322 REPORT OF NATIONAL MUSEUM, 1892. 



which it pertains not being complex the idea was readily dismissed. 

 The idea of the absoluteness of the geological scale now in general use 

 as a standard for the whole earth pertains to an unusually complex 

 subject, involving various concurrent lines of systematic thought. It 

 was probably for this reason that it was not fully dislodged eveu by 

 the great biological revolution which has been referred to in the last 

 essay, notwithstanding the fact that its subject is essentially a bio- 

 logical one. 



Originally the idea of correlation involved that of actual and com- 

 plete contemporaneity. That is, in accordance with their belief in 

 special creations the early geologists assumed that every fannal and 

 floral type, as well as every species, having been specially created* each 

 one of those types which characterize any given stage or substage of 

 the geological scale was simultaneously deposited. They also believed 

 that all the divisions and subdivisions of the geological scale were 

 divinely ordained and sharply definable, and their acceptance of that 

 scale as an absolute universal standard of correlation was a necessary 

 result of that belief. Notwithstanding the great revolution in methods 

 of biological thought and practice which has been referred to, paleon- 

 tological literature abounds with proof that the idea of absoluteness of 

 correlation is still held even by authors who ostensibly reject all the 

 beliefs which alone could have given origin to such an idea. 



Sufficient reasons have been given why formations as such can not 

 be considered in discussing correlation, but I again refer to the fact for 

 the purpose of emphasizing the statements that true correlation is es- 

 sentially a biological and not a physical matter, that its application is 

 necessarily restricted to divisions of the geological scale that are more 

 comprehensive than those which may be represented by even the great- 

 est of the formations as they are defined in Essay n, and that in the 

 determination of such correlation specific identity of fossils can rarely 

 be considered. It is therefore necessary to consider what divisions of 

 the scale may be satisfactorily correlated and what are the character 

 and attributes of the biological forms that constitute the criteria of 

 their correlation. 



Naturally the larger divisions. of the scale are more readily recogniz- 

 "able than are the smaller, because, besides other reasons, the fauna! 

 and floral characteristics of the former are more general than are those 

 of the latter, and their vertical range also is greater. Thus the sys- 

 tems, or stages, as I have designated them on preceding pages when 

 discussing the geological scale, are readily recognizable in widely sep- 

 arated parts of the world by means of more or less numerous general 

 types of fossil forms, while the characteristic types of their divisions are 

 fewer and more special. By means of those more general indications 

 the whole series of systems from the Cambrian to the Tertiary, inclu- 



* See pages 291-299. 



