INTERPOLATION OF A CONTINUOUS 



V. 



INTERPOLATION OF A CONTINUOUS 

 PULLULATION IN THE GENETIC CYCLE. 



1. To complete our survey of the Genetic Cycle in the 

 alternating species, it is necessary to advert more specifi- 

 cally than has yet been done to the fact, that this cycle is 

 at times much extended by a continuous succession of gem- 

 mations, all of the same general character. Thus we 

 find that in some of the best marked cases of alternation 

 of generations, the offsets produced in a particular stage of 

 development are not transformed immediately into the next 

 phase in the life-history of the species, but that there occurs 

 a continuous series of zooids [or phytoids], budded off from 

 each other, and all referable to one phase of development, 

 which is succeeded in due time by that next in order, as if 

 there had been but a single intervening link. In these 

 cases, instead of a regular alternation of free gemmae with 

 true embryos, we find the general propagation of the species 

 effected by gemmae alone ; the sexual zooids, which give 

 origin to the fecundated germs, recurring only at longer or 

 shorter intervals, which are occupied by continuous suc- 

 cession of non-sexual zooids, bearing a general resemblance 

 to each other, though with differences frequently in details. 

 In certain cases as among the Trematoda the links 

 are but few in number, and apparently fixed in each species, 

 and there are generally appreciable differences between the 

 forms which succeed each "other in the series ; in others we 

 have an indefinite succession of like forms, and the periodic 

 recurrence of the sexual zooids, which terminate the series, 

 seems to depend on circumstances, being a provision to 



