456 E. O. ULRICH- — CORRELATION OF THE STRAND-LINE 



lies in this — that it identifies geological horizons as positively and as 

 closely as it is possible to do so by means of fossil evidence. The new 

 criterion is based on the logical belief that combinations of biologically 

 unimportant characters can have existed but once, and that they endured 

 for only short periods of time. Accordingly, absolute identification of 

 such minor modifications of species in widely separated localities is re- 

 garded as establishing the essential contemporaneity of these occurrences. 



The application of this criterion involves the utmost refinement in the 

 discrimination of fossils, for the greater the detail and the smaller the 

 distinctions, the more exact the correlation. The basic idea is too obvious 

 to be called new, but, so far as known to me, it has not hitherto been 

 formally recognized as a principle in correlation. In my own practice, 

 however, and despite the fact that it subjected me to reproach as a "species 

 maker," the identification of minute organic differences has for many 

 years played the most important part in my work as a paleontologist and 

 stratigrapher. The method of minute discrimination of fossils was early 

 favored because it always led to dependable results. Later it seemed the 

 best, if not the only, safeguard against the confusion of recurring faunas 

 or species. Judging from the results of my work in stratigraphy during 

 the past 20 or 25 years, this method has proved vastly superior in definite- 

 ness and accuracy to all other methods of correlation by fossils. Strati- 

 graphic correlation by relative similarity in general fauna! aspect com- 

 monly is indecisive, and often, indeed, such comparisons are positively 

 misleading. In my opinion, then, "preponderance" of generic or broadly 

 specific affinities, however great, should never outweigh the dissenting 

 evidence of two or three exactly identified varieties. Subordinating the 

 latter would be like accepting hearsay evidence in preference to the testi- 

 mony of a photographic picture. 



Another new conception seeks to explain the not uncommon apparent 

 discordance of the evidence of marine animals on the one hand, and that 

 of land organisms on the other. It is explained by the reasonable assump- 

 tion that whereas land animals and plants flourished best when the lands 

 attained their greatest development, hence during and shortly preceding 

 and succeeding the intersystemic intervals, the shallow-water marine ani- 

 mals, on the contrary, experienced their least stressful periods when the 

 seas encroached on the lands and thereby increased the total area of 

 shallow seas. The greatest changes in the land organisms, therefore, 

 occurred in the middle ages of the geological periods, when their range 

 was most restricted and submergent conditions dominant, whereas the 

 marine animals changed most in the transition stages from one period to 

 the next, when emergent conditions prevailed and the lands, therefore, 



