238 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



did not vary greatly from region to region, and that they were nowhere torrid, 

 possibly not even fully tropical as a whole. Yet the action of breezes, so well 

 demonstrated by the abundant ripple-marks on the sands of the Coal Measures, is 

 also indicated by the equipment of many seeds and spores with wings (although 

 the latter may have been for gliding), and, more particularly, by the separation 

 of the male and female flowers in most of the flowering plants. 



"The action of sunlight may be inferred from the presence of palisade cells 

 to shade the mesophyl, the horizontal attitude of the leaves, and the rapid growth. 



" POTTSVILLE TIME. 



"Of the Pennsylvanian floras, those of the Pottsville and Allegheny time are 

 perhaps widest spread in relative entirety, though the flora of the upper Cone- 

 maugh and Monongahela have nearly equal homogeneity in migration. The 

 fact that these floras differ markedly, though the changes are somewhat transi- 

 tional, especially between the last-named stages the fact that plant life changed, 

 differentiating, eliminating, and adding types is probably due not merely to 

 kinetic evolution and exterminative competition; it was undoubtedly due in a 

 large part to changes in the climates as well as in other environmental elements. 

 As to the degree of the climatic change we have little knowledge, but, as is later 

 suggested, they were probably of relatively small magnitude during this interval. 



"ALLEGHENY TIME. 



"As already suggested, it is possible that the maximum uniformity of climate 

 occurred in the upper Pottsville. In the Appalachian trough this was perhaps 

 the period of greatest and most evenly distributed rainfall. In the Rocky Moun- 

 tains some coal was laid down at this time. The Allegheny, which includes the 

 topmost Westphalian, is marked by the disappearance of many of the climbing 

 and clambering types, whereas the membranaceous and laciniate-leaved forms 

 are much rarer, and the pinnules of the filicoid types are growing larger. Coal 

 formation, which seems to have occurred wherever the adjustment of topography 

 and water-level was favorable, appears, however, to have been general. The 

 woods show no trace whatever of seasonal interruption of growth, and the con- 

 clusion that there was no winter frost to cause a periodic stage of arrest of growth 

 seems well founded. That there were, however, times when during certain 

 seasons, possibly exceptional or extraordinary, the water-level was reduced, is 

 nevertheless indicated by the increasing development of pseudoxerophytic char- 

 acters. As they later become more prominent in the Conemaugh and Permian, 

 they are discussed in connection with the paragraphs referring to those periods. 



" CONEMAUGH TIME. 



"The Conemaugh (of lower Stephanian age) time witnessed several changes 

 in the floras which may be of climatic cause. Most prominent among these are 

 a rapid decrease, approaching extinction, of the colossal lycopods (Lepidoden- 

 dreae), and the rapid development of the group of gigantic tree ferns, such as 

 Psaronius, whose supposed fronds, Pecopteris, became highly varied, very large, 

 and more or less distinctly villous in most species. The evidence therefore 

 points to the occurrence of short, dry seasons. The reduction in the lepidophytes 

 is attributable to occasional unusual failures or disappearance of the water in 

 which their spores must fall in order to insure reproduction of the species. Pro- 

 vision for spells of unusual evaporation may account also for the tremendously 

 thick bark of the Psaronii, with their abundant intracortical ramentum; for the 



