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, for 
example, with Dictoyota 1 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. 2 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 Dictyota, 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 
1 See Chapter XVII. 
* See Chapter XIV. 
