4SZ ORIGIN OF SPECIES 



retrograde, for the male is a mere sack, which lives for a 

 short time and is destitute of mouth, stomach, and every 

 other organ of importance, 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 contin- 

 gent 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 character 

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

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

 having their proper forms, whilst the marine m.embers 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 in- 

 sects 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 the development of 

 this insect, by Professor Huxley, we see hardly any trace 

 of the vermiform 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 sim.ple 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 crusta- 

 ceans belong, no other member is as yet known to be first 

 developed under the nauplius-form, though many appear as 

 zoeas; nevertheless Miiller assigns reasons for his belief, 

 that if there had been no suppression of development, all 

 these crustaceans would have appeared as nauplii. 



How, then, can we explain these several facts in embry- 



