Chap. VI.] AFFINITIES AND GENEALOGY. oqi 



manner of development ; for Professor Turner informs me, 

 on tlie authority of Kolliker and Lauger, that in the em- 

 bryo the mammary glands can be distinctly traced before 

 the nipples are in the least visible ; and it should be borne 

 in mind that the development of successive parts in the 

 individual generally seems to represent and accord with 

 the development of successive beings in the same line of 

 descent. The Marsupials differ from the Monotremata by 

 possessing nipples ; so that these organs were probably first 

 acquired by the Marsitpials after they had diverged from, 

 and risen above, the Monotremata, and were then trans- 

 mitted to the placental mammals. No one will suppose 

 that after the Marsujiials had approximately acquired 

 their present structure, and therefore at a rather late pe- 

 riod in the development of the mammalian series, any of 

 its members still remained androgynous. We seem, there- 

 fore, compelled to recur to the foregoing view, and to con- 

 clude that the nipples were first developed in the females 

 of some very early marsupial form, and were then, in ac- 

 cordance with a common law of inheritance, transferred 

 in a functionally imperfect condition to the males. 



Nevertheless, a suspicion has sometimes crossed my 

 mind that long after the progenitors of the whole mam- 

 malian class had ceased to be androgynous, both sexes 

 might have yielded milk and thus nourished their young ; 

 and, in the case of the Marsupials, that both sexes might 

 have carried their young in marsupial sacks. This will 

 not appear utterly incredible, if we reflect that the males 

 of syngnathous fishes receive the eggs of the females in 

 their abdominal pouches, hatch them, and afterward, as 

 some believe, nourish the young ; "* that certain other 



-'' Mr. LocliTvood believes (as quoted in ' Quart. Journal of Science,' 

 April, 1868, p. 269), from what he has observed of the development of 

 Hipp'ocampus, that the vralls of the abdominal pouch of the male in some 

 way afford nourishment. On male fishes hatching the ova in their 



