RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 317 



Ibssil remains of certain kinds of land animals and plants may be and 

 often arc found only within the limits of a given formation, ami in that 

 respect they may bo regarded as characteristic of it; but that docs not 

 affect the accuracy of the foregoing statements. 



The foregoing comparisons of the relative value of fossil remains of 

 aquatic and terrestrial animals can not be of unlimited application be- 

 cause no remains of terrestrial animals have been found in the forma- 

 tions of the earlier geological periods, and it is generally believed that 

 none existed then. Indeed, such comparisons are of little applicability 

 in the case of any formations earlier than the Mesozoic, and they mainly 

 pertain to tin- Cenozoic. 



The facts mentioned in the foregoing paragraphs have far-reaching 

 significance other than that which directly relates to the identification 

 of formations. They arc to be again mentioned, but only for the pur- 

 pose of showing that whatever of value fossil remains of land animals 

 and plants may possess other than that which has been referred to, 

 they are, as a rule, of much less value in identifying and characterizing 

 formations than are those of aquatic animals. 



Marine waters having always been of world-wide extent, and marine 

 conditions more nearly uniform or subject to far less vicissitude than 

 either nonmarine or terrestrial conditions, the fossil remains of the 

 marine faunas naturally constitute a better medium for a continous 

 chronological record by organic evolution than do those of terrestrial 

 faunas and floras or those of fresh-water faunas. This fact, however, 

 does not make the fossil remains of fresh-water faunas any less trust- 

 worthy than are those of marine faunas for the identification of the 

 formations which they characterize.* 



With the exception of fishes and a few mammals and reptiles verte- 

 brates of the kinds whose remains are available in geological studies 

 are mostly land animals, while those of invertebrates of the kinds whose 

 remains are thus available are mostly aquatic animals. The superior 

 value of the latter has already been pointed out ; but it should be added 

 that remains of fishes, in case of equally complete faunal representation, 

 are of no less value than are those of aquatic invertebrates. The migra- 

 tory habits of a greater or less proportion of fishes, however, has in 

 some cases caused their remains to be commingled with those of more 

 than one fauna and to be deposited in more than one formation. 



The remains of other vertebrate aquatic animals are sometimes avail- 

 able in the identification of formations, but they are generally of less 

 value than are the remains of fishes, because the aerial respiration and 

 amphibious habits of most of them generally restricted their range within 

 narrower limits. Furthermore, their range in geological time is even 

 less extended than that of fishes. That is, air-breathing vertebrates 

 are not yet known to have existed earlier than the Carboniferous, and 



* For discussions <>t this subject see pages 281-283. 



