INTERPRETATION OF ENVIRONMENTAL CONDITIONS 191 



marked by distinct environmental conditions, and the correlation of the 

 strata within this time interval depends largely upon the recognition of the 

 results of the operation of climatic factors. 



The ideas here suggested are in decided variance with accepted methods 

 of correlation, especially in the conception of the limitation and retardation 

 of animal and plant migration by unfavorable conditions. 



Ulrich says:^ 



"I have strong convictions respecting the great possibilities of correlation 

 by a judicious application of organic criteria. Their greatest value in this connec- 

 tion arises from the demonstrable fact that, as a rule, the migration, and to a con- 

 siderable extent also the evolution, of species, however slow, is yet relatively 

 rapid as compared to the inconceivable length of geologic time. 



"As to marine faunas, with which the student of Paleozoic stratigraphy is 

 chiefly concerned, their migrations, when not prohibited by physical barriers, 

 usually proceeded with such rapidity that their progress can not be expressed in 

 recognizable units of the geologic time scale. Hence, unquestionable correlations 

 by fossil evidence, fully checked by physical criteria, may be said to establish, so 

 far as the practical purposes of geology are concerned, the essential contem- 

 poraneity of the beds so identified." 



Ulrich's remarks are based upon the action of marine invertebrates 

 almost entirely, but it will be noticed that the only influence that he recog- 

 nizes as able to deter the rapid spread of such forms is the effect of " physical 

 barriers." That there is an abundance of other barriers to migration and 

 evolution will be evident at once to anyone who looks at the fossils as forms 

 of life, subject in their time to influences strictly similar to those affecting 

 living forms to-day. 



The discovery of vertebrate fossils belonging to identical or closely 

 related genera, and the evidence of fossil plants, has led to the suggested 

 correlation of the red beds of Kansas, Oklahoma, Texas, and New Mexico 

 with the Dunkard series of Ohio and Pennsylvania and with the isolated 

 deposits carrying vertebrate fossils near Danville, Vermillion County, 

 Illinois. Such suggestions of correlation, however, do violence to the 

 probabilities indicated by the stratigraphic position of the beds in which the 

 fossils are found. Though the correlation of widely separated areas must 

 be largely accomplished by fossil evidence, it is becoming increasingly 

 evident to all workers in stratigraphy, as well as paleobiology, that fossils 

 must be regarded and interpreted as once-living things, and the problem of 

 their distribution is inextricably associated with the problem of their living 

 conditions. 



The method of evolution is as yet unknown, but all biologists concede 

 the directive influence of environment, once a line is started. In other 

 words, the evolution of life follows and responds to the changed conditions 



^ Ulrich, E. 0., Revision of the Paleozoic Systems, Bull. Geol. Soc. Amer., vol. 22, p. 507, 191 1. 



