76 The Royal Society, London. 



the original algal form, led to the evolution of the sporophyte. 

 The altered conditions in this series of experiments are of a 

 similar kind to thosc whicli are assumed by Professor Bower to 

 have occurrcd on the spread of algal forms to the land, and to 

 have conduced to antithetic alternation. 



The results may now be used in picturing the manner in 

 which alternation of generations might have come about by the 

 modification of originally similar individuals into gametophyte 

 and sporophyte. It is assumed for this purpose that the sporo- 

 phyte of the vascular cryptogams did not arise by the elaboration 

 of a structure resembling a bryophytic sporogonium. It is 

 recoguised that the theory of antithetic alternation, as elaborated 

 by Professor B o w c r , affords a consistent and satisfactory 

 explanation, if th(! assnmptions necessitated by the theory are 

 granted. The present theory, which is pnt forward merely as a 

 provisional hypothesis, is founded on anolher class of facts. 



With the spread of algal organisms to the land, where in the 

 absence of any Vegetation affording shade, some at least would 

 be cxposed to more intense illumination, the fiattened form Avould 

 probably be assumed. Prolonged drought and the influence of 

 direct sunlight, inducing directly a change of forms into a 

 cylindrical body, might be acconipanied by the Substitution of a 

 reproductive organ forming dry rcproductivc cells (spores) for 

 those adapted to an aquatic existence. The acquisition of more 

 highly developcd absorbent organs (primitive roots) would further 

 the existence and growth of this modified gametophyte. This 

 spore-producing stage would at first follow the sexual stage in 

 any individual exposed to dry conditions. It is possible to 

 imagine, however, how the association of the asexual with the 

 sexual individual might come about. Absence of fluid water would 

 prevent the liberation of motile spores from the Zygote. The 

 latter would be obliged to germinatc in situ, and the fact that 

 it did so under dry conditions would tend to the shortening of 

 the sexual stage, and the speedy assumption of the sporophytic 

 form and mode of reproduction. From the spore, which would 

 always separate from the parent, a sexual individual would arise, 

 since germination could only take place in a damp spot. As 

 soon as, with the increase in size and complexity of the spore- 

 bearing plant, a Vegetation capable of affording shade came into 

 existence, the conditions suitable for the persistence of the more 

 primitive, alga-like. sexual stage in the life history would be 

 present. The latter has, of course, also been modified in 

 various ways. 



In the concluding portion of this paper, the thcories of anti- 

 thetic and homologous alternation are compared by considering 

 the explanations they afford of the facts. The general conclusion 

 reached is that, while both afford a possible explanation of the 

 facts of alternation in archegoniate plants, any evidence which 

 would render one or the other untenable is wanting. The reasons 

 on which either is considered more probable depend on the views 



