336 T. C. CHAMBERLIN 



perhaps, than were witnessed by the preceding ten generations. 

 That there was a special biologic cause for this only intensifies 

 its importance as a subject of special study. It is such critical 

 events in history that should put us on our guard, for similar 

 critical events probably characterized all the past, as LeConte 

 so wisely urged on our attention. 



Co-ordinate with this a like radical change has taken place in 

 the rates of erosion, sedimentation, and the surficial aspects of 

 the land. These climacteric stages in the extinction of species, 

 in the introduction of species, and in surficial evolution furnish 

 stimulating grounds for revision of ideas and a reconstruction of 

 working tenets. They offer a point of departure of most sug- 

 gestive value. 



A very vital phase of this recent physico-dynamico-bio-psychic 

 combination is human art and industry. This becomes a very 

 important additional resource in discrimination. The evolution 

 of the arts and industries is usually far more rapid and rnuch 

 more sensitive to conditions than the evolution of biotic species. 

 It is in like degree a more refined criterion of historical progress. 



(3) The third line of approach is an endeavor to find in the 

 record of work done by the chief terrestrial agencies a basis for 

 the natural events that mark off the progress of the ages. These 

 agencies are more or less rhythmical in action and so reach climaxes 

 here and there that serve for dividing lines or points of reference. 

 To a large degree these agencies are the basal causes of the leading 

 events of the geologic record. As such basal causes they are 

 eminently fit for true criteria. Concretely, the physico-dynamic 

 agencies record themselves in changed relations of land and sea, 

 in changes of surface attitude, in changes of soil growth, of denuda- 

 tion, of configuration, and of sedimentation, as also in changes 

 of climate, and of most other environing features that condition 

 life. We have already remarked that glacial invasions and glacial 

 oscillations have already in large measure replaced the older cri- 

 teria on which are based the divisions of the Pleistocene Period 

 and its separation from the Recent Period. A greatly enlarged' 

 use of these newer physico-dynamic criteria is scarcely less than 

 imperative to conclusions that will stand the tests of the future. 



