520 W. D. MATTHEW CORRELATION BY FOSSIL VERTEBRATES 



vertebrate evidence is important. These formations are peculiarly diffi- 

 cult to correlate. Difference in facies due to different conditions of en- 

 vironment and deposition results in wide diversity of contemporary strata 

 and faunas. This is also true of marine formations. But terrestrial 

 faunas are much more provincial than marine, especially during and fol- 

 lowing epochs of submergence and isolation of the continental masses, 

 where the marine faunas are most widespread and cosmopolitan. Direct 

 correlation through identical species is possible only within limited re- 

 gions, and in most instances, where there seems to be a correspondence of 

 species, a rigorous investigation is necessary to find out how far this is 

 merely due to hasty identification of fragmentary material and not valid 

 for precise correlations. 



The only methods practicable when dealing with fossil mammals of 

 two widely separated regions are, first, the equivalence in stage of evolu- 

 tion of related or corresponding species; second, the first appearance of 

 identical genera new to both regions; third, the extinction of identical 

 genera or families common to both faunas, and, fourth, the degree of 

 difference between the extinct and the modern faunas of the two regions 

 compared. All of these methods are more or less untrustworthy unless 

 they are combined with a knowledge of the evolutionary history and dis- 

 persal of each group of species or genera compared, which will in varying 

 ways modify the interpretation of the data regarding its distribution. 



Summary 



Correlation difficulties are of two kinds — one practical, the other theo- 

 retical. 



The practical difficulties are the scarcity or fragmentary character of 

 the fossils, doubts as to their true stratigraphic position, as to the accu- 

 racy of identification. 



The theoretical difficulties may be summed up as the distinction of 

 homotaxis from true synchronism. The assumption that closely related 

 equivalent or identical species appeared and became extinct at the same 

 time in all parts of the world has long been recognized as theoretically 

 untrue. Yet in practice we base our correlation work chiefly on this false 

 assumption. It is true that the error is negligible in all broad correlation 

 work. But in the more precise correlations which are now being worked 

 out it is not negligible, and the discrepancies may in some cases be very 

 considerable. 



In dealing on the above principles" with Tertiary mammal faunas the 

 following conclusions must be emphasized: 



