THE PHYLUM CHORDATA 



TO MAMMALS 



Figure 58. Adaptive Radiation in Reptiles. (From Romer, 

 brates," University of Chicago Press, 1946.) 



'Man and the Verte- 



not able to adapt themselves to conditions of cold. Just the opposite sug- 

 gestion has been made by Cowles, largely on the basis of studies on the 

 reproductive physiology of living reptiles. It is well known that the testis 

 is heat-sensitive. Mammals are sterilized by a temperature only slightly 

 above the normal temperature of the scrotom. Birds, which characteristi- 

 cally have a higher body temperature than mammals, show spermatogenic 

 activity principally in the early morning hours, when the body tempera- 

 ture is lowest. Cowles has demonstrated that the optimum temperature 

 for normal activity of living reptiles is only a shade below the sterilizing 

 temperature. Reptiles, of course, are "cold blooded," meaning that they 

 do not maintain a constant body temperature. But large bodies cool oflE 

 much more slowly than do small ones, and Cowles has suggested that, in 

 an increasingly hot climate, reptiles so large as many of the dinosaurs 

 might only rarely cool o£f suflBciently to permit spermatogenesis. Thus 

 the great size of the Ruling Reptiles, in combination with an increasingly 

 hot climate, could lead to their extinction by sterilization of the males. 



169 



