the brain] in the whole
organization is subject to the law of decrease in size from disuse"
(p. 129). He remarks that in birds of the oceanic islands "not
persecuted by any enemies, the reduction of their wings has
probably been caused by gradual disuse." After comparing one of
these, the water-hen of Tristan d'Acunha, with the European
water-hen, and showing that all the bones concerned in flight are
smaller, he adds--"Hence in the skeleton of this natural species
nearly the same changes have occurred, only carried a little
further, as with our domestic ducks, and in this latter case I
presume no one will dispute that they have resulted from the
lessened use of the wings and the increased use of the legs" (pp.
286-7). "As with other long-domesticated animals, the instincts of
the silk-moth have suffered. The caterpillars, when placed on a
mulberry-tree, often commit the strange mistake of devouring the
base of the leaf on which they are feeding, and consequently fall
down; but they are capable, according to M. Robinet, of again
crawling up the trunk. Even this capacity sometimes fails, for M.
Martins placed some caterpillars on a tree, and those which fell
were not able to remount and perished of hunger; they were even
incapable of passing from leaf to leaf" (p. 304).

Here are some instances of like meaning from volume ii.

"In many cases there is reason to believe that the lessened use of
various organs has affected the corresponding parts in the
offspring. But there is no good evidence that this ever follows in
the course of a single generation.... Our domestic fowls, ducks,
and geese have almost lost, not only in the individual but in the
race, their power of flight; for we do not see a chicken, when
frightened, take flight like a young pheasant.... With domestic
pigeons, the length of the sternum, the prominence of its crest,
the length of the scapulae and furcula, the length of the wings as
measured from tip to tip of the radius, are all reduced relatively
to the same parts in the wild pigeon." [After detailing kindred
diminutions in fowls and ducks, Mr. Darwin adds] "The decreased
weight and size of the bones, in the foregoing cases, is probably
the indirect result of the reaction of the weakened muscles on the
bones" (pp. 297-8). "Nathusi