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Show 434 DARWINISM CHAP. exclusive n.nd unmodified action n.re nowhere to be found in nn.ture. It mn.y be allowed to rank as one of ~ho~c "laws of crrowth" of which so many have now been mdiCated, ancl ~hich 'were always recognised by Darwin as underlying all variation · but unless we bear in mind that its action must always b~ subordinated to natural selection, and that it i · continually checked, or diverted, or even reversed by the necessity of adaptation to the environment, we shall be liable to fall into such glaring errors as the imputing to " ebbing vitality" alone such a, widespread phenomenon as the occurrence of , pines and thorns, while ignoring altogether the influence of the organic environment in their production.1 The sketch now given of the chief attempts that have been made to prove that either the direct action of the environment or certain fundamental laws of variation are independent causes of modificn.tion of species, shows us that their authors have, in every case, failed to establish their contention. Any direct action of the environment, or any characters acquired by use or disuse, can have no effect whatever upon the race unless they are inherited; and that they are inherited in any case, 1 The general arguments and objections here set forth will apply with eqnal force to Profe~so r G. H enslow's th eory of the origin of the v ari o u ~ forms and structures of flow ers as due to ''the responsive actions of the protoplasm in consequence of tlie irritations set up by the weights, pressures, thrusts, tensions, etc., of the insect visitors" (The Origin of Ji'toml Structu1·es tht'OI'(fh Insect and othe1· A rtencies, p. 340). On the assumption that acquire<l characters are inh erited, such irritations may have had something to do with the initiation of variations and with the production of certain deta ils of structure but th ey are clearly incompetent to have brought about tho more im~1ortant structural au<l functional moclifi.cations of fl owers. Such are, the various adju s tment~ of length and position of th e stamens to l>riug the pollen to the in sect an<l from the i11Rect to the stigma ; the various motions of tamens and styles at the right time and the right di rcrtiou ; the physiological a(ljustment: bringing about fertility or sterility ~ n helerostyled plants ; the trap~, springs, and complex movements of van om; parts of orchids · ancl innumerable other remarkable phenomena. For the' explanation of the. e we have no resource hut variation allll sl'lection, to the effects of wlli ch, acting alternately with regre.- ~ ion or degradation as nbove explaine<l (p. 328) must be imputed the development of t he ronutless floral structures we now behold. Even the primitive flowers, wl1osc initiation may, perhaps, have been caused, or rendered possible, by the irritation .·et up by insects' visits, must, from th eit~ ~cry origin, have l1c.en modified, in accordance with the supreme law of utLhty, by means of vanation and survival of 'the fittest. X IV FUNDAMENTAL PROBLEMS 435 except when they directly affect the reproductive cells, has not been proved. On the other hand, as we shall presently show, there is much rea. on for believing that such acquired characters are in their nature non-heritable. Va1'itttion and Selection Ove1power the Effects of Use and Disuse. But there is another objection to this theory arising from the very nature of the effects produced. In each generation the effects of use or disuse, or of effort, will certainly be very small, while of this small effect it is not maintained that the whole will be ::Llways inherited by the next generation. How small the effect is we have no means of determining, except in the case of disuse, which Mr. Darwin investigated carefully. He found that in twelve fancy breeds of pigeons, which are often kept in aviaries, or if free fly but little, the sternum had been reduced by about one-seventh or one-eighth of its entire length, and that of the scapula about one-ninth. In domestic ducks the weight of the wing-bones in proportion to that of the whole skeleton had decreased about one-tenth. In domestic rabbits the bones of the legs were found to have increased in weight in due proportion to the increased weight of the body, but those of the hind legs were rather less in proportion to those of the fore legs than in the wild animal, a difference which may be imputed to their being less used in rapid motion. The pigeons, therefore, afford the greatest amount of reduction by disuse-one-seventh of the length of the sternum. But the pigeon has certainly been domesticated four or five thousand years; and if the reduction of the wings by disuse has only been going on for the last thousand years, the amount of reduction in each generation would be absolutely imperceptible, and quite within the limits of the reduction due to the absence of selection, as already explained. But, as we have seen in Chapter III, the fortuitous variation of every part or organ usually amounts to one-tenth, and often to onesixth of the average dimensions-that is, the fortuitous variation in one generation among a limited number of the individuals of a species . is as great as the cumulative effects of disuse in a thousand generations ! If we assume that the effects of use or of effort in the individual arc equal to the .effects of disuse, or even ten or a hundred times greater, they |