s view to
widely distinct structures and to whole classes. The fore-limbs, for
instance, 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, 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 such 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 from their following the same habits of life with their
parents; for in this case it would be indispensable for their existence
that they should be modified in the same manner as their parents. Again,
with respect to the singular fact that many terrestrial and fresh-water
animals do not undergo any metamorphosis, while marine members of the
same groups pass through various transformations, Fritz Muller has
suggested that the process of slowly modifying and adapting an animal
to live on the land or in fresh water, instead of in the sea, would be
greatly simplified by its not passing through any larval stage; for it
is not probable that places well adapted for both the larval and
matu