DAWSON, W. R. 



of temperature regulation and with the requirements of development 

 until the young are well on their way to mature size, and because it 

 requires elaborate patterns of parental care following the hatching 

 of the eggs. The evolution of small size in birds appears to have been 

 contingent upon the development of the altricial condition. 



While the details of the evolution of temperature regulation in 

 birds are obscure, they are less complex than those of mammals. 

 Birds appear tobe a monophyletic group in which there was probably 

 but one development of homeothermy. Mammals on the other hand 

 are almost certainly polyphyletic (Olson, 1959; Simpson, 19 59), and 

 homeothermy could have developed independently in each of several 

 evolutionary lines as they traversed the boundary between mammal- 

 like reptiles and mammals. Theprototherians (monotremes) and the 

 therians (marsupials and placentals), the surviving groups of mam- 

 mals, appear to have been separately derived from the mammal- like 

 reptiles (Simpson, 1959), complicating considerations of the evolu- 

 tion of homeothermy. 



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