140 THE OLD RED SANDSTONE. 



period during which vertebrate life made its decided appear- 

 ance on our planet, and during the continuance of which 

 several new distributions of sea and land were effected. We 

 say new distributions of sea and land, for there is no other 

 way of accounting for the differences that exist between its 

 lower, middle, and upper portions without supposing that 

 they were deposited in seas of different depths, and in seas 

 that derived their sediments from different directions. And 

 as these varying distribuions of sea and land necessarily 

 imply variations in climate and external conditions, we can 

 readily perceive how the plants and animals of the lower 

 portion differ from those of the middle, and these again 

 from those of the uppermost division. Nature is incessant 

 in her operations, and while the system of Waste and Re- 

 construction, described in our Sketch No. 2, endures, new 

 distribution of sea and land will be brought about in the 

 course of ages, varying conditions of climate will be effected. 

 and under the new conditions, some plants and animals 

 will shift their ground, some will flourish more luxuriantly, 

 and others again become altogether extirpated. But this^is 

 not all : under these ever-varying conditions, and as time 

 rolls on, some forms of life seem to run their appointed 

 course and die out, and other and newer forms, in the 

 course of creation, seem to make their appearance. It ig 

 thus that some forms of life are peculiar to the Old Eed 

 Sandstone that is, do not occur in earlier systems, and are 

 not found beyond the close of the period. Many forms oi 

 coral, several genera of shell-fish, some trilobites, the gigan- 

 tic crustaceans, pterygotus arid stylonurus, the cephalaspis. 

 pteraspis, coccosteus, pterichthys, and other fishes, have 

 never been detected beyond the limits of the Old Red' for- 

 mation. They came in during the system, and died ou1 

 before its close; thus implying not only long lapses oJ 

 growth, and reproduction, and decay, but an onward marcl] 



