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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 and spiders, and some members of the great class of insects, as with Aphis. With respect to the final cause of the young in such groups not passing through any metamorphosis, or closely resembling their parents from their earliest age, we can see
which served as legs in the parent-species, may have 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 of our new species, the embryonic fore-limbs will differ greatly from the fore-limbs in the mature animal; the limbs in the later 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 disuse may have had in modifying an organ, such influence will mainly affect the mature animal, which has come to its full powers of activity and has had to gain its own living; and the effects thus produced will be inherited at the 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, land-shells, fresh-water crustaceans, spiders, and with a few members of the great class of insects. .. .. .. With respect to the final cause of the young in these cases not undergoing any metamorphosis, ... we can see