14 HOMOIOTHERMISM 



earth. Perhaps the mammals ate their eggs, perhaps 

 they were unable to compete with the mammals on ac- 

 count of their lack of ability to endure cold, perhaps they 

 passed away partly on account of their inferior loco- 

 motor mechanisms and brains, and because of wasteful 

 methods of reproduction. At any rate, they were exter- 

 minated or greatly reduced in numbers and reptiles to- 

 day hold a rather unimportant place. Eepresentatives 

 of very primitive Cretaceous mammals have recently 

 been discovered in Mongolia (Gregory, 1927). These 

 early insectivore-carnivore ancestors of modern mam- 

 mals were already far from the beginnings of their 

 careers; already differentiated into marsupials and 

 placentals. 



Among primitive, living mammals, the monotremes 

 are probably not distinct from other Theria but are ^ * the 

 much specialized descendants of very early mammals'' 

 (Watson, 1916). They probably represent primitive an- 

 cestral conditions in their egg-laying habits, the arrange- 

 ment of their genitalia in relation to a cloaca, and their 

 somewhat variable body temperature. Among many 

 types of placentals there are rather elaborate mechan- 

 isms for maintaining constant temperature, and these 

 are not so well developed in more primitive mammals 

 (Martin, 1903). Marsupials depend largely on evapora- 

 tion through the lung membranes for the lowering of 

 body temperature, and are the lowest animals to show 

 heat polypnea. A rabbit also uses polypnea as a means 

 of temperature regulation and depends largely on vaso- 

 motor regulation for the dissipation of heat ; carnivores 

 have somewhat more varied and effective mechanisms. 

 Among homoiothermic animals there are various means 

 and degrees of efficiency in the regulation of temperature. 



