THE NATURE OF ALTERNATION, ETC. 341 



dividuals of the ancestral algal form, but has had a distinct 

 phylogenetic history as an interpolated stage in the life 

 history. On the former view, the two generations of a moss 

 are equivalent to two independent individuals of, e.g., Qido- 

 gonium, on the latter to one individual and the zygote which 

 produces the spores. If the first neutral generation of the 

 ancestral form had, as Pringsheim's comparisons would 

 suggest, become reduced to a group of spore-producing cells, 

 the methods of advance in the complexity of the sporophyte 

 need not have differed from those assumed by the advocates 

 of the antithetic theory. But it is also possible that the 

 differentiation of the two generations proceeded at first in 

 free living individuals which only later became united to 

 one another in almost invariable sequence. A provisional 

 hypothesis as to how this might have occurred in such a 

 group as the Ferns has been suggested by the author. 



The views held as to the probability that one or the 

 other course has been followed are intimately related to 

 those on the connections by descent that exist between the 

 different phyla of the vegetable kingdom. The usual view 

 is that forms like the simpler liverworts descended from 

 green algal forms which are represented by such existing 

 Algae as Ulothrix, (Edogonium and ColeocJicete. From the 

 simple sporophyte of these forms those of the more com- 

 plex liverworts and the mosses on the one hand and of 

 the ancestors of the vascular cryptogams on the other were 

 evolved. 1 Even such a view involves the independent 

 origin in the different groups of common characters of the 

 sporophyte. But it is not inconsistent with any known 

 facts to go a step farther, and to consider the possibility of 

 a number of somewhat similar developments having taken 

 place from the algal ancestry leading to various forms of 

 simple sporophyte, some of which were physiologically 

 independent after a time, while others were wholly depen- 

 dent on the gametophyte. In particular, the possibility of 

 the sporogonium of Bryophyta and the sporophyte in the 



1 Goebel while tracing the Pteridophyta to forms resembling the liver- 

 worts states clearly his opinion that their asexual generation proceeded on 

 a different line from the commencement. Goebel (2), p. 401. 



