736 HISTORICAL GEOLOGY. 



group tliat is represented among, and make, modern coral reefs. Even the 

 old straight Nautiloid, the Orthoceras, had its later species. 



The Insects lost, as has been said, a Paleozoic feature at this time ; but 

 the tribes are still the same as before in their more fundamental characters. 

 Fishes, although their period of culmination had passed, still continued under 

 the tribes of Ganoids and Dipnoans. Amphibians and Reptiles held on, and 

 the latter became the ruling life of Mesozoic time. So it was with the 

 greater part of the tribes of the Paleozoic. There was no break in the stream 

 of life, but for the most part only seeming interruptions ; and many of these 

 owe their prominence in geological history to the culminations and declines 

 of types that were in progress. 



But it was an epoch of relatively abrupt change ; and if chiefly due to the 

 progressive evolution of new species, as has been urged by some geologists, 

 there must have been for the result a great acceleration in such changes in 

 consequence of the physical conditions produced by the orogenic disturbance. 

 But the orogenic movements were local, and the biologically transforming 

 effects from such a cause should have been confined to the countries where 

 these movements were in progress. The universality and abruptness of the 

 disappearances cannot therefore be so explained. Very much is left for the 

 destructive effects, direct and indirect, that is, the exterminations attending 

 the mountain-making. 



The causes of the exterminations suggested by the changes are two. 

 (1) A colder climate over the land, and colder waters in the extra-tropical 

 oceans ; for the emergence of the eastern semi-continent of !N"orth America and 

 of large lands in the other continents could not fail to lower somewhat the 

 temperature of the whole globe. With a lower temperature, the currents from 

 the north sweeping along the coasts would have been destructive to the marine 

 species living in the waters. (2) Earthquake waves produced by orogenic 

 movements. If North America from the west of the Carolinas to the Mis- 

 sissippi valley can be shaken in consequence of a little slip along a fracture in 

 times of perfect quiet, and ruin mark its movements, incalculable violence 

 and great surgings of the ocean should have occurred and been often repeated 

 during the progress of flexures, miles in height and space, and slips along 

 newly opened fractures that kept up their interrupted progress through thou- 

 sands of feet of displacement. The Acadian upturning took place on the 

 ocean's border ; and the Appalachian was not far distant from it. Arkansas, 

 moreover, added to the extent of the belt of disturbance. Under such circum- 

 stances the devastation of the sea border and the low-lying land of the period, 

 the destruction of their animals and plants, would have been a sure result. 

 The survivors within a long distance of the coast-line would have "been few. 

 The same waves would have swept over European land and seas, and there 

 found coadjutors for new strife in earthquake waves of European origin. 

 These times of catastrophe may have continued in America through half of 

 the following Triassic period ; for fully two thirds of the Triassic period are 

 unrepresented by rocks and fossils on the Atlantic border. 



