THE PHYSIOLOGICAL EVOLUTION, ETC. 



393 



With these birds the average cloacal temperature is only 

 about 42°. In the Passeres, however, which form the 

 highest order of the class Aves, the temperature is, as a 

 rule, a degree or two higher. 



Why there should be this gradual rise of temperature 

 accompanying the increasing complexity of morphological 

 structure is by no means clear. That the two conditions 

 do not necessarily go hand in hand is shown by the above 

 quoted data on the temperatures of certain of the higher 

 mammals. From these it may be seen that the temperature 

 of man is lower than that of any of the other mammals men- 

 tioned, and no less than 3-2° lower than that of the sheep. A 

 high body temperature is certainly an advantage in one way. 

 Thus the more it is raised above that of the environment, 

 the easier is it for the nervous system to keep it approxi- 

 mately constant. On the other hand it obviously entails 

 a higher metabolism and increased consumption of food, a 

 weighty drawback in view of the ever present struggle for 

 existence. Probably the temperature is to some extent 

 bound up with the size of the animal. Thus the smaller 

 this is the more would variations in the external tempera- 

 ture tend to produce variations in body temperature, and 

 the more sensitive would the heat regulating mechanism 

 need to be in order to keep the body temperature constant. 

 Thus if the above data on birds be re-examined, it will be 



