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the fore-limbs will not have been much modified in the embryos of these several forms; although in each the embryonic fore-limb will differ greatly from that in the adult. ↑ Whatever influence, moreover, long-continued use or disuse may have had in modifying the limbs or other parts of animals, this will chiefly or solely have affected them when mature and when they had to use their full powers ... to gain their own living; and the effect thus produced will be transmitted to the offspring at a corresponding mature age. Thus the young will not be modified or will be modified in a less degree.
In other cases .. 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 case, as we have seen with the short-faced tumbler, the young or embryo would closely resemble the mature parent-form. And this is the rule of development in certain whole groups or sub-groups, as with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and some members of the great class of insects. .. .. .. With respect to the final cause of the young in these groups not passing through any metamorphosis, ... we can see that this would follow from the .. following contingencies; namely, from the young ... having to provide at a very early age for their own wants, and
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 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 respect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the two following contingencies: firstly, from the young, during a course of modification carried on for many generations, having to provide for their own wants at a very early stage of development, and secondly,