DERIVATION OF ORGANIC BEINGS. 11 



relation was indicated between the two processes in such 

 cases, than that a high development of the one was usually 

 accompanied by a proportionate abeyance of the other. 



Now, in so far there can be no doubt that it is only 

 in the lower forms that gemmation is met with as an ob- 

 vious phenomenon ; but the idea can no longer be enter- 

 tained that sexual reproduction is confined in the same 

 way to the higher species, the tendency of recent investiga- 

 tions being rather in favour of greatly extending the limits 

 of both forms. In particular, an origin from two parents is 

 now known to have place in many species, in which it had 

 long been overlooked on account of its recurring only at 

 intervals, the ordinary mode of propagation being by offsets 

 from a single stock. Thus, in some of the lower Algse, the 

 ordinary mode of increase is by the formation of new cells, 

 which, becoming detached from the parent frond, may form 

 each the nucleus of a new and independent plant. This 

 goes on with more or less vigour during the whole season ; 

 but when such a change of external circumstances super- 

 venes, as to interfere with continuous growth, a new mode 

 of propagation is brought into play in the process termed 

 conjugation, or the coalescence of two cells in separate 

 fronds, so as to form a seed-like body, which, after lying 

 dormant till the return of conditions favourable to veo-eta- 

 tion, gives origin to a new frond, like one of those previously 

 produced by detached cells. Here the process of digenesis 

 — represented by the fusion of two cells — comes in only at 

 intervals, to supplement, as it were, what is in this case the 

 more usual one of monogenesis — represented by the detach- 

 ment and o;ermination of single cells. 



In those of the lower species, in which both modes of 

 propagation are well marked features, we find that they 

 have a tendency to succeed each other in a regular order. 

 with corresponding differences in the immediate progeny, 

 to which the term alternation of generations has been 



