CHAPTER V 



MAMMALIAN SEXUAL CHARACTERS. EVIDENCE 

 OPPOSED TO THE HORMONE THEORY 



PERHAPS the most remarkable of all somatic 

 sexual characters are those which are almost uni- 

 versal in the whole class of Mammalia, the mammary 

 glands in the female, the scrotum in the male. We 

 have considered the evidence concerning the re- 

 lation of the development and functional action of 

 the milk glands to hormones arising in the ovary 

 or uterus, now we have to consider the origin of the 

 glands and of their peculiar physiology in evolution. 

 The obvious explanation from the Lamarckian point 

 of view, and in my opinion the true one, is that they 

 owed their origin at the beginning to the same 

 stimulation which is applied to them now in every 

 female mammal that bears young. There is, as we 

 have seen, a difficulty in explaining how the occurrence 

 of parturition causes the secretion of milk to begin, 

 but it is certain that the secretion soon stops if the 

 milk is not drawn from the glands by the sucking 

 action of the offspring, or the artificial imitation of 

 that action. A cow that is not milked or milked 

 incompletely ceases to give milk. When the 

 stimulus ceases, lactation ceases. The pressure 

 of the secretion in the alveoli causes the cells to 

 cease to secrete, much in the same way that pressure 

 in the ureters injures the secretory action of the renal 



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