THE EVOLUTION OF PLANTS 575 



500. Another Hypothesis of Alternation of Genera- 

 tions. — Some of the facts of paleobotany support the 

 hypothesis that the modern sporophyte has not been 

 gradually developed from a simple structure like the moss 

 sporogonium (derived, in turn, by progressive steriliza- 

 tion), but that the gametophytic and sporophytic stages 

 were at the first vegetatively or somatically equivalent 

 except for chromosome number (as is the case now, foi 

 example, with Dictoyota^ and Polysiphonia); and that, 

 in the course of evolution, the sexual phase became more, 

 and the asexual phase less, important in certain forms 

 (e.g., mosses), and the asexual phase more, and the sexual 

 phase less, important in other forms (e.g., ferns). This 

 is the hypothesis of homologous alternation, as opposed to 

 that of antithetic alternation.^ The structural differences 

 in the two generations are, on the basis of this hypothesis, 

 considered as due almost, if not entirely, to differences 

 in environment, the main factor being the gradual transi- 

 tion from aquatic to dry-land surroundings. Where the 

 environment is uniform and the same for both genera- 

 tions, as for Diclyota, the gametophyte and sporophyte are 

 identical in external organs and general appearance (Fig. 

 177). In any event the hypothesis postulates a homology 

 between the various organs of the two generations, how- 

 ever much these parts may differ in external appearance 

 as a result of individual variation and environmental 

 influence. 



501. Lang's Ontogenetic Hypothesis. — Viewing the 

 matter from the standpoint of individual development 

 (ontogeny), Lang has developed the ontogenetic hypothesis 



> See Chapter XVII. 

 2 See Chapter XIV. 



