246 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



The interpretation of conditions from the presence of plants showing 

 xeromorphic structures, in Pennsylvanian or Permo-Carboniferous time, is 

 thus rendered very uncertain. Especially in the Permo-Carboniferous time, 

 with its continuous approach to aridity, the interpretation is complicated by 

 the evident close juxtaposition of a great variety of plant habitats. The need 

 for caution in this particular is well illustrated in Spalding's discussion of 

 the distribution of plants in an arid habitat.^ He shows clearly how purely 

 aquatic plants assembled along water-courses may exist in close association 

 with a purely desert vegetation, and it is evident that in such chance accumu- 

 lations as might easily occur the fossilized remains might lead to very con- 

 fusing and erroneous results if not correctly interpreted. 



In this connection, David White says:^ 



"Irregular temporary reductions or withdrawals of the water cover, possibly 

 seasonal or perhaps less frequent, are, in the writer's judgment, causally related 

 to the ordinary type of lamination of much of our coal, and the sheeting of the 

 latter by fragments of ' mineral charcoal ' (' mother of coal '). To their occurrence 

 is probably due also the development of xerophytic and water-storage devices for 

 the protection of so many of the coal plants of the Carboniferous swamps. Such 

 periods of water reduction and evaporation appear generally to have been attended 

 by concentration of the hydrocarbon solutes resulting from the putrefaction 

 process in the form of paste, which now constitutes the jetlike 'binder' of the 

 coal. Conversely, the alternate periods of rise of the water-level and the attend- 

 ant dilution of the water cover favored to some extent not only the extraction, 

 and, in cases of flushing, the removal of some of the putrefaction products from 

 the upper part of the peat-forming debris, but also promoted the oxygenation 

 and, consequently, the revival of decay wherever the asepticity was neutralized." 



Turning to another phase of biologic evidence, Schuchert^ says: 



"A climatic change naturally must affect the land life more quickly and pro- 

 foundly than that of the marine waters, for the oceanic areas have stored in them- 

 selves a vast amount of warmth that is carried everywhere by the currents. 

 The temperature of the ocean is more or less altered by the changes of climate, 

 be they of latitude or of glaciation. The surface temperatures in the temperate 

 and tropical regions, however, are the last to be affected, and only change when 

 all of the oceanic deeps have been filled with the sinking cold waters brought 

 there by the currents flowing from the glaciated area. We therefore find that 

 the marine life of earlier Permic time was very much like that of the Coal Meas- 

 ures, and that it was not profoundly altered even in the temperate zones of Middle 

 Permic time (Zechstein and Salt Range faunas). Our knowledge of Upper 

 Permic marine life is as yet very limited and will probably always remain so 

 because of the world-wide subtraction of the seas from the lands at that time. 

 It was a period of continued arid climates, and the marginal shallow sea pans 



•Spalding, V. M., Present Problems of Plant Ecology: Problems of Local Distribution in 

 Arid Regions, Amer. Nat., vol. 43, 1909. Reprinted in Annual Report Secretary Smith- 

 sonian Institution for 1909, p. 453. 



' White, David, Origin of Coal, Bureau of Mines Bull. 38, p. 64, 1913. 



' Schuchert, Chas., Climates of Geologic Time, in Huntington, The Climatic Factor as 

 Illustrated in Arid America, Carnegie Inst. Wash. Pub. No. 192, p. 279, 1914. 



