THE PENNSYLVANIAN PERIOD 651 



tions. Nearly all the genera, and about one-third of the species, 

 were identical. 



Climatic Implications of the Coal-plants 



What suggestions do the Coal-plants give relative to the atmos- 

 pheric conditions under which they grew? Two partly antago- 

 nistic views relative to these conditions have been held. The older 

 one regards the thick deposits of coal as evidence of a very luxuriant 

 growth of vegetation, which in turn has been thought to imply a 

 warm, moist atmosphere, heavily charged with carbon dioxide. 

 The great size of many of the trees, the succulent nature of many of 

 the plants, and the abundance of aerial roots, are appealed to as 

 evidence of mildness of climate, while the absence of rings, and, 

 above all, the great geographic distribution of the floras in relative 

 unity throughout very diverse latitudes, point strongly to equa- 

 bility, an equability which was more pronounced early in the 

 period than toward its close. This view was formerly about the 

 only one, and still predominates, and it is clear that it has much 

 support. 



The alternative view which has grown up in recent years postu- 

 lates less warmth and moisture, and more diversity; in other 

 words, a somewhat nearer approach to the present conditions. It 

 assumes, however, a somewhat higher percentage of carbon dioxide 

 than now, and a climate milder and more uniform than that of 

 to-day. The basis of this view is found in the following consider- 

 ations: (1). Great thicknesses of coal do not necessarily imply 

 rapid accumulation, any more than great thicknesses of limestone 

 do. Given favorable conditions of preservation, a slow growth 

 will produce great thicknesses. (2). At present the accumulation 

 of peat, the nearest analogue of coal formation, is most favored 

 in cool climates, and is taking place chiefly in high latitudes. (3) . 

 The dominant plants of the coal flora had narrow leaves with their 

 breathing pores confined to deep furrows on the under side, devices 

 common to plants of dry regions. (4). The trees had unusually 

 thick corky bark, as though protection from external conditions 

 was needed. 



The thickness of the bark, and the form and structure of the 



