s. In this case "the new parts are not moulded upon the inner
surface of the old ones. The plastic force has changed its course of
operation. The outer case, and all that gave form and character to the
precedent individual, perish and are cast off; they are not changed into
the corresponding parts of the new individual. These are due to a new and
distinct developmental process," &c.[887] Metamorphosis, however, graduates
so insensibly into metagenesis, that the two processes cannot be distinctly
separated. For instance, in the last change which Cirripedes undergo, the
alimentary canal and some other organs are moulded on pre-existing parts;
but the eyes of the old and the young animal are developed in entirely
different parts of the body; the tips of the mature limbs are formed within
the larval limbs, and may be said to be metamorphosed from them; but their
basal portions and the whole thorax are developed in a plane actually at
right angles to the limbs and thorax of the larva; and this {367} may be
called metagenesis. The metagenetic process is carried to an extreme degree
in the development of some Echinoderms, for the animal in the second stage
of development is formed almost like a bud within the animal of the first
stage, the latter being then cast off like an old vestment, yet sometimes
still maintaining for a short period an independent vitality.[888]

If, instead of a single individual, several were to be thus developed
metagenetically within a pre-existing form, the process would be called one
of alternate generation. The young thus developed may either closely
resemble the encasing parent-form, as with the larvae of Cecidomyia, or may
differ to an astonishing degree, as with many parasitic worms and with
jelly-fishes; but this does not make any essential difference in the
process, any more than the greatness or abruptness of the change in the
metamorphoses of insects.

The whole question of development is of great importance for our present
subject. When an organ, the eye for instance, is metagenetically formed in
a part of the body where during the previous stage of development no eye
existed, we must look at it as a new and independent growth. The absolute
independence of new and old structures, which correspond in structure and
function, is still more obvious when several individuals are formed within
a previous encasing form, as in the cases of alternate generation. The same
important principle probably c