3IO T. C. CHAMBERLIN 



(3) there are more vital questions than the simple Pleistocene or 

 non-Pleistocene age of man in America. 



Distinct foreshadowing of these points appeared in the confer- 

 ences and symposia. The most significant of these was a rather 

 distinct challenge of the trustworthiness of some of the criteria 

 in so far as applicable to the case in hand, though heretofore re- 

 garded as good; modifications of the accepted criteria in certain 

 vital aspects were suggested. This of course really means that 

 they are not criteria at all, as used, and will become such only 

 when properly revised. The stress of skepticism was thereby put 

 on the criteria; and so the trustworthiness of these comes to the 

 front as really the most vital feature of the case. 



Now the criteria originally used in determining the divisions 

 named Eocene, Miocene, Pliocene, Pleistocene, and Recent were 

 made to rest on the numerical proportions of living species repre- 

 sented in the respective formations, viz. : in the Eocene, less than 

 5 per cent living species; in the Miocene, 20 to 40 per cent; in 

 the Pliocene, more than half living species ; and in the Pleistocene, 

 about 95 per cent.' The species counted were chiefly, though not 

 exclusively, marine. As the rates of change of marine forms dift'er 

 from those of land forms and those of the land forms differ 

 among themselves, this numerical basis was found unsatisfactory 

 from the outset. As the real needs of the case developed, these 

 original criteria were abandoned more or less completely and are 

 now used only in a residual way in certain modified forms and in 

 certain special situations. So far as the Pleistocene is concerned, 

 the numerical paleontological criteria have been replaced to a 

 large extent by physico-dynamic criteria drawn from the great 

 ice invasions. This replacement constitutes a radical and signifi- 

 cant departure. It is a part of the general movement to make 

 the causal and world-wide agencies the basis of classification rather 

 than their products, which vary with regional conditions. 



For the close of the Pleistocene, in particular, the retreat of 

 the continental glaciers from the plains of Europe and North 

 America and the accompanying climatic effects are now the more 

 generally accepted criteria. The substages of the Pleistocene are 



' See Dana's Manual of Geology, fourth edition (1895), pp. 879-880. 



