Marked Effects of Use and Disuse on the Individual
These are, to some extent, an argument against the cumulative inheritance of such effects. When a nerve atrophies from disuse, or a duct shrivels, or bone is absorbed, or a muscle becomes small or flabby, it proves, so far, that the average effect of use through enormous ages is not transmitted. When the fibula of a dog’s leg thickens by 400 per cent. to a size “equal to or greater than” that of the removed tibia which previously did the work,69 it shows that in spite of disuse for countless generations, the “almost filiform” bone has retained a potentiality of development which is fully equal to that possessed by the larger one which has been constantly used. When, after being reared on the ailanthus, the caterpillars of the Bombyx hesperus die of hunger rather than return to their natural food, the inherited effect of ancestral habit does not seem to be particularly strong. Neither is there any strongly-inherited effect of long-continued ancestral wildness in many animals which are easily tamed.