12 DERIVATION OF OKGANIC BEINGS. 



applied an expression which, though open to some ob- 

 jections, has now come into very general use. 



The recurrence of sexual reproduction, in some form or 

 other, has now been ascertained in so many species, ordi- 

 narily propagated in the way of gemmation, as strongly to 

 suggest the probability of the occasional interposition of the 

 other form being eventually detected, by close and long 

 continued observation, even in those cases in which deriva- 

 tion from a single source is the only mode of increase yet 

 known ; and the impression is gaining ground among 

 naturalists that the two are co-existent, and alternate with 

 more or less regularity and distinctness in many of the 

 lower Invertebrata, and probably still more universally in 

 the vegetable kingdom. It is unnecessary at present to go 

 into any argument on the point, as the sketch which will 

 presently be given of the modifications of the function of 

 reproduction in both kingdoms of nature will serve to show 

 the grounds on which this view is founded. 



5. I believe, indeed, that it falls short of the truth, 

 and that there are processes co-extensive with organic 

 nature which represent, in some degree, the gemmation of 

 the lower species, and alternate in the same way with the 

 sexual act. 



I must premise, however, that, even in cases where the 

 phenomena of alternation come out most clearly, I cannot 

 regard the processes as all of a parallel kind. They ap- 

 pear to me to fall into groups whose characteristic features 

 depend principally on the period of the life-history of the 

 species at which a process of gemmation is interpolated in 

 the genetic cycle. 



The gemmation sometimes occurs just before, and is, as it 

 were, ancillary to sexual reproduction ; sometimes it occurs 

 after it, when it is subservient rather to the progress of 

 development. In the former case, what may on the whole 

 be considered as the most typical of the diverse forms be- 



