The narrowing of the long central tail feathers of the motmot is
attributed to the inherited effects of habitual mutilation (_Descent of
Man_, pp. 384, 603). But in the specimens at South Kensington[61] the
narrowness extends upwards much beyond the habitually denuded part, and
the broadened end is the broadest part of the whole feather. If the
inherited effect of an inch or two of denudation extends from three to
six inches upwards, why has it not also extended two inches downwards so
as to narrow the broadened end? The narrowness seems to be a mainly
relative or negative effect produced by the broadening out of a long
tapering feather at its end under the influence of sexual selection.
Several other birds have similarly narrowed or spoon-shaped feathers and
do not bite them. Is it not more feasible to suppose that this
attractive peculiarity first suggested its artificial intensification,
than to suppose that the bird began nibbling without any definite cause?
Sexual selection would then encourage the habit. Anyhow, it is as
impossible to show that the mutilation preceded the narrowing as it is
to show that tonsure preceded baldness.