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.