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HISTORICAL GEOLOGY 



Problems of the Permian 



No other period of the earth's history offers so many unsolved 

 problems as the Permian. These problems have to do with the cli- 

 mate and the life of the period. 



(i) Why was the Permian so fatal to marine life? During this 

 time the invertebrate life, for the most part, either became extinct or 

 was much modified in important structural features. The impov- 

 erishment of the life is shown in the estimate that the number of 

 Permian species was only two per cent, of that of the combined 

 Mississippian and Pennsylvanian. One factor in the extinction of 

 such a large percentage of species was doubtless the emergence of 

 the continent and the consequent withdrawal of most of the epicon- 

 tinental seas. This drove the life of the warm, shallow seas into 

 the coastal waters of the ocean, where it was not only obliged to com- 

 pete with other species but was compelled to live under conditions to 

 which it was not accustomed. Such a radical change in environment 

 was, doubtless, fatal to many species, and it is consequently not 

 surprising that a large number disappeared. The faunas in the 

 restricted epicontinental seas were crowded, and competition was 

 severe. Where such seas persisted, as in India and California, the 

 change in the fauna was gradual and no satisfactory dividing line can 

 be drawn. 



A further result of the elevation of the continents (or withdrawal 

 of the seas) was probably the changing of the position of the ocean 

 currents, which were forced to take new courses. The extinction of 

 some of the plant food upon which the animals of the epicontinental 

 seas ultimately depended would have had a marked effect on the life. 

 These and other causes may have combined to bring about the sweep- 

 ing changes in the invertebrate life at the close of the Paleozoic. 



The land vertebrates did not suffer as did the invertebrates. This 

 may have been due (i) to the greater land area over which they could 

 spread and the greater variety of conditions open to them ; (2) to the 

 greater variety, or more suitable varieties, of plants and insects which 

 they could use for food ; or (3) to their better organization. During 

 this time, so fatal to the marine fauna, the amphibians continued in 

 abundance and the reptiles became supreme. 



The plant life also suffered a great change : the important Carbon- 

 ire ions groups became extinct, or nearly so, and their places were 

 taken by plants of a more modern type. Whether this was due to the 



