REVISED PRINCIPLES OF CORRELATION 475 



record, this can have been preserved only in the inaccessible floors of the 

 permanent oceanic basins where deposition presumably continued unin- 

 terruptedly. 



There seems, therefore, no ground for the belief held by some that the 

 expansion of waters in the epicontinental basins made them areas of 

 stimulated organic modification. If they had been, their deposits must 

 have been filled with such a wealth of intergrading mutations as to render 

 the efforts of the systematic paleontologist positively futile. 



Proceeding, it once more seems certain that what we call fossil species 

 and genera must have become extinct during, rather than before, the in- 

 tervals that separated the periodic marine invasions of the continental 

 seas. In other words, the final extinction of particular species or genera 

 must, as a rule, have been accomplished in the oceanic basins. There is, 

 therefore, no general warrant for the paleontologist's tacit assumption 

 that the apparent and perhaps actual last appearance of a species or genus 

 in the deposits of one continental basin or province marks at the same 

 time also its universal extinction. We know of innumerable instances of 

 recurrence — hence of survivals — in the same and other basins or prov- 

 inces. And the same is true, though in a smaller degree, as regards 

 earlier occurrences than were recognized in our standard of comparison. 



However, under proper limitations, the first and last appearances of 

 particular species and genera in a given province are usually excellent 

 correlation data within such province ; but be sure that the occurrence is 

 the first or the last, as the case may be. Thus confined, the instances 

 wherein these definitely located faunal appearances or invasions often 

 fail geographically occur chiefly among those organisms, especially corals 

 and Bryozoa, whose distribution is largely dependent on transportation 

 of larva by shore currents. Yet, so far as they go, these very animals are 

 of extraordinary value in correlation, for distribution by currents insures 

 rapidity of migration and corresponding accuracy of time determinations 

 based on their testimony. 



But I am in danger of .being misunderstood. My reference to the areal 

 limitation on the one hand and the extraordinary correlation value on 

 the other of organisms owing their distribution to current action includes 

 only those that are free-swimming in their larval stages and bottom 

 dwellers thereafter. It does not include the truly pelagic forms, like 

 graptolites, many pteropods, and certain cephalopods. For some reason — 

 perhaps because of greater stability of environmental conditions — these 

 pelagic organisms commonly seem to have been less mutable, hence longer 

 lived as species and genera, than other marine animals that existed under 

 more changeable conditions. 



