AFFINITIES AND GENEALOGY. 185 



placenta! mammals.* No one will suppose that the Mar- 

 supials still remained androgynous after they had approx- 

 imately 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 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 after- 

 ward, 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 incuba- 

 tion, 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 repro- 

 ductive parts, which are found in the one sex though proper 

 to the other. The mammary glands and nipples, as they 

 exist in male mammals, can indeed hardly be called rudiment- 

 ary; they are merely not fully developed and not functionally 



*Prof. Gegenbaur lias shown (" Jenaische Zeitsclirift," Bd. vii, p. 

 212) tliat two distinct types of nipples prevail throughout the several 

 mammalian 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 Monotreinata. See, also, a memoir by Dr. Max 

 Huss, on the mammary glands, ibid., B. viii, p. 176. 



f Mr. Lockwood 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. 

 78. Dr. Giinther has likewise described similar cases. 



