OCR Text |
Show 208 THE DESCENT OF MAN. PART l\. difficulty. In the mammalian class the males poss:ss in their vesicul:oe prostraticre rudiments of a uterus w1th the adjacent passage; they bear also rudiments of mammre, and some male marsupials have rudiments of a marsupial sack24 Other analogous facts could be added. Are we, then, to suppose that some extremely ancient mammal possessed organs proper to both sexes.7 that is, continued androgynous after it had acquired the chief distinctions of its proper class, and therefore after it had diverged from the lower classes of the vertebrate l~ingdom ? This seems improbable in the highest degree; for had this been the case, we might have expected that some few members of the two lower classes, namely fishes 25 and amphibians, "·ould still have remained androgynous. \Ve must, on the contrary, believe that when the five vertebrate classes diverged from their common progenitor the sexes l1ad already become separated. To account, however, for male mammals posse~sing rudiments of the accessory female organs, and for female mammals possessing rudiments of tlle masculine organs, we need not suppose that their early progenitors were still androgynous after they bad assumed their chief mammalian characters. It is quite possible that as the one sex gradually acquired the accessory organs proper to it, some of the successive steps or modifications were transmitted to the opposite sex. 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, 24 'fbe male 'l'hylacinus offers the best instance. Owen, 'Anatomy of Vertebrates,' vol. iii. p. 771. 2~ Serranus is well known often to be in an hermaphrodite condition ; but Dr. Gunther informs me that he is convinced that tuis is not its normal state. Descent from an ancient androgynous prototype would, however, naturally favour and explain, to a certain extent, the recurrence of this condition in these fishes. Cn.A.P. VI. AFFINITIES AND GENEALOGY. 209 and brilliant colours, acquired by male birds for battle or ornament, and transferred to the females in an imperfect or rudimentary condition. The possession by male mammals of functionally imperfect mammary organs is, in some respects, especially curious. 'fhe Monotremata have the proper milksecreting glands with orifices, but no nipples; and as these animals stand at the Yery base of the mam-· malian series, it is probable that the progenitors of the class possessed, in like manner, the 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 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 Marsupials after they had diverged from, and risen above, the Monotremata, and were then transmitted to the placental mammals. No one will suppose that after the Marsupials had approximately acquired their present structure, and therefore at a rather late period in the development of the mammalian series, any of its members still remained androgynous. vVe seem, therefore, compelled to recur to the foregoing view, and to conclude that the nipples were first developed in the females of some very early marsupial form, and were then, in accordance with a common law of inheritance, transferred in a functionally imperfect condition to the males. Nevertheless a suspicion has sometimes crossed my vor •. r. p |