484 E. O. ULRICH CORRELATION OP THE STRAND-LINE 



genus has as many more chances to survive as it comprises other con- 

 temporaneous species. 



We may conclude, then, that generic alliances are of very doubtful — 

 perhaps it would be better to say indefinite — value in correlation. Prop- 

 erly checked by other criteria, such identifications are useful in approxi- 

 mate correlations between distinct provinces. Within the same province, 

 however, where more definite results are desirable, they are to be used 

 only when organic remains are few, or when the species as well as the 

 genus is identified. 



For much the same reasons stages of evolution are often of uncertain 

 and always indefinite value in correlation. We do not know how fast or 

 slow evolution progressed. Certainly the apparent rate varied greatly in 

 different instances and at different times. Nor can we always decide 

 whether so-called primitive characteristics are really so chronologically. 

 Besides, a great deal of information is required before we can be sure that 

 the presence of "primitive characters" are not due to reversion or arrested 

 development. 



In this connection I am reminded of an instance wherein a paleontolo- 

 gist of the highest standing established a new genus on the ground that 

 its genotype possessed structures indicating a more advanced stage of 

 development than prevailed in the half dozen other species left in the 

 older genus to which it also had been referred. Unfortunately, however, 

 stratigraphic investigations proved that the supposedly more mature type 

 was really much older than the allied other species which were thought 

 to be the more primitive of the two. 



As a rule, I doubt whether correlation by stages of evolution is a de- 

 pendable criterion except in such cases in which the life history of a genus 

 or family is reasonably well worked out. Then, perhaps, we might go so 

 far as to decide that a stratigraphically unplaced new species which has 

 been found to agree best with species prevailing, say in the Hamilton 

 stage, is of Devonian age. But even then I should regard the assignment 

 as provisional. 



AGE DETERMINATION SOLELY BY PERCENTAGE OF SPECIES KNOWN 



ELSEWHERE 



Authors sometimes decide the age of formations by means of faunal 

 comparisons confined to those species which the unplaced formation holds 

 in common with standardized formations elsewhere. The other species 

 of the fauna, even though they comprise the greater part of it, are wholly 

 ignored. An example of this method is contained in a manuscript re- 



