5o6 



HISTORICAL GEOLOGY 



eastern United States, where the continent of Appalachia formerly 

 stood and of which the Piedmont Plateau is a part. During this 

 era the seas varied greatly in extent and in position. In only a few 

 areas, notably in the Appalachian trough and in the Great Basin 

 region, did they persist through the greater part of the Paleozoic. 



Evolution and Extinction of Life. — A study of the accompanying 

 table (Fig. 479) brings out some important points concerning the life 



Fig. 479. 



Table showing the distribution and relative abundance of the life 

 of the Paleozoic. 



of the era. It is seen that certain classes began, or at least are first 

 known, in the earlier periods, culminated in the later periods, and 

 then after several periods of struggle became extinct. If one were to 

 make a careful study of each of these classes it would be found that 

 the genera of each class had a shorter life than the class as a whole, 

 and that the species had a still briefer one. It would also be found 

 that striking evolutional changes took place during their life histories. 

 It is also to be seen that some classes gradually increased in impor- 

 tance throughout the era, while others were inconspicuous in the 



