234 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



to uniformity of climatic conditions, which, with a few exceptions, appear to 

 have lacked cold winters or severe frosts; (3) a generally high humidity, the rain- 

 fall being from moderately heavy to very heavy and fairly well distributed, 

 though in many cases there is evidence of the occurrence of dry periods which, 

 however, seem ordinarily to have been comparatively short and not severe; (4) 

 an amazingly wide geographical distribution of these genial and equable climates, 

 which occurred seemingly in almost uniform development simultaneously in the 

 high and in the low altitudes of both the northern and the southern hemispheres. 

 This shows either that the essentially uniform climatic conditions were truly 

 extraordinary in geographic extent, with little regard to modern climatic zones, 

 or that the formation of coal was mainly confined to the areas of the above- 

 prescribed climatic environment. 



" PALEOBOTANICAL CRITERIA AS TO CLIMATE. 



"The principal criteria as to climate offered by the fossil plant remains 

 preserved either in the coal or in the enveloping shales and sandstones, and 

 serving as a basis for the conclusions stated above, may be summarized as follows, 

 further particulars being noted in the discussion of the climates of the several 

 most important periods of the coal formation : 



" (i) Relative abundance or luxuriance and large size of terrestrial vegetation 

 that is, rankness of growth, indicating favorable conditions of temperature, 

 humidity, etc. 



" (2) Character, condition, and amount of the land-plant material preserved 

 as coal or carbonized in the rocks. The formation of xyloid coal of the ordinary 

 type, composed mainly of subaerial vascular-plant remains indicates humidity. 

 In regions of cool temperature the humidity required for the foimation of peat 

 the initial state of coal is moderate; in the warmer climates, where decay is 

 more rapid, not only must the humidity be greatly increased, in order to provide 

 the necessary wetness to retard decomposition, but there must be no long dry 

 seasons of the year for the too great reduction of the water cover. The observa- 

 tions of peat formation at the present day in tropical climates show that in order 

 to permit the decomposition of peat the rainfall must be both very heavy and 

 fairly well distributed through the entire year. 1 



" (3) Great radial distribution, seemingly over the greater part of the earth, 

 and especially over wide ranges of latitude, of identical species and genera in 

 characteristic association, indicating the extension of approximately uniform 

 climatic conditions in these regions. Floras identical, or essentially identical, 

 in remote or detached regions can owe their identity to no other cause than ap- 

 proximate continuity of the environment, whether that continuity is geographic 

 or chronographic. Conversely, the migration of a flora without change is possible 

 only through regions of essentially identical environmental conditions. Illus- 

 trations are found in the Carboniferous, Triassic, Jurassic, and lower Cretaceous 

 floras, and even to a remarkable degree in the upper Cretaceous and Tertiary 

 floras. 



" 1 Peat formation in the United States is not taking place, according to C. A. Davis (in 

 conversation with the author), in areas of less than 20 inches of rainfall. Practically 

 25 inches is the lower limit. Failure of peat in certain districts, such as the Pied- 

 mont Plateau of the South, would appear to be due in part to the occurrence of long 

 dry seasons; presumably the total rainfall in that region is also insufficient. Peat is 

 forming in Florida and in many tropical and subtropical regions of heavy and well- 

 distributed precipitation. 



