492 E. O. ULRICII REVISION OF THE PALEOZOIC SYSTEMS 



geology. That a species or genus ceased to exist in some continental 

 basin certainly does not mean that its existence was everywhere terminated 

 at the same time. Such total and approximately sim:ultaneoiis extinction 

 probably occurred only in the case of locally evolved species whose geo- 

 graphic range was limited to a single continental basin. Other forms, 

 especially those invading from the oceanic basins, not to mention such of 

 these as were common to two or more of the permanent basins, doubtless 

 continued to exist after their local extinction in continental seas. In the 

 meantime physical barriers may have arisen prohibiting their return to 

 resubmerged former habitats, while other barriers may have been let down 

 permitting their subsequent appearance in other continental basins. 



Certain faunas, like the Spergen (see pages 298 to 303), are known to 

 have invaded the same general area four or five times, being entirely ex- 

 cluded during long intervening ages. That some earth deformation and 

 sea withdrawal preceded each of these appearances is indicated by slight 

 unconformities and sometimes in addition by clastic deposits. During 

 all this time, however, the fauna continued to live in its permanent habi- 

 tat, being confined to it when physical conditions were unfavorable, but 

 taking advantage of every opportunity to spread beyond it. 



In their permanent and normal habitats the indigenous faunas modi- 

 fied very slowly. It is only when conditions changed in these, and when 

 new f aunal elements were introduced, that the inherent tendency to slowly 

 modify was appreciably augmented. Comparing successive faunas in a 

 given section, they often appear to have changed abruptly and wholly at 

 frequent intervals. I do not refer to modifications merely in the local 

 expression of a given fauna, but to changes affecting the fauna as a 

 whole. Unless the plane of change represents a great stratigraphic 

 hiatus, these abrupt transitions, as will be shown presently, may be due 

 to a quite different cause then evolution. It is only by comparing suc- 

 cessive invasions from the same oceanic basin that we get anything like a 

 true idea of the character and rate of evolutional modification. The ex- 

 cessive slowness with which the indigenous fauna of such a basin modi- 

 fied under ordinary circumstances is clearly shown when we compare the 

 successive invading stages of the Spergen fauna among themselves, and 

 further with long preceding and succeeding facies of the same. Except 

 for the diagnostic new things that mark each of the Spergen invasions 

 in the Tennessean and early Pottsville rocks of the Mississippi valley, 

 they could not be distinguished. Then, in certain later Pennsylvanian 

 zones we recognize species after species so little changed that only an 

 expert paleontologist can distinguish them from their Spergen ancestors. 

 The aspect of the fauna as a whole has become different, but the change 



