DAWSON, W. R. 



two classes probably reflects nothing more than differences in the 

 temperature relations of the independent reptilian stocks from which 

 they emanated. Variations in levels of activity and lethal body tem- 

 peratures comparable in extent to the differences separating birds 

 and mammals can be found among contemporary reptiles, desert 

 lizards and snakes for example, (Cole, 1943; and Cowles and Bo- 

 gert, 1944). 



The stabilization of body temperature at a high level in birds 

 may have demanded physiological adjustments beyond those con- 

 cerned with the establishment of thermoregulatory capacities, even 

 if the antecedents of the first homeotherms in the avian line had 

 utilized high body temperatures for activity in the manner of many 

 contemporary reptiles, particularly lizards. These animals, despite 

 their utilizing body temperatures similar to those of homeotherms 

 for activity (see Cowles and Bogert, 1944; Norris, 1953; and Fitch, 

 1956), apparently have not developed the capacity for prolonged 

 existence at a high thermal level. Wilhoft (19 58) found that main- 

 tenance of fence lizards (Sceloporus occidentalis ) at their activity 

 temperature (34 C) for approximately three months resulted in the 

 death of some animals, increased frequency of molting in some, and 

 increased thyroid activity in all. None of these changes was observed 

 in the control animals, which were allowed a more normal thermal 

 pattern in which warm body temperatures alternated with cooler 

 ones. The duration of the daily periods spent at warm body tem- 

 peratures by heliothermic lizards such as Sceloporus and Uma is 

 apparently controlled in partby the parietal eye (Stebbins and Eakin, 

 19 58). Elimination of this structure or shielding it from radiation 

 significantly increased the extentto which the lizards exposed them- 

 selves to sunlight. 



EVOLUTION OF THERMOREGULATORY PROCESSES 



The evolution of the complex array of processes on which home- 

 other my depends must have involved many steps. It has been possi- 

 ble to gain some insight into the probable nature and sequence of 



48 



