40 A HISTORY OF BIRDS 



fourth chamber the high temperature and phenomenal activity 

 of the birds came into being ; but for reasons for which no 

 explanation is yet forthcoming, the reptilian character of the 

 blood corpuscles has been retained. That is to say, the red 

 corpuscles still retain the nucleus in common with all the 

 lower vertebrates, while in the warm-blooded Mammalia — also 

 of reptilian descent — these nuclei have been lost. 



But whether these pro-aves are to be regarded as descended, 

 in common with the reptiles, as a collateral branch of the same 

 stock ; or whether they sprang from some primitive but true 

 reptile, is a point too subtle to be determined. 



