304 STUART WELLER 



It is a universally recognized fact today, that assemblages of 

 organisms are intimately related to the environment in which 

 they live. With a change in the environment there will be 

 changes among the associated organisms. 



In no geologic province, such as the continental interior of 

 North America during Carboniferous time, whose history must 

 be considered as a unit, are the physical conditions of the whole 

 area identical at any given time. Neither are the conditions of 

 any one limited portion of the province, identical through- 

 out an entire epoch or chapter in its history. Local changes in 

 the physical conditions, and consequently in the local assem- 

 blages of organisms are continually in progress. A stratigraphic 

 classification of rock strata is based upon these local changes in 

 the sediments and their contained organic remains, and conse- 

 quently can be of but local significance. In such a classification the 

 profound physical changes which affect the whole geologic prov- 

 ince in its relations with adjacent provinces, are given no greater 

 importance than the comparatively insignificant local changes. 



A natural classification of strata must be a faunal classi- 

 fication in its broadest sense. It is based not merely upon the 

 identity or lack of identity of fossil species in the different local 

 formations, but upon the minute study of the relationships of 

 the assemblages of fossils in the successive zones of particular 

 sections, and upon the study of the geographic distribution of 

 species. All fossil species are either indigenous or exotic to 

 the geologic province in which they are found preserved. They 

 are either evolution species or immigration species, and the sud- 

 den appearance of exotic or immigration species in the fauna of 

 a geologic province, shows, as nothing else can show, that the 

 relationship between the province and its neighbors is undergo- 

 ing a readjustment. In the history of any geologic province 

 the distinct epochs or chapters must be limited by these periods 

 of readjustment. Oftentimes these periods cover a considera- 

 ble lapse of time and alternate with periods of quiet, in which 

 case the periods of change and the periods of quiet are most 

 naturally considered as distinct epochs. 



