464 PENNSYLVANIAN PERIOD 



many of the trees, the succulent nature of many of the plants, and 

 the abundance of aerial roots, are appealed to as evidence of mild- 

 ness of climate, while the absence of rings in the wood, and, above 

 all, the distribution of similar floras through diverse latitudes, point 

 strongly to an equable climate, especially in the earlier part of the 

 period. It is clear that this view has much support. 



The alternative view postulates less warmth and moisture, and 

 more diversity; in other words, a 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 

 considerations: (i) Great thicknesses of coal do not necessarily 

 imply rapid accumulation, any more than great thicknesses of lime- 

 stone do. Given favorable conditions of preservation, 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 had narrow leaves with their breathing pores con- 

 fined to deep furrows on the under side, devices common to plants of 

 dry regions. (4) The trees had thick corky bark, as though protec- 

 tion from external conditions was needed. 



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

 leaves, give a xerophytic aspect to the overgrowth made up of 

 lepidodendrons, sigillarias, calamites, and cordaites. This is not 

 the case with the undergrowth, but this would not be expected of 

 shaded plants. The force of the inference from the xerophytic 

 aspect of the overgrowth is much weakened by the fact that the 

 vegetation of undrained swamps and bogs has many xerophytic 

 features. It is clear that a more critical study of the problem is 

 needed before a final conclusion concerning the climate of the period 

 is reached. 



Land Animals 



So far as the evolution of air-breathing vertebrates is concerned, 

 this is one of the most important periods in geological history. 1 

 Amphibians, insects, spiders, scorpions, and myriapods, lived on the 

 land at this time. The amphibians are perhaps of chief interest, 

 for they were the first land vertebrates. 



The rise of amphibians. Tracks attributed to amphibians are 

 found in the Devonian and Mississippian, but in neither of these 



1 Williston. Faunal Relations of Early Vertebrates, Jour. Geol., Vol. xvii, p. 389. 



