474 E. O. ULRICH CORRELATION OF THE STRAND-LINE 



And the geological column affords many other instances of faunas 

 commonly credited to particular geological ages on insufficient or quite 

 incorrect correlation data. 



CAUSES OF MUTATION AND EXTINCTION OF MARINE SPECIES 



Taking up another general proposition, it again seems certain that the 

 marine faunas of the continental seas consisted almost entirely of organ- 

 isms that periodically and very frequently migrated from their perma- 

 nent oceanic habitats into these inland seas. Because of their shallow- 

 ness and the consequent susceptibility to fatal changes in temperature to 

 which these inland seas must have been subjected, their faunas must 

 often have been exterminated in whole or part. However, the supply of 

 life in the great oceanic basins was inexhaustible and presumably ever 

 ready to replace the locally exterminated fauna. 



For obvious reasons these new invasions could never be exact duplica- 

 tions of the immediately preceding faunal facies. Some of the previously 

 dominant species always returned, and often the returned fauna consists 

 almost entirely of species that had at one or another preceding time in- 

 habited the same area. In practically all cases, however, the returned 

 fauna differs from the immediately underlying fossiliferous zone in two 

 respects : ( 1 ) some, and occasionally many, of the fossils of the preced- 

 ing zone are absent or have become rare, and (2) one to many species 

 that are wholly unknown in the next underlying zone have been added. 

 Some of these added forms may recur from still lower zones, whereas 

 others are seen here for the first time. 



Under this conception of frequent local extermination of life in the 

 shallow continental seas, and in view of the fact that the fossil remains 

 in the deposits of these inland seas are divisible into reasonably constant 

 specific and generic units, we are justified in concluding that the organic 

 remains found in the successive beds and formations constitute in each 

 case a "snapshot"-like representation of briefly enduring but specifically 

 completed stages of the evolutionary process; also that the many inter- 

 mediate mutations occurred before each of the at present accessible fos- 

 silized stages invaded the continental basins, as it were, in completed 

 form. In other words, these unknown, and just so far unknowable as 

 they are inaccessible, transitional modifications were accomplished in the 

 oceanic basins during relatively longer emergent periods, in which the 

 marine waters were withdrawn from the continental basins. These emer- 

 gent periods alternated with the submergent stages, in which, of course, 

 the stony record that is now in places bared to paleontological investiga- 

 tion could only have been laid down. As for the complete marine life 



