58 GEOLOGY 



cally revolutionary, it is important to note, in view of beliefs once 

 current, that it was transitional, and not an abrupt substitution of a 

 new fauna for an old one. Paleozoic types lived side by side with 

 the later forms, though usually represented by new genera. This 

 overlapping and commingling of old and new clearly indicates the 

 gradation of the earlier into the later. The transition was very ex- 

 traordinary, however, in the apparent rapidity of its progress, and in 

 the extent to which it affected all classes. The fact that most of the 

 new forms were already present in the earliest Triassic indicates that 

 the transition was chiefly made earlier, in the Permian, as already 

 noted. The fundamental cause was with little doubt the readjust- 

 ment of the earth's surface to internal stresses, and the physiographic 

 and climatic changes consequent upon this readjustment. 



