Of course in a certain sense this thickening of the sole has resulted
from use. In one sense or other, most--or perhaps all--of the results of
natural selection are inherited effects of use or disuse. Natural
selection preserves that which is of use and which is used, while it
eliminates that which is useless and is not used. The most confident
assertions of the effects of use and disuse in modifying the heritable
type, appear to rest on this indefeasible basis. Darwin's statements
concerning the effects of use and disuse in evolution can frequently be
read in two senses. They often command assent as undeniable truisms as
they stand, but are of course written in another and more debatable
sense. Thus in the case of the shortened wings and thickened legs of the
domestic duck, I believe equally with Darwin and Spencer that "no one
will dispute that they have resulted from the lessened use of the wings
and the increased use of the legs." "Use" is at bottom the determining
circumstance in evolution generally. The trunk of the elephant, the fin
of the fish, the wing of the bird, the cunning hand of man and his
complicated brain--and, in short, all organs and faculties
whatsoever--can only have been moulded and developed by use--by
usefulness and by using--but not necessarily by use-inheritance, not
necessarily by directly inherited effects of use or disuse of parts in
the individual. So, too, reduced or rudimentary organs are due to
disuse, but it by no means follows that the diminution is caused by any
direct tendency to the inheritance of the effects of disuse in the
individual. The effects of natural selection are commonly expressible as
effects of use and disuse, just as adaptation in nature is expressible
in the language of teleology. But use-inheritance is no more proven by
one of these necessary coincidences than special design is by the
other. The inevitable simulation of use-inheritance may be entirely
deceptive.
Darwin thinks that "there can be no doubt that use in our domestic
animals has strengthened and enlarged certain parts, and disuse
diminished them; and that such modifications are inherited." Undoubtedly
"such" or _similar_ modifications have often been inherited, but how can
Darwin possibly tell that they are not due to the simulation of
use-inheritance by natural or artificial selection acting upon general
variability? Of the inevitability of selection and of its generally
adaptive tendencies "there can be no doubt," and panmixia would tend to
reduce disused parts; so that there _must always_ remain grave doubts of
the alleged inheritance of the similar effects of use and disuse, unless
we can accomplish the extremely difficult feat of excluding both natural
and artificial selection as causes of enlargement, and panmixia and
selection as causes of dwindling.