by their well-developed
organs of sense, and to reach by their active powers of swimming, a
proper place on which to become attached and to undergo their final
metamorphosis. When this is completed they are fixed for life: their
legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes
are now reconverted into a minute, single, and very simple eye-spot.
In this last and complete state, cirripedes may be considered as
either more highly or more lowly organised than they were in the larval
condition. But in some genera the larvae become developed either into
hermaphrodites having the ordinary structure, or into what I have called
complemental males: and in the latter, the development has assuredly
been retrograde; for the male is a mere sack, which lives for a short
time, and is destitute of mouth, stomach, or other organ of importance,
excepting for reproduction.

We are so much accustomed to see differences in structure between the
embryo and the adult, and likewise a close similarity in the embryos of
widely different animals within the same class, that we might be led to
look at these facts as necessarily contingent in some manner on growth.
But there is no obvious reason why, for instance, the wing of a bat, or
the fin of a porpoise, should not have been sketched out with all the
parts in proper proportion, as soon as any structure became visible in
the embryo. And in some whole groups of animals and in certain members
of other groups, the embryo does not at any period differ widely from
the adult: thus Owen has remarked in regard to cuttle-fish, "there is no
metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed;" and again in spiders, "there
is nothing worthy to be called a metamorphosis." The larvae of insects,
whether adapted to the most diverse and active habits, or quite
inactive, being fed by their parents or placed in the midst of proper
nutriment, yet nearly all pass through a similar worm-like stage of
development; but in some few cases, as in that of Aphis, if we look to
the admirable drawings by Professor Huxley of the development of this
insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology,--namely
the very general, but not universal difference in structure between the
embryo and the adult;--of parts in the same individ