GENERAL PRINCIPLES. 



281 



dinates them to the higher principles which form its own character- 

 istic, and thus the social organism becomes ever higher, more complex, 

 and varied. 



So it is also in geologic history. When the dominance of any class 

 declines at the end of an age, the class does not disappear, but remains 

 subordinate to the next succeeding and higher dominant class, and the 

 organic kingdom, as a whole, becomes successively more and more com- 

 plex and varied. This is graphically represented by the accompanying 

 diagram, in which we have five successive ages determined by the cul- 

 mination of as many successive dominant classes. 



Fig. 253.— Diagram illustrating the Rising, Culmination, and Decline of Successive Dominant 

 Classes, and the Increasing Complexity of the whole. 



5. There are two modes of determining and limiting eras, ages, peri- 

 ods, etc., in geology, viz., unconformity of the rock-system and change 

 in the life-system. In written human history, the divisions of time are 

 recorded in separate volumes, chapters, sections, with boards or blank 

 spaces between. These divisions in the record ought to correspond to 

 conspicuous changes in the character of the most important contents. 

 So, in the history of the earth, the rock-systems, rock-series, rock-forma- 

 tions, are volumes, chapters, sections, respectively, more or less com- 

 pletely separated from each other by unconformity, indicating blanks 

 in the known record ; and the most important changes in the contents, 

 i. e., in the life-system, ought to, and usually do, correspond with the 

 unconformity of the rock-system. But if there should be (as there is in 

 some limited localities) a discordance between these two, we should fol- 

 low the life-system rather than the rock-system, the contents rather 

 than the artificial divisions of the record. 



6. As in human history there is a general onward movement of the 

 race, and yet special modifications in character and rate in each coun- 

 try ; so in geology there has been a general march of evolution of the 

 whole earth and the organic kingdom, and yet special modifications in 

 character and rate in each continent, and to a less extent in different 

 portions of the same continent. The great eras, ages, and periods, be- 

 long to the whole earth alike, and are the same in all countries, but the 

 epochs and the smaller divisions of time, though similar, are probably 

 not contemporaneous in different countries. This fact has probably 

 been too much overlooked by geologists. The term homotaxy is used 

 to express identity in the stage of evolution, as synchronism is used j 

 for identity of time. 



Great Divisions and Subdivisions of Time. — Eras.— It is upon these 



