236 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



dendron, Sigillaria, and many CalamaricC, a characteristic of dense tropical 

 forests at the present time, and {b) the presence in many ferns of Aphlebiae, 

 which to-day are unknown except in tropical types. 



"Evenness of Climate in Coal-forming Periods. 



"It must be borne in mind that the climatic conditions here described as 

 having prevailed during the deposition of the groups of coal were not necessarily 

 conditions persisting without change from one period to another; in fact, the 

 characters of the successive floras and their changes are found to indicate the 

 occurrence of many climatic changes, some within the limits of a single peiiod 

 in the geologic record. To these changes in the element of the environment are 

 largely due the important steps in the evolution of the higher plant groups, such 

 as the approximate disappearance of heterospory, and the better protection of 

 the megaspore observed in the cycadofilices of the Carboniferous; the origin of 

 the dicotyledonous leaf, and of delayed germination, the former developed to 

 give a maximum vegetative efficiency in a growing season shortened, as indicated 

 by concurrent evidence by the occurrence of seasons of winter cold, the other to 

 enable the plant to survive several seasons that might be unfavorable for the 

 sprouting and successful start of the plant. The periods of coal formation have, 

 however, been for the most part confined to the long geologic intervals of relatively 

 uniform climate, the principal features of which are outlined later. 



"The evidence afforded by the presence of coal in thick and extensive beds 

 in various regions is mostly valuable as indicating that during a long period of 

 time there were no wide variations of either temperature or, especially, humidity. 



"But absence of coal or lignite is far from furnishing a certain basis for 

 conclusions as to opposite climatic conditions. The recurrent deposition of coal 

 of large areal extent and thickness postulates a base-level subsidence so adjusted 

 that at various times the necessary close relationship between water-level and 

 the peat-formation surface may be maintained for considerable intervals, to 

 permit peat deposition of the required thickness. The formation of coal (peat) 

 in extensive deposits (always continental) is rare in regions undergoing erosive 

 dissection, on the one hand, and, on the other, it is likely to fail when the sub- 

 sidence is too rapid or the water-level reaches the region of topographic relief 

 so that the coast is bold. It should be repeated that in successively warmer 

 climates the formation of peat requires not only a heavier rainfall, but also a 

 more even distribution of the same, so as to obviate the occurrence of long dry 

 seasons. Even then its formation is possible only by the great rapidity of plant 

 growth, which exceeds, under favoring circumstances, such as maintenance of 

 the water cover, the rate of rapid decay. 



" Pennsylvania (' Upper Carboniferous') Coal Measures.' 



"Judged by the criteria outlined above, the climate of the principal coal- 

 forming intervals of the Pennsylvanian was mild, probably near-tropical or 

 subtropical, generally humid, and equable. The evidence may be outlined in 

 summary form as follows: 



"Abundant humidity and condensation are shown by: (o) succulency of the 

 growth, large medullary development, and large intercellular spaces; {b) presence 

 of many hydathodes or water-pores on the leaves (possibly due to an aquatic 

 environment), and abundant lacunose tissue; (c) dissected or laciniate forms of 



1 White, David, loc. cit., page 74. 



