APPLICATION OF BIOMETRIC METHOD 297 



progress in the discovery of the minute steps by which modification of 

 form is accomplished in evolution. 



Application of biometric Method 



My paper is therefore not only a statement of some facts, but an appli- 

 cation of the biometric method to the interpretation of fossils. 



The genus Rldpidomella Oehlert was selected for first application of 

 the method, almost at random among a number of similar series, chiefly 

 because as a morphologic form it can be easily and certainly distinguished 

 from all associated genera, and, further, because it offers clear and dis- 

 tinct characters for measurement. 



Prerequisites to the Study of Evolution oe Fossils 



Before it is possible to interpret the laws of evolution from a geological 

 series of specimens so that the results can be regarded as scientifically 

 established, a rigid control of the conditions of selecting the samples is 

 essential. The samples must all be of one genus; the fauna in which they 

 occur must be unchanged and continuous ; the length of time and the 

 change of environmental conditions must be sufficient to furnish the 

 opportunity for evolutional modification ; the effects of geographic distri- 

 bution and of artificial selection of the specimens must be eliminated and 

 some mode of mathematical expression of the facts must be devised which 

 will not only be exact, but will eliminate the changes of dimension inci- 

 dent to ontogenetic growth of the individual. 



Faunae Continuity 



In order to obtain a continuous series of samples of the same genus, 

 running through a period of geologic time long enough to satisfy the 

 conditions for the evolution of new species, it was necessary to establish 

 faunal continuity, for the reason that where there is a faunal break of 

 continuity in succession there can be no certainty that the specific repre- 

 sentatives of the same genus in the two faunas have the same origin. 

 They may have had widely different origin and represent evolutional races 

 whose ancestry had been distinct for long periods of preceding time. 



Environmental Change sufficient to permit evolutional 

 Modification 



Again, in order to establish the fact that the genus had suffered a 

 sufficient amount of environmental change to give it an occasion for 



