THE PALEOPHYTIC ERA. 



417 



nants in all but the initial stages of algae, lichens, and mosses. Its general 

 course may best be sketched by considering the development in the three 

 zones already indicated, though this must be recognized as identical with their 

 development in the cosere of any region, as the vegetation passed from the 

 climax of gjrmnosperms through mictium and into the final climax of angio- 

 sperms. The part played by the two floras in the respective zones depends 

 upon the genetic relation of angiospermous trees and shrubs. If we assume, 

 as seems to be the usual opinion, which is confirmed in so far as the record 

 can do this by the great abundance of trees and shrubs in it, that trees generally 

 preceded herbs, then the sere of the angiosperm climax along the Altantic 

 must have been characterized for much of the Comanchean period by serai 

 dominants of the preceding gymnosperm climax. In the northernmost 

 portion the evolution of herbs may have accompanied or even preceded that of 

 trees in a large degree. In the latter event, especially, the serai dominants 

 may have been furnished by the new flora, while the climax still belonged to 

 the old one. This is essentially what has happened in the case of the coniferous 

 climaxes of to-day, in which the sere is chiefly marked by angiospermous 

 -associes. The outstanding consequence in any event throughout the Atlantic 

 zone was the addition of a postclimax of angiosperms to those seres which had 

 reached the normal gymnosperm climax, or its substitution for the latter, 

 in those developed after the competition was decided against the gymnosperms. 

 Sooner or later the continued evolution of flowering plants must have provided 

 angiosperm dominants and subdominants for the entire sere, and the materials 

 of succession must have approximated those of to-day, at least in so far as the 

 genera are concerned. 



In the case of monocotyledons, it seems clear that herbs were primitive 

 and trees derived. This is supported by the fact that aquatic aUsmals and 

 arals, as well as sedges and grasses, are recorded in the Cretaceous. As a 

 consequence, when westward migration brought the angiosperms to the more 

 arid, wind-swept interior, there must have been a rapid evolution of grasses 

 and xeroid sedges. Accordingly, it is not improbable that a grassland climax 

 replaced the cycadean vegetation directly, though valleys and basins were 

 doubtless occupied by scrub and forest, much as is the case to-day, but to a 

 larger degree. 



THE PALEOPHYTIC ERA. 



The flora. — ^While the distinction between an Eophytic and a Paleophytic 

 era is vaUd theoretically, it is difficult to date such a distinction in geological 

 time. Though the Sudden appearance of the Paleophytic flora in the Devo- 

 nian suggests this period as the real beginning of the era, it seems highly prob- 

 able that land plants existed at least as early as the Cambrian, and not improb- 

 able that the evolution of the dominant pteridophytes was a result of Protero- 

 zoic or Cambrian glaciation and the resulting expansion of habitats during 

 the Ordovician. More or less doubtful fossils of land plants are indicated 

 for the Silurian, but the record is of little value until the Upper Devonian is 

 reached. Here all of the dominant types of the era, ferns, calamites, lepido- 

 dendrons, and cordaites, appear, and the vegetation assimied the character 

 which marked the Mississippian and Pennsylvanian, and continued into the 

 Permian. The flora was apparently very uniform as well as world-wide in dis- 

 tribution, but there is evidence that its differentiation had already begun (fig. 44) . 



