MAMMALIAN SEXUAL CHARACTERS 137 



caused both growth and secretion, for at first there 

 was nothing but sebaceous or sweat glands, and 

 although a mutation might be supposed to have 

 produced larger glands, no mutation could explain 

 the influence of hormones on the growth and function 

 of such glands. Then heredity of the effect of stimulus 

 took place to some slight degree, and this would occur, 

 according to my theory, only in the presence of the 

 hormone from the ovary in the same condition as that 

 in which the modification was first caused. This would 

 be of course after ovulation, and after hatching of 

 the eggs. In the next stage, if we adopt the modern 

 view that Marsupials are descended from Placental 

 Mammals, the eggs would be retained for increasing 

 periods in the uteri, and would be born in a well- 

 developed condition, since lactation would demand 

 active sucking effort on the part of the young. The 

 early Placentalia would inherit from the Monotreme- 

 Hke ancestors the development of the milk glands 

 after ovulation, although no sucking was taking 

 place while the young were inside the uterus. It 

 seems probable that the relation between parturi- 

 tion and actual milk secretion originated with the 

 sucking stimulus of the young after birth. 



There is good evidence that the secretion of milk 

 may continue almost indefinitely under the stimulus 

 of sucking or milking. Neither menstruation nor 

 gestation put an end to it. Cows may continue to 

 give milk until the next parturition, and if castrated 

 during lactation will continue to yield milk for years. 

 Women also may continue to produce milk as long 

 as the child is allowed to suck, and this has been in 

 some cases two or three years or even more. More- 

 over, lactation may be induced by the repeated act 



