CHAPTER XX. 



THE LAWS OF EVOLUTION EMPHASIZED BY THE STUDY 

 OF THE GEOLOGICAL HISTORY OF ORGANISMS. 



Testimony of Vertebrates. The vertebrates might be used 

 with great force to illustrate the general laws of evolution. 

 No better example than the vertebrates could be selected to- 

 illustrate the fundamental law of the gradual inciease, in 

 differentiation and in rank, of the great classes of a branch in 

 the order of their successive appearance and dominance in the 

 geological formations. 



In the lowest system of stratified rocks, the Cambrian, no- 

 trace of vertebrates has yet been found. In the Ordovician 

 and Silurian only the lowest type of fishes, and they very 

 rare, have been seen. Fishes were abundant in the Devonian. 

 The Lower Carboniferous shows the first amphibians; and 

 large-sized and extinct types of amphibians prevailed in the 

 Carboniferous era. In this era also a few traces of true 

 reptiles have been found. In the Triassic the great Dino- 

 saurian reptiles were abundant on the land. In the Jurassic 

 the shallower seas swarmed with the Enaliosaursor sea-lizards, 

 and in the lower Jurassic (Lias) the flying reptiles infested the 

 air and culminated the reptilian domination of the Mesozoic 

 time. 



While reptiles were the masters of sea, land, and air, the 

 lower types of mammals the marsupials, and probably 

 monotremes began to appear in feeble representatives as 

 early as the Triassic, and in the Cretaceous birds, too, make 

 their appearance : though true birds in structure, they com- 

 pete with the flying reptiles in their use of reptilian teeth 

 for offence and defence. 



Remarkable and Extreme Evolution of the Mammals in the 

 Eocene. As we examine the earlier beds of the Tertiary rocks 

 we observe for the first time the dominance of mammals ; and 



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