338 SCIENCE PROGRESS. 



(the sex-producing generation C). The male and female 

 reproductive cells and the zygote resulting from their union 

 constitute the androgynous generation (D). This after a 

 period of rest produces swarm spores, each of which 

 develops into and represents the first stage of an individual 

 of the sexually produced generation (A). The alternation 

 is thus expressed 



A. B!-B B . C. D. 



The sexual generation of the moss (protonema and moss 

 plant) is regarded as corresponding to the repetition genera- 

 tions and the sex-producing generation in Ulothrix; the 

 sporophyte to the androgynous generation and the sexually 

 produced generation. Using the same symbols as before, 

 the life history of the moss is represented bv 



(B 1 -B,+C) (D+A). 



While both in the antithetic and homologous theories the 

 correspondence between the group of spores formed from 

 the zygote in Ulothrix or CEdogonhmi and the moss sporo- 

 gonium is recognised, on Naegeli's view the result of 

 division of the zygote in these Algae is not regarded as a 

 generation. This introduces confusion into the comparison 

 of the life history of the green Algae with that of archegoniate 

 plants. The application of these views to the problem of 

 the probable course of evolution followed in the origin of 

 Bryophyta and Pteridophyta from Algal ancestors appears to 

 accord rather with the antithetic theory, though, as the 

 comparison of the life histories given above show, points in 

 common with the homologous theory also exist. 



Some of the facts bearing on the nature of alternation 

 which have been investigated by Bower have already been 

 referred to ; it remains, however, to indicate the main lines 

 on which his elaboration of the antithetic theory has pro- 

 ceeded, and the views as to the phylogenetic relations exist- 

 ing between Algae, Bryophytes and Pteridophytes expressed 

 by him. 1 Agreeing with the distinction drawn by Cela- 

 kovsky between antithetic and homologous alternation, and 

 regarding the former as having originated by the inter- 



1 Bower (4), (7), (8), (9), (10), (13). 



