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, from
causes of which we are wholly ignorant, at a very early period of life,
or each step might be inherited at an earlier period than that at which
it first appeared. In either case (as with the short-faced tumbler) the
young or embryo would closely resemble the mature parent-form. We have
seen that this is the rule of development in certain whole groups of
animals, as with cuttle-fish and spiders, and with a few members of the
great class of insects, as with Aphis. With