184 THE DESCENT OF MAN. 



androgynous, after it had acquired the chief distinctions of 

 its class, and, therefore, after it had diverged from the 

 lower classes of the vertebrate kingdom? This seems very 

 improbable, for we have to look to fishes, the lowest of all 

 the classes, to find any still existent androgynous forms.* 

 That various accessory parts, 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 of sexual 

 selection we shall meet with innumerable instances of this 

 form of transmission as in the case of the spurs, plumes, 

 and brilliant colors, 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 imper- 

 fect 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 proba- 

 ble 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 Prof. 

 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 individual 

 generally represents and accords with the development of 

 successive beings in the same line of descent. The Mar- 

 supials-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 



* Hermapliroditism has been observed in several species of Ser- 

 ranus, as well as in some other fishes, where it is either normal and 

 symmetrical, or abnormal and unilateral. Dr. Zouteveen has given 

 me references on this subject, more especially to a paper by Prof. 

 Halbertsma, 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 verified' the observations made by Cavolini 

 on Serranus. Prof. Ercolani has recently shown (" Accad. delle 

 Scieuze," Bologna, Dec. 28, 1871) that eels are androgynous. 



