XIV FUXDAMEXTAL TROBLEMS 435 



excej^t when they directly affect the reproductive cells, has 

 not been proved. On the other hand, as we shall presently 

 show, there is much reason for belie\ang that such acquired 

 characters are in their nature non-heritable. 



Variation and Selection Overpoicer 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, tv^II certainly be very 

 small, while of this small effect it is not maintained that the 

 whole will be always inherited by the next generation. How 

 small the effect is we have no means of determining, except 

 in the case of disuse, which Mr. Dar-sdn 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 ths 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 gi^eatest 

 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 ^ings 

 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 HI, the fortuitous variation of every 

 part or organ usually amounts to one-tenth, and often to one- 

 sixth of the average dimensions — that is, the fortuitous varia- 

 tion in one generation among a limited number of the in- 

 dividuals 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 are equal to the 

 effects of disuse, or even ten or a hundred times greater, they 



