early period of life, and have been inherited by the offspring at a
corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles which latter,
though not proved true, can be shown to be in some degree probable to
species in a state of nature. 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. Whereas the
young will remain unmodified, or be modified in a lesser degree, by the
effects of use and disuse.
In certain cases the successive steps of variation might supervene,
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