FOSSIL PEDIGREES 93 



should unite these two ancient families? Even a mesalliance 

 is inconceivable." {Science, Sept. 17, 1920, p. 260.) 



Certainty, therefore, with respect to the comparative an- 

 tiquity of the fossiliferous strata is the indispensable presup- 

 position of any palaeontological argument attempting to show 

 that there is a gradual approximation of ancient, to modern, 

 types. Yet, of all scientific methods of reckoning, none is 

 less calculated to inspire confidence, none less safeguarded 

 from the abuses of subjectivism and arbitrary interpretation, 

 than that by which the relative age of the sedimentary rocks 

 is determined! 



In order to date the strata of any given series with reference 

 to one another, the palaeontologist starts with the principle 

 that, in an undisturbed area, the deeper sediments have been 

 deposited at an earlier period than the overlying strata. Such 

 a criterion, however, is obviously restricted in its application 

 to local areas, and is available only at regions of outcrop, 

 where a vertical section of the strata is visibly ex- 

 posed. To trace the physical continuity, however, of the strata 

 (if such continuity there be) from one continent to another, 

 or even across a single continent, is evidently out of the 

 question. Hence, to correlate the sedimentary rocks of a given 

 region with those of another region far distant from the for- 

 mer, some criterion other than stratigraphy is required. To 

 supply this want, recourse has been had to index fossils, which 

 have now come into general use as age-markers and means 

 of stratigraphical correlation, where the criterion of super- 

 position is either absent or inapplicable. Certain fossil types 

 are assumed to be infallibly indicative of certain stratigraphi- 

 cal horizons. In fact, when it comes to a decision as to the 

 priority or posteriority of a given geological formation, index 

 fossils constitute the court of last appeal, and even the evi- 

 dences of actual stratigraphical sequence and of physical tex- 

 ture itself are always discounted and explained away, whenever 

 they chance to conflict with the presumption that certain fossil 

 forms are typical of certain geological periods. If, for ex- 



