ALTERNATION OF GENERATIONS. 12U 



the completeness of the whole organization, according to 

 the type of the species, as a juster criterion of the culmi- 

 nating point of development than the possession of any one 

 function, however important. It is true that where the in- 

 dividuality is much broken up, there may exist a difficulty 

 in determining what particular zooid does really represent 

 most correctly the typical organization of the species a 

 difficulty which can, of course, have no place, where, such 

 zooids not being budded off, all the structures characteristic 

 of the species are united as organs of one individual. Yet 

 as it rarely, if ever, happens that alternation prevails so 

 universally in any natural family as wholly to exclude the 

 more common course of reproduction, the cases whether 

 many or few in which this function is performed without 

 disturbance of the integrity of the organism, generally 

 furnish us with sufficient data for selecting the truly typical 

 form, and for determining whether it does or does not coin- 

 cide with that phase of development characterised by the 

 sexual peculiarities. 



Now the viviparous Trematoda, in which there is no 

 " alternation" the ovum being at once developed into the 

 sole form of the species clearly indicate that in this group 

 the typical form is that represented by the Distoma ; and 

 this is never dissociated from the sexual phase, though in 

 the great majority of the order the culminating point of 

 development is not attained till after a more or less pro- 

 longed course of gemmation in a rudimentary stage. 



Among the Polypifera again, we have the case of the 

 common Hydra to determine the polype form as the typi- 

 cal, because in that species the only form ; and therefore I 

 conceive we must hold it to be the typical form also in the 

 Sertularian and other alternating species, though no longer 

 that of sexual completeness.* 



* A mucli more perplexing fact if it be really a fact is the occasional 



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