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Show 416 DARWINISM CHAP. birds whose powers of flight were n.lready somewhat rc~uced, and to whom, there being no enemies to e cape from, th01r usc wa,s only a source of danger. vVe may thus, perhaps, account for the fact that many of these birds retain small but useless wincrs with which they never fly ; for, the wings having been redt~ced to this functionless condition, no power could reduce them further except correlation of growth or economy of nutrition causes which only rarely come into play. The ~omplete loss of eyes in some cave animals may, perhaps, be explained in a somewhat similar way. 'Yhcnever owincr to the total darkness, they became usclc . , they micrht also l:\ecome injurious, on account of their delicacy of or;anisation and liability to accidents and disease; in which case natural selection would begin to act to reduce, and finally abort them ; and this explains why, in some cases, the rnd i mentary eye remains, although completely covered by a pro tective outer skin. Whales, like moas and cassowaries, carry us back to a remote past, of whose conditions we know too little for safe speculation. We are quite ignorant of the ancestral forms of either of these groups, and are therefore without, the materials needful for determining the steps by which th<' chancre took place, or the causes which brought it about. 1 0~ a review of the various examples that ha,ve been giv n by Mr. Darwin and others of organs that have been rcclnccd or aborted, there seems too much diversity in the resul ts for all to be due to so direct and uniform a cause as the individunJ effects of disuse accumulated by heredity. For if that were the only or chief efficient cause, and a cause capable of producing a decided effect during the comparatively short period 1 The idea of the non-heredity of acquired variations was suggested hy the. ummary of Professor vVeismann's views, in Nat1t1·e, referred to latPr 011. But since this chapter was written I have, through the kindness of Mr. K H. Poulton, seen some of tbe proofs of the forthcoming translatio~1 of Woisn.Iann's E ·says on Heredity, in which he sets forth an explanatwn very smular to that here given. On the diffi cult question of the aln_1ost entire <lisappearance of organs, as in the limbs of snakes and of some lizards, l1e adduc~s "a certain form of correlation, which Roux calls 'the struggle of tl1e parts m the organism,' " as playing an important par~. Atrophy following eli sus~ is nearly always attended by the corrcspondmg mcreas~ of other organs: l>lmd animals possess more developed organs of touch, heanng, and smell ; the loss of power in the wings is accompanied hy increa.sed strength of ~h~ legR, etc. Now as these latter characters, being u ·eful, will be selected, 1t IS easy to understand that a congenital increase of these will be accompanied by a cor- XIV FUNDAMENTAL PROBLEMS 417 of the existence of animals in a state of dome tication, we should expect to find that, in wild species, all unused parts or or(J'ans had been reduced to the smallest rudiments, or had wholly disappeared. Instead of this we find various grades of reduction, indicating the probable result of several di. tinct C<tu e , sometimes acting separately, sometimes in combination, such as those we have already pointed out. And if we find no positive evidence of clisuse, acting by it.s direct c:frcct on the individual, being transmitted to the ofi'spring, still less can we find such evidence in the case of the ~tse of organs. For here the very fact of use, in a wild state, implies 1dility, and utility is the constant subject for the action of natural selection; while amo11g domestic animals those parts which arc exceptionally used arc so used in the :ervice of man, and have thus become the subjects of artificial selection. Thus "the great and inherited development of the udders in cows and goats," quoted by Spencer from Darwin, really affords no proof of inheritance of the increase due to use, because, from the earliest period of the domestication of these animals, abundant milk-production has been highly esteemed, and has thus been the subject of selection ; while there are no Gases among wild animals that may not be better explained hy variation and natural selection. Dijjiculty as to Co-adaptation of Parts by Variation ancl Selection. Mr. Spencer again brings forward this difficulty, as he did in his PTinciples of Biolog.1J twenty-five years ago, and urcrcs that all the adjustments of bones, muscles, blood-vessels, - and nerves which would be required during, for example, the development of the neck and fore-limbs of the giraffe, could responding congenital diminution of the unu. eel organ ; and in cases where the means of nutrition are defici ent, every diminution of these useless parts will be a gain to the whole organi m, and thus their complete eli apvearanec will, in some cases, be brought about directly by natural selection. 'fhis corresponds with what we know of these rudimentary org.ans. . 1t must, however, be pointed out that the non-heredity of acqmred characters was maintained by Mr. Francis Galton more than twelve years ago, ?n theoretical consicleration ·almost identical with those urged by Profes. or Weismann ; while the insufficiency of the evillence for their hered~tary transmission was shown by similar arguments to those used above ancl m the work of Professor Weis~ann already referred to ( ee "A 'l'heory of Heredity," in Journ. AntMop. Instit., vol. v. pp. 343-345). 2 E |
