REVIEWS 



179 



tions. These conditions spread very slowly from east to west and, 

 therefore, have left a record oblique to stratigraphic lines (see Fig. i). 

 The environment of an organism, "the sum total of all its contacts 

 with the external world," determines to a great extent its structural 

 changes and the distribution of its kind. The equable, humid climate 

 and topographic uniformity of the typical (lower) Pennsylvanian pro- 

 duced an abundant though fixed food supply — vegetation. Since new 

 forms arise only through isolation (not necessarily geographic isolation) 

 the monotony of this environment acted as a repressive force, checking 

 the expansion of the amphibians and reptiles into new forms. "En- 

 vironmental monotony would result in the persistence of older and 

 simpler types, because the variants, possibly being constantly produced, 

 would not have a chance to develop." 



lexas, Okla. Kane 



MlSfiourlan 



Dunkard 



Monongahela 



Conem&Mgh 



Fig. I. — "Diagram illustrating in a schematic way the relative position of the 

 sediments formed under Permo-Carboniferous conditions. The land was rising from 

 east to west, but there was continuous sedimentation in the eastern region at the 

 western edge of the rising land of Appalachia. As the land rose slowly the red beds 

 spread toward the west, occupying relatively higher positions in the stratigraphic 

 column. It is difficult to illustrate the actual conditions in the diagram, because the 

 'red beds conditions' were advancing, but the wavy lines indicate the surface of the 

 ground relative to these conditions. In Pennsylvania and West Virginia deposition 

 was continuous during the conditions. In lUinois and Indiana deposition had ceased 

 by the time the conditions reached that far west; in Kansas, Oklahoma, and Texas 

 'red-bed conditions' reached the region in time to affect only the uppermost Paleozoic 

 deposits. The upper limit of the red-bed conditions is not known, and so the upper 

 limit of the wedge is indicated by a dotted Hne" (from Case, p. 192). 



Permo-Carboniferous conditions included a cool to cold, arid or 

 semiarid climate, resulting from deformation throughout various parts 

 of the world, the presence of volcanic dust in the air, and a diminution 

 of the carbon-dioxide content of the atmosphere. This made for a 

 great variety in environmental conditions and destroyed the repressive 

 bounds to vertebrate expansion. "The fauna, long restrained from 

 any expression of evolutionary tendencies, full fed, and in the vigor 

 of its youth, responded at once to the change, and new forms 

 appeared so suddenly as to be unheralded in the preserved remains." 



