138 MAMMALIAN SEXUAL CHARACTERS 



of sucking without any gestation. This has happened 

 in mares, virgin bitches, mules, virgin women, and in 

 one woman lactation was continued uninterruptedly 

 for forty-seven years, to her eighty-first year, long 

 after the ovary had ceased to be functional. Lacta- 

 tion has also been induced in male animals, e.g. in a 

 bull, a male goat, male sheep, and in men.^ We may 

 conclude, therefore, that the secretion of milk 

 normally begins by heredity after parturition, and 

 this, in accordance with what we have learned 

 about hormones in connexion with the reproductive 

 system, is probably the consequence of the with- 

 drawal of the hormone absorbed from the foetus. I 

 do not think it is necessary to suppose, as do Lane- 

 Claypon and Starling, that the hormone physio- 

 logically inhibits the dissimilative process and aug- 

 ments the assimilative, and that the withdrawal of 

 the hormone at parturition therefore causes the 

 dissimilative process, i.e. secretion of milk. My 

 conclusion is that the process of secretion set up by 

 the mechanical stimulus of sucking is inherited as it 

 was acquired, so that it only begins to take place in 

 the individual in the absence of the hormone from 

 the foetus, which was absent when the process was 

 acquired. The growth of the gland during gestation 

 would then be due to the postponement of the process 

 of secretion in consequence of the presence of the 

 foetal hormone, and in this way this hormone has be- 

 come in the course of evolution at once the stimulus 

 to growth and the cause of the inhibition of secretion. 

 This interpretation does not, however, agree with 

 the case of Dasyurus. If the foetal hormone is 



1 Knott, 'Abnormal Lactation,' American Medicine, vol. ii. (new 

 series), 1907. 



