I 



STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 495 



by the evolution of the organisms is strongly supported by stratigraphic 

 data. 



The greater breaks in faunal succession noted in local stratigraphic 

 studies are commonly indicative of important interruptions in the sedi- 

 mentary record. There are, however, other cases in which this is not so. 

 These are most frequently found in areas subject to submergence by 

 waters from different oceanic basins. The Arbuckle, Ozark, and Adiron- 

 dack uplifts offer very notable examples. Likewise the Appalachian 

 Valley. Comparing, for instance, the successive Ordovician faunas in 

 the vicinity of Mercersburg, Pennsylvania (see pages 325-328), we find 

 that the Stones Eiver fauna is succeeded by a totally different late Chazy 

 fauna. The first invaded through the Mississippi embayment, the second 

 from the north middle Atlantic. The latter was followed by another 

 Gulf fauna — the Lowville — which resembles the first, but is very different 

 from the second. The fourth and fifth faunas have a few forms in com- 

 mon, but both are entirely distinct from all of the preceding faunas. 

 The sixth fauna again is wholly unlike the fourth and fifth. Those in- 

 raded from the east; this from the northwest. The seventh fauna, like 

 the fifth, came in from the Atlantic, and is altogether distinct from the 

 intervening sixth fauna. Good illustrations of similar changes in the 

 composition of faunas is given in the discussion of currents in the conti- 

 nental seas (pages 367 to 371). 



EFFECT OF EXPANSION AND RESTRICTION OF CONTINENTAL SEAS ON 



EVOLUTION 



I can not permit this opportunity to pass without expressing my con- 

 viction that the prevalence of so-called expansional evolution is overesti- 

 mated in certain quarters. The idea advanced by Chamberlin of ''expan- 

 sional evolution of shallow water life ... in broad epicontinental 

 seas of nearly uniform depth,"®^ seems a plausible conception, but so far 

 as 1 can learn it is not being generally accepted by practical paleontolo- 

 gists who should naturally be the best judge of its value. It is true that 

 species which have migrated from Europe and Asia to America, or vice 

 versa, have rather commonly sustained sufficient modification to be dis- 

 tinguishable. It is probably true also that some expansional evolution 

 took place in the newly invaded continental seas. Yet, after all, when we 

 note the distribution of the fossil species in America, and when we com- 

 pare the near and distant occurrences that we may reasonalily regard 

 as geologically contemporaneous, the facts point strongly to the conclu- 



"T. C. Chamberlin: A systematic source of evolution of provincial faunas. Journal 

 Geology, vol. vi, 1898, pp. 597-609. 



