RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 303 



mains have been preserved is also of great importance in this connec- 

 tion. Some of them Lave been preserved only in marine deposits and 

 others only in nonmarine. Marine deposits have been continuous 

 through the whole of geological time, while nonmarine deposits are not 

 known among the older formations, and those which are known have 

 necessarily been isolated and have had little or no direct relation either 

 to one another or to marine deposits. The various conditions of habi- 

 tat of the animals and plants which have furnished fossil remains also 

 have much significance with reference to these discussions because of 

 their essential relation to the differences between the kinds audto their 

 differences of relation to one another. 



It was shown in the preceding essay that while the general ad- 

 vancement in biological rank of organic forms which has occurred dur- 

 ing geological time constitutes the ultimate standard of measure for 

 that time, there has not been a uniform secular advancement for all 

 kinds, but that there has been much difference in the rate of evolu- 

 tional advancement for the various kinds of both animals and plants. 

 Again, the differences of reciprocal relation between the various kinds 

 of animals and plants which have furnished fossil remains are inti- 

 mately connected with the causes which have produced the differences 

 of chronological value of those remains. That is, certain kinds were 

 not ODly radically different from others, but they lived under such wide 

 differences of condition and were so nearly free from reciprocal rela- 

 tion to others, that they could not have produced a closely similar chro- 

 nological record. 



The facts thus briefly stated are of themselves clearly suggestive of 

 the subject of this essaj', that is, of wide difference in the relative 

 value of the different kinds of fossil remains as means of characterizing 

 the different stages of the geological scale and of determining the geo- 

 logical age of the strata in which they are found. These differences, 

 however, will be discussed at some length on the following pages, but 

 it is proper to say at the outset that while certain of the kinds men- 

 tioned are much more valuable for the purpose indicated than are 

 others, it is inexcusable iu any geologist, in attempting to determine 

 the geological age of formations, to reject any kind as valueless, or to 

 fail to give due weight to every accessible relevant fact, whether bio- 

 logical or physical. 



A special grouping of the different kinds of fossil remains is more 

 appropriate for these discussions than is a strictly systematic one, and 

 1 have therefore adopted the following: (a) marine invertebrates, (b) 

 nonmarine and land invertebrates, (c) fishes, (<l) batrachians and rep- 

 tiles, (e) birds, (/) mammals, and (g) land plants. For convenience of 

 reference our present knowledge of the time-range of these kinds may 

 be presented in tabular form. The accompanying table, Plate XIV, rep- 

 resenting the whole of geological time by its height, indicates in a 

 general way by perpendicular lines the time range of the kinds just 



