Let us take a genus of
birds, descended on my theory from some one parent-species, and of
which the several new species have become modified through natural
selection in accordance with their diverse habits. Then, from the many
slight successive steps of variation having supervened at a rather
late age, and having been inherited at a corresponding age, the young
of the new species of our supposed genus will manifestly tend to
resemble each other much more closely than do the adults, just as we
have seen in the case of pigeons. We may extend this view to whole
families or even classes. The fore-limbs, for instance, which served
as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles--namely
of each successive modification supervening at a rather late age, and
being inherited at a corresponding late age--the fore-limbs in the
embryos of the several descendants of the parent-species will still
resemble each other closely, for they will not have been modified. But
in each individual new species, the embryonic fore-limbs will differ
greatly from the fore-limbs in the mature animal; the limbs in the
latter having undergone much modification at a rather late period of
life, and having thus been converted into hands, or paddles, or wings.
Whatever influence long-continued exercise or use on the one hand, and
disuse on the other, may have in modifying an organ, such influence
will mainly affect the mature animal, which has come to its full
powers of activity and has to gain its own living; and the effects
thus produced will be inherited at a corresponding mature age.
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