^ STRATIGRAPHIC TAXONOMY 575 



quence of events and in drawing the time boundaries every criterion, be 

 it physical or organic, should be employed. In correlation each set of 

 criteria is found to have its limitations, and none is alone sufficient for 

 the purpose of the taxonomist. Nor can we justly say that the paleonto- 

 logic criteria are more important than the lithologic or diastrophic. The 

 organic criteria doubtless are the most useful and in many cases are by 

 themselves determinative where the physical evidence by itself may be 

 quite incompetent. But as each method of correlation commonly re- 

 quires the corroboration of the other criteria before a detailed final deter- 

 mination of the geologic history of a given area becomes possible, and as 

 each fills its own place and carries out indispensable functions, they are 

 all equally important. As a rule the several methods are logically sup- 

 plementary. In studying a section the fossils give us the general position ; 

 the lithologic criteria checked by fossil evidence narrows the limit of 

 possible error and the diastrophic criteria finally establish the exact loca- 

 tion of the boundary sought. 



Regarding such ideas as "shifting'' and migration of faunas and floras, 

 providing their conception contemplates time consumed in migration of 

 sufficient duration to be definitely expressed in the geologic time scale, or 

 if they are promulgated with a view to show the futility of endeavors to 

 establish the contemporaneity of geographically separated occurrences, 

 such views, it seems to me, are practically groundless. They do not suffi- 

 ciently take into account the immensity of geological time. I tried to 

 illustrate this with the case of a living shell, Littorina littorea (page 

 295), which has extended its range in the past fifty jears so fast that at 

 the same rate it might encircle the globe many times before the smallest 

 stratigraphic unit now thought correctable over wide areas could be 

 laid down. The groundlessness of the fear that shifting of faunas may 

 seriously affect the accuracy of correlation by fossils is all the more ap- 

 parent if my belief that marine faunas were modified and developed only 

 to a very limited extent in the continental seas is accepted. (See pages 

 495 to 501.) In my opinion, the evidence as a whole points very strongly 

 to the conclusion that faunas passed from the oceanic basins into and 

 about in the continental seas so rapidly that their appearance in New 

 York and central Kentucky or Tennessee is to all intents and purposes 

 of stratigraphic correlation practically simultaneous. 



The presence or absence of a fauna in a given continental sea at a cer- 

 tain time is wholly dependent on whether or not its migration is opposed 

 by physical obstacles. When conditions for expansion of range were 

 favorable, then the fauna instantly took advantage of them ; if they were 

 not propitious, then the organisms either succumbed or they retreated to 



