258 



HISTORICAL GEOLOGY. 



ture as a whole. This is represented in the diagram (Fig. 

 162), in which A B is the course of time, and the rising 

 and declining curved lines the successive culminations of 

 five great dominant classes of animals. 



Fie. 162. Diagram illustrating ihu rising, culmination, and decline of successive 

 dominant classes, and the increasing complexity of the whole. 



5. As in human history, while the whole race, or at 

 least Christendom, advances together, and yet there are 

 special differences in rate or direction of advance peculiar 

 to each country and constituting its national civilization ; 

 so also in geology, while the whole earth and its inhabi- 

 tants in every part are affected with a common onward 

 movement in evolution, yet there are special differences 

 in rate or direction of evolution, characteristic of each 

 great division of the earth. The most marked example 

 of this is Australia, which is far behind other continents 

 in the march of evolution. 



6. In a written human history, there are two ways in 

 which we may judge of the subdivisions, viz. : 1. By the 

 artificial divisions of the record, i. e., volumes, chapters, 

 etc.; or, 2. By the nature of the most important con- 

 tents. In a well-written history these will correspond 

 with each other. So also in geology there are two modes 

 of separating and determining the limits of the great 

 divisions and subdivisions of earth-history, viz. : 1. By 

 unconformity of the rock record ; or, 2. By the change 

 in the organic contents. These usually correspond, be- 

 cause they are produced by the same cause. But, if 

 there be a discordance (as there may be locally), then we 

 follow the changes in the organic forms, rather than the 

 unconformity of the rocks, 



