158 THE DESCENT OF MAN. 



of Kolliker and Langer, that In the embryo the mammary glands 

 can be distinctly traced before the nipples are in the least 

 visible; and the development of successive parts in the indi- 

 vidual generally represents and accords with the development of 

 successive beings in the same line of descent. The Marsupials 

 differ from the Monotremata by possessing nipples; so that 

 probably these organs were first acquired by the Marsupials, 

 after they had diverged from, and risen above, the Monotremata, 

 and were then transmitted to the placental mammals.^" No one 

 will suppose that the Marsupials still remained androgynous after 

 they had approximately acquired their present structure. How 

 then are we to account for male mammals possessing mammae? 

 It is possible that they were first developed in the females and 

 then transferred to the males; but from what follows this is 

 hardly probable. 



It may be suggested, as another view, that long after the 

 progenitors of the whole mammalian class had ceased to be 

 androgynous, both sexes yielded milk, and thus nourished their 

 young; and in the case of the Marsupials, that both sexes carried 

 their young in marsupial sacks. This will not appear altogether 

 improbable, if we reflect that the males of existing syngnathous 

 fishes receive the eggs of the females in their abdominal pouches, 

 hatch them, and afterwards, as some believe, nourish the 

 young;" — that certain other male fishes hatch the eggs within 

 their mouths or branchial cavities; — that certain male toads 

 take the chaplets of eggs from the females, and wind, them round 

 their own thighs, keeping them there until the tadpoles are 

 born; — that certain male birds undertake the whole duty of 

 incubation, and that male pigeons, as well as the females, feed 

 their nestlings with a secretion from their crops. But the above 

 suggestion first occurred to me from the mammary glands of 

 male mammals being so much more perfectly developed than 

 the rudiments of the other accessory reproductive parts, which 

 are found in the one sex though proper to the other. The mam- 



=0 Prof. Gag-enbaur has shown ('Jenaische Zeitschrift,' Bd. vii. p. 212) 

 that two distinct types of nipples prevail throughout the several mam- 

 malian orders, but that it is quite intelligible how both could have 

 been derived from the nipples of the Marsupials, and the latter from 

 those of the Monotremata. See, also, a memoir by Dr. Huss, on 

 the mammary glands ibid. B. viii. p. 176. 



™ Mr. Lookwood believes (as quoted in 'Quart. Journal of Science,' 

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

 Hippocampus, that the walls of the abdominal pouch of the male in 

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

 their mouths, see a very interesting paper by Prof. Wyman, in 'Proc. 

 Boston Soc. of Nat. Hist.' Sept. 15, 1857: also Prof. Turner, in 'Journal 

 of Anat. and Phys.' Nov. 1, 1866, p. 7S. Dr. Gunther has likewise de- 

 scribed similar cases. 



