342 Causes and Course of Organic Evolution 



In passing from the genealogy of the Hepatic se to that of 

 the Musci or leafy mosses, a question presents itself that has 

 been considerable debated, specially by Wettstein (1£0). This 

 bears on the fact that in the Hepaticfe from Riccia and Sphoero- 

 carpus upward the growth of the sexual plant is dorsi ventral, 

 and so each forms an expanded flattened thallus, as in Riccia, 

 or an inclined axis with dorsiventral bilaterally placed leaves, 

 as in the scale mosses. In the true or leafy mosses on the 

 other hand the growth is typically radial, and so upright axes 

 bearmg spirally arranged leaves are characteristic of the group. 



But in referring the mosses as well as the hepatics back to 

 a coleochsetoid ancestry ample explanation is got. For the 

 living species of Coleochcete evidently represent survivors not 

 merely of a group of species, but even of genera that varied 

 considerably amongst themselves in general structure and life- 

 history. Now the surviving species of Coleochcete may be 

 said to fall under three heads as to their mode of vegetative 

 growth. First, C. divergens forms elongated cellular threads 

 that branch at varying angles, while many of the branches 

 grow upward as green processes. This seems to be the most 

 l)rimitive type that approaches most nearly to forms like 

 Chcetophora. Second, in C. pidvinata the protonemal tube of 

 the swarmspore produces branches that grow out in one plane 

 as a dorsiventral expanse, but from these branches others 

 arise that grow upward and branch repeatedly in radial man- 

 ner. From this or some related type of organism we would 

 consider that the mosses originated. Third, in C. soluta and 

 specially in C. scutata, that we regard as nearest in proto- 

 riccioid characters, the protonemal tube early gives rise to a 

 dorsiventral type of growth, and to branches that arise by 

 equal or unequal dichotomy, as in Hepaticse. 



So even the living species of Coleochcete exhibit that degree 

 of variability and adaptability to environal substrate and sub- 

 aerial stimuli that would enable us to account satisfactorily 

 for evolution of the Hepaticse and Musci from them, or from 

 a related extinct group. 



A greatly wider morpliologic and taxonomic gap now exists 

 between Coleochcete and the simplest mosses, than between it 



