The embryo in the course of development generally rises in
organisation: I use this expression, though I am aware that it is
hardly possible to define clearly what is meant by the organisation
being higher or lower. But no one probably will dispute that the
butterfly is higher than the caterpillar. In some cases, however, the
mature animal is generally considered as lower in the scale than the
larva, as with certain parasitic crustaceans. To refer once again to
cirripedes: the larv in the first stage have three pairs of legs, a
very simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage,
answering to the chrysalis stage of butterflies, they have six pairs of
beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antenn ; but they have a closed and
imperfect mouth, and cannot feed: their function at this stage is, to
search 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
antenn , 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 larv 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
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