76 GEOLOGICAL BIOLOGY. 



tain genera which are characteristic of the fauna and flora 

 for a long period, and thus serve as arbitrary marks of 

 these great periods. The individual continuing beyond a 

 certain specific zone in one section does not interfere with 

 the general law that there are grand divisions of time which 

 are characterized by peculiar types of organisms. 



Although we cannot, in the present state of knowledge, 

 draw sharp lines which shall be universal, between the for- 

 mations, or between the several species represented in them, 

 it is convenient to recognize these systems, and in each 

 country the lines can be arbitrarily fixed, and the sub-divi- 

 sions locally recognized. 



SUMMARY. 



Reference has been made to the difference between the 

 history of the organism (Ontogeny) and the history of organ- 

 isms (Phylogeny). It has been shown that there is a natural 

 history of the development of the individual, and that there 

 may be a history of organisms as a whole — a history in which 

 all the species of the same kind are but as a unit in a great 

 complex of organic life which has evolved with the geological 

 ages. In this latter history time and the conditions of en- 

 vironment have played very important parts; but ordinary 

 time-scales are practically useless, because they are not divided 

 into long enough periods, and because they do not reach back 

 far enough, A special time-scale was needed. This has 

 been constructed by an analysis of the classification of rock 

 formations. In this analysis we have seen that progress of 

 science is as much a progress of ideas as it is an increase of 

 known facts; that the accumulation of confirmatory facts has 

 followed rather than preceded the formulation of speculative 

 theories ; that the theories about the earth have dominated 

 in each proposed scientific classification of facts, and thus in 

 the formulated science of each period. 



The result of the analysis emphasizes a few laws, which 

 may be stated as established, regarding the chronological as- 

 pects of the rocks. 



First. There is a natural succession in the original forma-^ 



