FITNESS OF TEMPERATURE CONSTANCY 91 



within that tolerated by protoplasm, but the temperature 

 of the air may vary much beyond such limits. Homoio- 

 thermism has been attained by the cooperation of nerv- 

 ous, metabolic, and vascular activities. Through it opti- 

 mum conditions are ensured for the processes that must 

 go on for the maintenance of the individual. 



Homoiothermism apparently had its origin at the 

 time when the great dinosaurs were becoming extinct 

 and the newer but wiser mammals were rising. Variable 

 climates were associated with, and perhaps stimulated, 

 the development of a more precisely regulated internal 

 temperature. The aridity of the earth at the end of the 

 Paleozoic had its influence upon the terrestrial tj^es, 

 especially upon the more progressive of them. The strife 

 between various types for food and water likely led to a 

 more intensified living — a higher rate of metabolism — 

 which implied a higher and more stable temperature. 

 Then with increasing cold a premium was put upon such 

 animals as could maintain their activity beyond the limits 

 of the shortening summers. This was accomplished by 

 the development of a mechanism whereby a relatively 

 constant temperature could be maintained within the or- 

 ganism regardless of external conditions. 



Thus it appears that the evolution of the bird and 

 mammal, particularly the latter, was permitted by the 

 concurrence of two factors — aridity and cold. Homoio- 

 thermism was not suddenly attained. It was a relatively 

 slow process, just as was the emergence from aquatio 

 habitats. Indications of imperfectly adjusted mechan- 

 isms are still to be seen in existing animals. An investi- 

 gation of a comparative nature was conducted by Martin 

 (1903), who states that Echidna is the lowest in the scale 

 of warm-blooded animals. The attempt of this animal 

 to attain homoiotherism fails to the extent of 10 deg. C. 



