146 INTERPOLATION OF A CONTINUOUS 



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 sino^le intervenino- 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 mth 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 tenninate the series, 

 seems to depend on circumstances, being a provision to 



