RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 311 



plants falls very far short of the full range of the scale, while the range 

 of each of the more important of these kinds is at best through only a 

 portion of the later half of the scale. Besides this, the incongruity 

 which has been shown to have existed between the different kinds of 

 these nonmarine and land animals and land plants respectively, and the 

 difference in the rate of evolution of each, were such that their respec- 

 tive time ranges could not have been so coraplemental of one another 

 as to constitute of themselves a consecutive and harmonious paleonto- 

 logical record for that portion of the geological scale in which they oc- 

 cur. At least a record thus produced could not have been so complete 

 as is that which has been produced for the whole scale by the combined 

 ranges of the different kinds of marine invertebrates. 



If the fossil remains of the nonmarine and land faunas and land 

 floras can not be used conjointly as a standard for the characterization 

 and recognition of the divisions of the geological scale it is evident 

 that none of the kinds which they embrace can be separately so used. 

 It has been claimed by some authors that, although their complexity of 

 structure was the predisposing cause, the exciting cause of the re- 

 markably rapid progressive, and the wide differential, evolution of the 

 placental mammals was their sensitiveness to physical changes which 

 w T ere so slight that they produced little or no effect upon associated 

 faunas. They further claim that this sensitiveness to slight physical 

 changes has made the remains of those animals more valuable as indi- 

 cators of the divisions of geological time than are any other fossil 

 remains. 



Whatever may have been the cause of the rapid changes which took 

 place among those mammals it is true that their remains are often 

 valuable for distinguishing subordinate horizons which other fossils do 

 not clearly indicate. It is plain, however, that a chronological classi- 

 fication based upon such rapidly changing forms alone will not har- 

 monize with that which we are obliged to use for all that great earlier 

 portion of the scale in the strata of which such remains do not occur, 

 nor with the continuation of that classification which is necessarily 

 used for the remaining portion of the scale. 



If the remains which the placental mammals have left had shown 

 any such approach to a direct .succession of faunas as have the marine, 

 and especially the nonmarine, invertebrates they would be much more 

 valuable in the wayjust mentioned than they are now found to be. That 

 is, there are great faunal breaks among themselves so far as their suc- 

 cession is known, and an especially wide faunal hiatus between the ear- 

 liest ot them and the nonplacental mammals and the dinosaurs which 

 preceded them, while the known succession or continuous existence of 

 species of gill-bearing mollusks show that the stratigraphic record is 

 continuous. 



For example, certain species of gill-bearing fresh water mollusks are 

 found associated with dinosaurian remains in the interior region of 



