136 MAMMALIAN SEXUAL CHARACTERS 



functional periodicity of the milk glands is proved 

 by their presence in the Monotremes, which are 

 oviparous. It is evident from the conditions in 

 these mammals that both hair and milk glands 

 were evolved before the placenta. 



It may also be pointed out here that, according 

 to the evidence of Steinach, in the milk glands at 

 least among somatic sexual characters there is no 

 difference between the male and female in the 

 heredity of the organs. The zygote therefore, 

 whether the sex of it is determined as male or female, 

 has the same factor for the development of milk 

 glands. On the chromosome theory as formulated 

 by Morgan this factor must be in the somatic chromo- 

 somes and not in the sex-chromosomes, and must be 

 present in every zygote. All the cells of the body, 

 assuming that somatic segregation does not occur, 

 must possess the same chromosomes as the zygote 

 from which it developed, and whether the sex 

 chromosomes are XX or XF or X, there must be 

 at any rate one chromosome bearing the factor for 

 milk glands. The functional development of these 

 depends normally, according to the evidence hitherto 

 discovered, on the presence or absence of hormones 

 from the ovary or from the uterus. 



If we attribute, as in my opinion we must, the 

 primary origin of the milk glands in evolution to 

 the mechanical stimulus of sucking, we may attempt 

 to reconstruct the stages of the evolution of the 

 present relation of the glands to the other organs 

 and processes of reproduction. In the earliest stage 

 represented by the Monotremata or Prototheria, 

 there was no intra-uterine development. We must 

 suppose that in the beginning the sucking stimulus 



