WILLIAMS.] CONCLUSIONS. 269 



The undisputed correlations from one proviuce to another, as from 

 the European sections to those of New York, from the Appalachian to 

 the Mississippian provinces, or from either of these to the Acadian 

 province, rest entirely upon biologic evidence — coal beds and masses of 

 coral and crinoidal limestone are of biologic not geologic origin. Such 

 correlations are generally satisfactory so far as they pertain to the gen- 

 eral equivalency of systems or series ; but all attempts to correlate with 

 precision the limits of such divisions or to establish uniformity in the 

 subdivisions of two separate provinces has proved forced and artificial, 

 and the history of American geology shows that after the determina- 

 tion of the general equivalence of age, in matters both of classification 

 and of nomenclature, little attempt has been made to attain uniformity 

 with outside standards. Paleontologists have discussed the relations 

 between the fossil faunas of America and the European standards, but 

 the cases have been rare in which the differences have not been as con- 

 spicuous as the agreements. 



The principles involved in correlations made by use of fossils are 

 purely biologic and are intimately concerned with the laws of structure 

 and growth of the individual, with the effects of environment and geo- 

 graphical distribution, with the laws of heredity and evolution, and 

 with the laws of relationship of organisms to each other and to geo- 

 logic time. The discussion of these matters would be out of place here; 

 but it may be said that the great advance attained in the accuracy and 

 in the general methods of geologic correlation during the last twenty 

 years is mainly due to the changed concex>tions regarding the nature 

 of the organic species. 



The Cuvierian notion of species was entirely consistent with the no- 

 tion of sharply defined, uniform delimitations and u universal " forma- 

 tions. Each species was supposed to belong to one, and how it could 

 appear in two formations was not explained. The Darwinian notion of 

 species is not consistent with sharply defined lines in the classification 

 either of organisms or of formations. 



According to this notion the modification of organic form is conceived 

 as not an arbitrary matter, but as correlated with difference of environ- 

 ment and of genetic relationship, so that the lesser variations of spe- 

 cific form are of as great value to the modern paleontologist for pur- 

 poses of correlation as is the ideutity of species. Comparison of allied 

 species in the same genus exhibits to him the rate and direction of mod- 

 ification taking place in the genetic history of the genus, and in the 

 plastic or variable characters he finds a sensitive indicator of the 

 stage of development attained by the race when the particular indi- 

 vidual lived. Biological study shows him that fossils must contain in- 

 trinsic evidence of their geologic age independent of the formations in 

 which they were buried, and his chief work is to learn what this evi- 

 dence is and how to interpret it. To such evidence the final appeal 

 must be made in all cases of the correlation of geologic formations. 



