GEOLOGY — WILLIS . 211 



series of processes, were first used as means of correlation. The 

 "Old Red" of Scotland was correlated with that of New York 

 because they were both red sandstones. The comparison of spe- 

 cies, genera, or faunas is the means now relied upon. It has the 

 advantage over stratigraphic evidence that in the course of organic 

 evolution there has been no repetition, but it is not without the diffi- 

 culties which result from the dependence of life upon conditions that 

 follow from successive processes. The differentiation of faunas, their 

 migration, their appearance in new districts, their retreat, reap- 

 pearance, and extinction, are not simply facts of life history ; they 

 are conditioned by environment and depend upon the transiently 

 fixed or shifting geographic conditions. The advent of closely 

 related species in districts remote from one another may or may not 

 be contemporaneous. The episode of migration, commonly neglected, 

 may or may not be negligible, and its value can not be reckoned until 

 we have compared the geographic histories of different lands by 

 the aid of other criteria of correlation. The interdependence of 

 stratigraphy and paleontology follows from their common relation to 

 the progress of geographic variation, back of which lies the general 

 cause, earth-warping. 



A broader basis of correlation has been sought by Chamberlin 

 in world-wide physical phenomena, immediately in fluctuations of 

 climate, more remotely in variations of continental extent, the under- 

 lying assumption in each case being a periodicity of activity in earth- 

 warping. This hypothesis is a relatively recent contribution to the 

 philosophy of geology and stands to be tested. Not only it, but also 

 the data of paleontology, will be tried when we are able to place in 

 evidence the parallel history of continents. Correlation is thus both 

 a method of interpretation and an object of investigation in the pro- 

 posed research. 



Deli7icatio7i and discussion. — The contribution which may be made 

 to science should be embodied in maps and reports. The available 

 maps should be issued from time to time in the progress of the work 

 in the form of atlases, each of which should contain related deduc- 

 tions, and with each atlas should be issued a volume of discussion 

 summarizing the evidence and giving the grounds of inference with 

 qualifications. 



In assembling the results on maps, the methods adopted should pro- 

 vide for adequate expression of the range of inference from doubt to 

 conclusion. Each map will represent the condition of a given region 

 during a certain geologic age. Areas known to have been land and 

 sea will be distinguished from those supposed to have been one or 



