398 THE ORIGIN OF SPECIES 



motive organs, a 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 an- 

 tennae; but they have a closed and imperfect mouth and cannot 

 feed: their function at this stage is, to search out 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, simple eye-spot. In this last and complete state, 

 cirripedes may be considered as either more highly or more lowly 

 organized than they were in the larval condition. But in some 

 genera the larvae become developed into hermaphrodites having 

 the ordinary structure, and into what I have called complemental 

 males ; and in the latter the development has assuredly been retro- 

 grade, for the male is a mere sac, which lives for a short time and 

 is destitute of mouth, stomach, and every other organ of impor- 

 tance, excepting those for reproduction. 



We are so much accustomed to see a difference in structure 

 between the embryo and the adult, that we are tempted to look at 

 this difference as in some necessary manner contingent on growth. 

 But there is no reason why, for instance, the wing of a bat, or the 

 fin of a porpoise, should not have been sketched out with all their 

 parts in proper proportion, as soon as any part became visible. In 

 some whole groups of animals and in certain members of other 

 groups this is the case, and 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 char- 

 acter is manifested long before the parts of the embryo are com- 

 pleted." Land-shells and fresh-water crustaceans are born having 

 their proper forms, while the marine members of the same two 

 great classes pass through considerable and often great changes 

 during their development. Spiders, again, barely undergo any 

 metamorphosis. The larvae of most insects pass through a worm- 

 like stage, whether they are active and adapted to diversified 

 habits, or are inactive from being placed in the midst of proper 

 nutriment, or from being fed by their parents; but in some few 

 cases, as in that of Aphis, if we look to the admirable drawings of 



