We may extend this view to widely distinct structures and to
whole classes. The fore-limbs, for instance, which once served as legs to
a remote progenitor, may have become, through a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; but on the above two principles the
fore-limbs will not have been much modified in the embryos of these several
forms; although in each form the fore-limb will differ greatly in the adult
state. Whatever influence long continued use or disuse may have had in
modifying the limbs or other parts of any species, this will chiefly or
solely have affected it when nearly mature, when it was compelled to use
its full powers to gain its own living; and the effects thus produced will
have been transmitted to the offspring at a corresponding nearly mature
age. Thus the young will not be modified, or will be modified only in a
slight degree, through the effects of the increased use or disuse of parts.
With some animals the successive variations may have supervened at a very
early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases the
young or embryo will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of development in
certain whole groups, or in certain sub-groups alone, as with cuttle-fish,
land-shells, fresh-water crustaceans, spiders, and some members of the
great class of insects.
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