THE RECAPITULATION THEORY 319 



earlier ajjdearlier stages, as in the gonophores of 

 the Hydromedusae, until finally the extreme condi- 

 tion seen in HycTra is produced. We do not know 

 the causes that determine the period, whether late 

 or early, at whicji the reproductive organs ripen, 

 but the question is one of great interest and 

 importance,__and, deserves careful attention. The 

 suggestion "has been made that entire groups of 

 animals, such as the Mesozoa, are merely larvae, 

 arrested through such precocious acquiring of re- 

 productive power, and it is conceivable that this 

 may be the case. Mesozoa are a puzzling group 

 in which the life history, though known with toler- 

 able completeness, has as yet given us no reliable 

 clue concerning their affinities to other animals, 

 a tantalising distinction that is shared with them by 

 Rotifers and Polyzoa. 



Distortion of a curious kind is seen in cases of 

 abrupt metamorphosis, where, as in the case of 

 many Echinoderms, of Phoronis, and of the meta- 

 bolic insects, the larva and the adult differ greatly 

 in form, habits, mode of life, and very usually in 

 the nature of their food and the mode of obtaining 

 it ; and the transition from one stage to the other 

 is not a gradual but an abrupt one, at any rate so 

 far as external characters are concerned. Sudden 

 changes of this kind, as from the free swimming 

 Pluteus to the creeping Echinus, or from the slug- 

 gish leaf-eating caterpillar to the dainty butterfly, 

 cannot possibly be recapitulatory, for even if small 

 jumps are permissible in nature, there is no room 

 for bounds forward of this magnitude. Cases of 



