62 



The Descent of Man. 



Part I 



proper to each sex, are found in a rudimentary condition in the 

 opposite sex, may be explained by such organs having been 

 gradually acquired by the one sex, and then transmitted in a 

 more or less imperfect state to the other. When we treat o f 

 sexual selection, we shall meet with innumerable instances cl 

 this form of transmission, — as in the case of the spurs, rjlumes, 

 and brilliant colours, acquired for battle or ornament by male 

 birds, and inherited by the females in an imperfect or rudimentary 

 condition. 



The possession by male mammals of functionally imperfect 

 mammary organs is, in some respects, especially curious. The 

 Monotremata have the proper milk-secreting glands with orifices, 

 but no nipples ; and as these animals stand at the very base of 

 the mammalian series, it is probable that the progenitors of 

 the class also had milk-secreting glands, but no nipples. This 

 conclusion is supported by what is known of their manner of 

 development ; for Professor Turner informs me, on the authority 

 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. 29 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. 



where it is either normal and sym- 

 metrical, or abnormal and uni- 

 lateral. Dr. Zouteveen has given 

 me references on this subject, more 

 especially to a paper by Prof. Hal- 

 bertsma, in the ' Transact, of the 

 Dutch Acad, of Sciences,' vol. xvi. 

 Dr. Giinther doubts the fact, but 

 it has now been recorded by too 

 many good observers to be any 

 longer disputed. Dr. M. Lessona 

 writes to me, that he has veri- 

 ned the observations iraJe by 

 Cavolini on Serranus. Prof. Erco- 

 Inni has recently shewn (/ Accad. 



delle Scienze,' Bologna, Dec. 28, 

 1871) that eels are androgynous. 



29 Prof. Gegenbaur has shewn 

 ('Jenaische Zeitschrift,' Bd. vii. p. 

 212) that 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 Monotre- 

 mata. See, also, a memoir by Dr. 

 Max Huss, on the mammary giaii^s, 

 .bid. B. viii. p. 176. 



