e modification supervening at a rather late age, and being
inherited at a {447} 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 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 {448} of development, and secondly, from
their following exactly the same habits of life with their parents; for in
this case, it would be indispensable for the existence of the species, that
the child should be modified at a very early age in the same manner with
its parents, in accordance with their similar habits. Some further
explanation, however, of the embryo not undergoing any metamorphosis is
perhaps requisite. If, on the other hand, it profited the young to follow
habits of life in any degree different from those of their parent, and
consequently to