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 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 are equal to the
effects of disuse, or even ten or a hundred times greater, they will
even then not equal, in each generation, the amount of the fortuitous
variations of the same part. If it be urged that the effects of use
would modify all the individuals of a species, while the fortuitous
variations to the amount named only apply to a portion of them, it may
be replied, that that portion is sufficiently large to afford ample
materials for selection, since it often equals the numbers that can
annually survive; while the recurrence in each successive generation of
a like amount of variation would render possible such a rapid adjustment
to new conditions that the effects of use or disuse would be as nothing
in