MAMMALIAN SEXUAL CHARACTERS 139 



absorbed from the pouch, as I have suggested in order 

 to explain the persistence of the corpora lutea during 

 lactation, then the secretion of milk after parturition 

 ought not to take place. But in this case the sucking 

 stimulus has been applied to the glands after a very 

 short gestation, while the hormone from the foetus 

 is being absorbed in the pouch, and therefore the 

 hereditary correlation between secretion and absence 

 of foetal hormone may be assumed to have been lost 

 in the course of evolution. 



We have next to consider the question of the 

 evolution of the corpora lutea. If these bodies are 

 formed only in Mammals which have uterine gesta- 

 tion, and not in Prototheria, they cannot be the only 

 essential source of the hormone which stimulates the 

 development of the milk glands, since the latter 

 develop in Prototheria. Again it is difficult, it 

 might be said impossible, to believe that an acci- 

 dental mutation gave rise to corpora lutea the 

 secretion of which caused uterine gestation and 

 ultimately the formation of the placenta. It seems 

 more probable that the retention of the originally 

 yolked ova within the oviduct, however this retention 

 arose, was the essential cause of the formation of the 

 placenta and all the changes which the uterus under- 

 goes in gestation. The absorption of nutriment 

 from the walls of the uterus, and the chemical and 

 mechanical stimulation of those walls, might well be 

 the cause of the diversion of nutrition from the 

 ovary, leading gradually to the decline of the process 

 of secretion of yolk in the ova. 



The conceptions and the mode of reasoning of the 

 physiologist are very different from those of the 

 evolutionist. The former concludes from certain 



