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University of California. 



[Vol. i. 



usually recognized as such. On the contrary, I regard it as the 

 type, as the best proof of the fact of critical periods, and as throwing 

 abundant light on the true character of such periods, and espe- 

 cially on the causes of the enormous changes in organic forms, dur- 

 ing such times. These four, then, are the great landmarks of time. 

 All other and lesser divisions are more or less local. If we recog- 

 nize the primary divisions, then these become the fixed points for 

 correlating the lesser divisions, as is now being so successfully done 

 by the U. S. G. S. in its admirable series of correlation papers. 



PROGRESSIVE CHANGES IN SUCCESSIVE CRITICAL PERIODS. 



It may be interesting to compare the successive critical periods 

 with one another and show in then! also progressive change or evo- 

 lution. 



1. Critical periods have gradually become shorter and shorter, 

 the changes in physical geography less and less, and therefore the 

 changes in organic forms, in so far as they are connected with these 

 latter, also less and less. The shortest in duration, the least in geo- 

 graphical changes and the least complete and sweeping in changes 

 of organic forms is the last. It is for this reason that it is not usu- 

 ally recognized. But this is exactly what we ought to expect from 

 a comprehensive survey of the whole history of the earth, i. e., from 

 the point of view of evolution. 



2. But on the other hand the effect of the introduction of new 

 dominant types in producing changes in the whole organic kingdom 

 has been steadily increasing. It was great in the introduction of 

 the great reptiles, much greater in the introduction of the Eutheria, 

 and by far the greatest of all in the introduction of man. Thus 

 this last critical period is not a whit behind the others in importance, 

 and therefore deserves no less than the others to be used as separa- 

 ting Primary Divisions of Time. 



3. The oscillations of temperature characteristic of critical peri- 

 ods have probably gradually increased with the course of time- 

 Local oscillations of temperature doubtless occurred frequently. 

 At critical periods they were both greater and more general, but I 

 suppose that glacial conditions at sea level in middle latitude regions 

 was reached but once, viz., in the last. This is illustrated by the 



