254 THE ORIGIN OF SPECIES 



tempted to look ut 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 por- 

 poise, 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 character is manifested 

 long before the parts of the embryo are completed." 

 Land -shells and fresh- water crustaceans are bora 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 nutri- 

 ment or from being fed by their parents; but in some 

 few cases, as in that of Aphis, if we look to the ad- 

 mirable drawings of the development of this insect, by 

 Professor Huxley, we see hardly any trace of the vermi- 

 form stage. 



Sometimes it is only the earlier developmental stages 

 which fail. Thus Fritz Miiller has made the remarkable 

 discovery that certain shrimp-like crustaceans (allied to 

 Penoeus) first appear under the simple nauplius-form, and 

 after passing through two or more zoea-stages, and then 

 through the mysis-stage, finally acquire their mature 

 structure: now in the whole great malacostracan order, 

 to which these crustaceans belong, no other member is 



