302 REPORT OF NATIONAL MUSEUM, 1892. 



V. THE RELATIVE CHRONOLOGICAL VALUE OF FOSSIL REMAINS. 



The discussions in the preceding essay of the positive value of fossil 

 remains in systematic and historical geology have made it apparent 

 that there is much difference in their relative value, especially as regards 

 their use in characterizing the different stages of the geological scale and 

 in determining the geological age of the strata in whichvfchey may be dis- 

 covered. It is this subject, the relative chronological value of fossil 

 remains, which I propose to discuss in this essay. 



The basis of comparison of such values is a matter of much impor- 

 tance. The early geologists believed that all kinds of animals and 

 plants were specially endowed at their creation with a certain chrono- 

 logical impress, but that this impress, being more distinctly recogniz- 

 able in the fossil remains of some kinds than in those of others, they are 

 consequently of greater chronological value. According to this idea 

 the relative value of fossil remains consists only in the greater or less 

 distinctness with which that impress can be recognized. The accept- 

 ance of the theory of the evolutional development of organic forms 

 necessitates the rejection of this idea of the early geologists, which 

 was one of the strongest influences in shaping their views of historical 

 geology, but as already intimated its adverse influence is still observ- 

 able in the practice of certain modern authors, even though they may 

 theoretically disavow it. 



Rejecting the idea of special endowment just referred to, we must 

 consider the relative chronological value of fossil remains with refer- 

 ence to the natural laws which have produced their characteristics and 

 governed the various conditions of their origin. Much may profitably 

 be said concerning the comparative chronological value of the different 

 genera, families, etc., belonging to one and the same class of any 

 branch of either the animal or vegetable kingdom, or to different classes, 

 but I propose to discuss only the broader relations to one another of 

 the more general kinds of fossil remains. These discussions will re- 

 late to the time range of each of those general kinds, the various con- 

 ditions under which they have been preserved, the various conditions 

 of habitat of the animals and plants which they represent, the relative 

 rate of evolutional development of the different kinds and their differ- 

 ences of reciprocal relation to one another. 



No fact in historical geology is more conspicuous than that of the 

 great differences in time range of the various kinds of organic forms, 

 some of them having ranged through the whole of the time represented 

 by the geological scale, while others, and among them some of the bio- 

 logically most important kinds, ranged through only a comparatively 

 small part of it. 



The various conditions under which the different kinds of fossil re- 



