CLARKE— THE AGE OF THE EARTH. 281 



the ages before the beginning of the Hfe record in the rocks. How 

 often the student of the past of the earth has exclaimed at the wonder 

 that Man came through to his excellence, in a world permeated with 

 ever-increasing conditions of degeneration. 



III. 



With such propositions as the foregoing we are confronted by an 

 impressive requirement of time necessary to the development of life 

 on the earth. It is a requirement that seems to roll back and ever 

 backward into the undifferentiated ages of our planetary history. It 

 is a magnitude that takes on proportions before which the outstanding 

 estimates of time based on processes of rock building would seem to 

 dwindle, and it partakes more and more of the magnitudes in which 

 the radiologist has been wont to speak. The question for us now is 

 whether our present knowledge affords any basis for an estimate or 

 calculation of this time or any part thereof into a concrete expression. 

 If it were possible to estimate by any or all approaches, the length of 

 the life of a single extinct species in any part of the world, there 

 would then lie a possibility of determining what fraction this given 

 quantity might be of the whole. For more than two generations the 

 evidence has been sought, paleontologists endeavoring first to establish 

 the endurance of a given or index species as the basis of a geologic or 

 stratigraphic element — a zone. 



Into the discussion of the Zone — its meaning in time and space — has 

 entered a very long list of eminent names in the science. The Zone 

 has been looked upon as a sedimentary element in which a datum 

 species slowly coming to its acme suddenly culminates and abruptly 

 disappears ; as such sedimentary unit in which not a species, but a 

 mutation, or an entire fauna rises and falls. To Oppel the Zone was 

 a space-unit. Buckman has embodied the time conception of the 

 Zone in the word hemera. The double combination of time and space 

 makes a hiosone. The time unit has also been termed scecuhim by 

 Jukes-Brown, moment and phase by the International Geological 

 Congress. In the recent summary of these expressions and their in- 

 terpretations as given by Diener, in order to determine a proper basis 

 for his discussion, he employs the term Zone for the spatial, that is 



