XIV 
FUNDAMENTAL FROBLEMS 
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 reason for believing that such acquired 
characters are in their nature non-heritable. 
Variation and Selection Overpower 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 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. 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 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 
