etc. The first directly physiological factor brought 

 into the picture of homeothermy seems to have been a variation in 

 metabolic heat production, thus for the first time releasing animal 

 life somewhat from the environment. As discussed above such regu- 

 lation has serious limitations, and collapse occurs at severely high 

 air temperatures and outside the tropical stability. The next step 

 toward advanced homeothermy seems to have been the appearance 

 and development of the regulation of physical he at exchange. We can 

 trace a gradual improvement of such function along the phylum but 

 also closely correlated to the thermal stress imposed by the envi- 

 ronment. We know from the important works of Irving and Scho lander 

 and associates that under the extreme conditions confronting the 

 arctic mammals, homeothermy is first of all accomplished and 

 maintained by adjustment of the shell-core temperature gradient— 

 or in other words, by adjustments of the insulation. The efficient 

 regulation of temperature under changing conditions in the environ- 

 ment must ultimately be entirely dependent upon an integrated con- 

 trol of heat loss and heat exchange by thermostatic arrangements. 

 These thermostatic arrangements have reasonably developed grad- 

 ually becoming increasingly complex and accurate. The data pro- 

 vided by Bartholomew on Setonix , as well as my own data on the 

 armadillo, I think, provides an example of animals whose tempera- 

 ture regulating ability has reached a stage where the degree of 

 thermostatic control is a factor limiting the efficiency with which 

 the animals maintain homeothermy. Efficient central nervous 

 thermostatic control seems thus to have been the last factor devel- 

 oped to perfection in homeothermy. 



If you will grant me another minute, I will admit that there is 

 an obvious trend in the lower forms of mammals for body tempera- 

 ture to be characterized by a large range and a lower set average. 

 In all treatments on the subject, however, this fact is regarded as 

 typifying primitive forms and thus the evolution of homeothermy. 

 It has been stated that the least variable factor in the whole pic- 

 ture of homeothermy is the body temperature. From my own data, 

 as well as other information accumulated recently, I am strongly 

 opposed to this view. In my mind it is entirely conceivable that a 



125 



