PRlNCirLES OF STRATTGRAPHIC CORRELATIONS 507 



denied. On the other hand it must be admitted that this evidence, like 

 any other, is liable to misinterpretation. The fault lies not with the 

 fossils — they are always right — but with us. Local faunal associations, 

 whether fossil or living, are but imperfect snapshots of a long, continuous, 

 and infinitely complex process of mutation; and the exact determination 

 of the relations of the various pictures to each other becomes a relatively 

 simple matter only when the general plan of faunal migration has been 

 worked out. This plan even is not uniform in its operation, but changes 

 from time to time with the varying movements of the land and sea. To 

 meet this difficulty the paleontologist must be wary and unremittingly on 

 the lookout for exceptions to the accepted rules. He must also realize 

 that all rules are but temporary, and that the exceptions may presently 

 reverse the rules. This may seem discouraging to the paleontologist, but 

 after all there is no cause for misgivings as to the present and ultimate 

 value of organic evidence in correlation. Though the working plans of 

 yesterday are riddled with exceptions today and those of today may be 

 similarly modified tomorrow, the comforting fact yet remains that we 

 are steadily progressing. 



I have strong convictions respecting the great possibilities of corre- 

 lation by a judicious application of organic criteria. Their greatest 

 value in this connection arises from the demonstrable fact that, as a 

 rule, the migration, and to a considerable extent also the evolution, of 

 species, however slow, is yet relatively rapid as compared to the incon- 

 ceivable length of geologic time. As to marine faunas, with which the 

 student of Paleozoic stratigraphy is chiefly concerned, their migrations, 

 when not prohibited by physical barriers, usually proceeded with such 

 rapidity that their progress can not be expressed in recognizable units of 

 the geologic time scale. Hence, unquestionable correlations by fossil 

 evidence, fully checked by physical criteria, may be said to establish, 

 so far as the practical purposes of geology are concerned, the essential 

 contemporaneity of the beds so identified. The trend of the whole mass 

 of correlation data in hand gives unqualified support to this contention. 

 Its probability is raised almost to complete demonstration by such ob- 

 served instances of recent migration of species as that of Littorina littorea 

 on the Atlantic coast (see page 295). 



Summary statement of principles. — With the citation of a few exam- 

 ples the principles of correlation by fossils may be summarily stated as 

 follows : 



(1) Systematic paleontology. — Systematic paleontology without a 

 stratigra'pliic basis is regarded as an absurdity. 



