294 REPOBT OF NATIONAL MUSEUM, 1892. 



illations and of the order of occurrence of geological events, such as the 

 resting of one formation upon another, lava overflows, faults and other 

 displacements, sub aerial erosion, etc. All such indications, however, 

 are of service only in local, or at best in regional, investigations, and 

 although they may be numerous and of great local value, they are 

 always so disconnected that they can never be reduced to a general 

 system of chronological classification, or even to a part of it, without 

 the aid of fossil remains. It, therefore, can not be too earnestly asserted 

 that the general advancement which has occurred during geological 

 time in the biological rank of organic forms, notwithstanding its varia- 

 tions and numerous discrepancies, together with their multifarious dif- 

 ferentiation, constitutes the only means of measuring that time as a 

 whole or of any considerable portions of it. It is to this abstract meas- 

 ure of time that material form has been given in the construction of the 

 geological scale. 



Notwithstanding the indispensability and general trustworthiness 

 of this time-measure in the study of historical geology, it cau not be 

 denied that it is not comparable in precision with the standard of 

 sidereal time, because the latter is mathematically definable, while the 

 former is based upon past biological conditions which were subject to 

 infinite and often great variation. 



(3) The chronological features which fossils possess are not of a special character 

 as such, but they are amoug those upon which their biological classification is based, 

 all of which features have resulted from both progressive and differential evolution. 



Progressive, and differential evolution were more or less completely 

 concurrent, but it was sometimes the case that the latter was greatly in 

 excess of the former. From progressive evolution we have successive 

 stages in biological rank, and from differential evolution the infinite 

 variety of forms which occur in approximately the same rank. Both 

 are often exemplified by one and the same series of fossil forms, but 

 in the study of historical geology the results obtained from each are 

 of different applicability. Those of progressive evolution are directly 

 chronological in character, and therefore of broader significance, than 

 are those of differential evolution, the results of the latter being only 

 indirectly chronological in character and of empirical applicability in 

 geological studies. 



Progressive evolution has produced from the great mass of life which 

 has continuously existed upon the earth variously connected genetic 

 lines of organic forms, the aggregate of which lines extended through 

 the whole of geological time. The varying structure of these forms 

 exhibits grades of biological rank, which, by their continuity and their 

 relation to one another, become chronological in character as well as 

 constituting the basis of their biological classification. 



Differential evolution has produced a great diversity of forms in each 

 of the principal grades of biological rank which have successively ex- 

 isted during geological time, and these are found by empirical study to 



