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GEOLOGICAL BIOLOGY. 



A GEOLOGICAL TIME-SCALE, PREPARED FOR THE COMPARATIVE 

 STUDY OF THE LIFE-HISTORY OF ORGANISMS. 



The periods are taken as the smallest divisions of time which can be uni- 

 versally recognized, and hence it is assumed that they are units of equal length. 

 This assumption probably exaggerates the length of the more recent periods. 



Importance of a Standard Time-scale. — For the comparative 

 study of the history of organisms this time-scale may be used 

 irrespective of estimates of actual length of time represented 

 by each period. 



The division of the eras into twenty successive periods is 



