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 feed and never see the daylight. {465} This corresponds to the _Origin_, Ed. i. p. 441, vi. p. 607, where, however, the example is taken from the Cirripedes. _Attempt to explain the facts of embryology._ I think considerable light can be thrown by the theory of descent on these wonderful embryological facts which are common in a greater or less degree to the whole animal kingdom, and in some manner to the vegetable kingdom: on the fact, for instance, of the arteries in the embryonic mammal, bird, reptile and fish, running and branching in the same courses and nearly in the same manner with the arteries in the full-grown fish; on the fact I may add of the high importance to systematic naturalists{466} of the characters and resemblances in the embryonic state, in ascertaining the true position in the natural system of mature organic beings. The following are the considerations which throw light on these curious points. {466} _Origin_, Ed. i. p. 449, vi. p. 617.