e of
all orders of insects more resembling the simpler articulate animals
than their parent insects{463}; and from such other cases as the embryo
of the jelly-fish resembling a polype much nearer than the perfect
jelly-fish; it has often been asserted that the higher animal in each
class passes through the state of a lower animal; for instance, that the
mammal amongst the vertebrata passes through the state of a fish{464}:
but Mueller denies this, and affirms that the young mammal is at no time
a fish, as does Owen assert that the embryonic jelly-fish is at no time
a polype, but that mammal and fish, jelly-fish and polype pass through
the same state; the mammal and jelly-fish being only further developed
or changed.

{461} _Origin_, Ed. i. p. 439, vi. p. 604.

{462} The uselessness of the branchial arches in mammalia is
insisted on in the _Origin_, Ed. i. p. 440, vi. p. 606. Also the
uselessness of the spots on the young blackbird and the stripes of
the lion-whelp, cases which do not occur in the present Essay.

{463} In the _Origin_, Ed. i. pp. 442, 448, vi. pp. 608, 614 it is
pointed out that in some cases the young form resembles the adult,
_e.g._ in spiders; again, that in the Aphis there is no "worm-like
stage" of development.

{464} In the _Origin_, Ed. i. p. 449, vi. p. 618, the author speaks
doubtfully about the recapitulation theory.

As the embryo, in most cases, possesses a less complicated structure
than that into which it is to be developed, it might have been thought
that the resemblance of the embryo to less complicated forms in the same
great class, was in some manner a necessary preparation for its higher
development; but in fact the embryo, during its growth, may become less,
as well as more, complicated{465}. Thus certain female Epizoic
Crustaceans in their mature state have neither eyes nor any organs of
locomotion; they consist of a mere sack, with a simple apparatus for
digestion and procreation; and when once attached to the body of the
fish, on which they prey, they never move again during their whole
lives: in their embryonic condition, on the other hand, they are
furnished with eyes, and with well articulated limbs, actively swim
about and seek their proper object to become attached to. The larvae,
also, of some moths are as complicated and are more active than the
wingless and limbless females, which never leave their pupa-case, never