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Show 210' THE DESCENT O'F MAN. PART r .. mind that long after the progenitors of the whole mammalian class had ceased to be androgynous, both sexes. might have yielded milk and thus nourished their young ; and in the case of the lVIarsupials, that both sexes might have carried their young in marsupial sacks. This will not appear utterly incredible, if we reflect that the males of syngnathous :fishes receive the eggs of the females in their abdominal pouches, hatch them, and afterwards, as some believe, nourish the young ; 26-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 suspicion first occurred to me from the mammary glands in male mammals being developed so much more perfectly than the rudiments of those other accessory reproductive 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 rudimentary; they are simply not fully developed and not functionally active. They are sympathetically affected under the influence of certain diseases, like the same organs in the female. At birth they often secrete a few drops of milk ; and they have 26 Mr. Lockwood believes (as quoted in' Quart. Journal of Science,' April, 1868, p. 269), from what he bas 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. GUnther has likewise described similar cases. CUAP. VI. AFFINITIES AND GENEALOGY. 211 been known occasionally in man and other mammals to become well developed, and to yield a fair supply of milk. Now if we suppose that during a former prolonged period male mammals aided the females in nursing their offspring, and that afterwards from some cause, as from a smaller number of young being producAd, the males ceased giving this aid, disuse of the ?rga~s during maturity would lead to their becoming mactive; and from two well-known principles of inheritance this state of inactivity would probably be transmitted to the males at the corresponding age of maturity. But at all earlier ages these organs would be left unaffected, so that they would be equally well developed in the young of both sexes. Oonclusion.-The best definition of advancement or progress in the organic scale ever given, is that by Von Baer; and this rests on the amount of differentiation and specialisation of the several parts of the same being, when arrived, as I should be inclined to add, at maturity. Now as organisms have become slowly adapted by means of natural selection for diversified lines of life, their parts will have become, fro~ the advantage gained by the division of physiologiCal labour, more and more differentiated and specialised for various functions. The same part appears often to have been modified first for one purpose, and then long afterwards for some other and quite distinct · purpose; and thus all the parts are rendered more and more complex. But each organism will still retain the general type of structure of the progenitor from which it was aboriginally derived. In accordance with this view it seems, if we turn to geological evidence, that ' organisation on the whole has advanced throughout tpe world by slow and interrupted steps. In the great p 2 |
