420 MESEOSERE AND PALEOSERE. 



a relatively mild climate. Toward the close of this epoch, however, erogenic 

 movements brought about a second cooUng and consequent differentiation of 

 marine faunas, and probably of the unknown land flora. The climate again 

 became warmer dining the Upper Cambrian, and continued generally mild and 

 uniform throughout the Ordovician and earlier Silurian, though the three 

 periods of mountain-making which close the corresponding epochs of the 

 Ordovician can hardly have been without climatic effect. The Silurian was 

 closed by a much more marked period of deformation and vulcanism which 

 extended into the Lower Devonian. Arid climates in North America are 

 attested by red beds, gypsvmi, and salt, and cold or arid climates are indicated 

 for Scotland and South Africa in the late Silurian or early Devonian. In spite 

 of orogenic movements during the Devonian and Mississippian, the climate 

 seems to have been warm and moist for the most part. Moimtain movements 

 on a large scale occurred toward the close of the Mississippian, together with 

 a widespread emergence of sediments from the sea. This must have been fol- 

 lowed by a marked climatic cooling, though the evidence of the latter, so far as 

 the land flora was concerned, is furnished by the poverty of the fossil record 

 rather than by any known change in the flora. 



The general sequence of climates for the Eophytic and Paleophytic eras 

 may be best inferred from the curve of aridity in Schuchert's chart (p. 321). 

 This shows that the Paleozoic era began and closed with times of maximum 

 cold and dryness, most nearly comparable to the third great glaciation, that 

 of the Pleistocene. During this geological era the change of the generally 

 warm, moist climate to a cooled or arid one is thought to have occurred five 

 times, viz, at the end of the Cambrian, Ordovician, Silurian, Devonian, and 

 Mississippian. Since we have no trustworthy record of the land flora before 

 the Devonian, the effect of climatic changes upon the unknown vegetation of 

 the Eophytic era is purely inferential, and need not be further considered. As 

 to the Paleophytic, whUe the record of land plants for the Devonian and Missis- 

 sippian is far from satisfactory, it does afford some idea of the development 

 of the vegetation. 



THE PALEOSERE. 



The sere. — ^The general course of serai development throughout the Paleo- 

 phytic era must have been essentially the same, since the flora changed but 

 Uttle during the three periods. In so far as there was a differentiation into a 

 Glossopteris or other clunax, the later stages of succession must have diverged, 

 but over much of the globe the course of the sere must have been uniform or 

 practically so. Since coal-beds constitute the characteristic deposits of the 

 era, the hydrosere must have been equally typical. The xerosere was doubt- 

 less present, even during the Pennsylvanian, but it must have been much less 

 frequent and important, in addition to being so remote from opportunities for 

 fossiUzation that no record of it has been found. While the Glossopteris 

 climax was xeroid in nature, it probably resembled all climaxes in developmg 

 upon hydrophytic as well as xerophytic bare areas. 



As has been previously suggested, the hydrosere apparently began with a 

 submerged associes of charads, with which were associated immersed Uver- 

 worts and mosses, such as Ricda and FontinaKs of to-day. The occurrence 

 of the usual floating associes is not certain, but the existence of floating hydrop- 



