ZOOLOGY AND PALEONTOLOGY 187 



same rules of nomenclature to the representatives of 

 several superposed geological faunas. We here find 

 ourselves confronted by phyletic branches composed 

 of forms or mutations following each other step by 

 step through time, and only separated from one 

 another by shades, the slighter as we dissect with 

 more minuteness the stages, the positions, or even 

 the strata of each find-spot. On the other hand, 

 if the differences are slight between a mutation 

 and the one immediately following it, these differ- 

 ences, generally tending in the same direction, 

 add themselves together, and end in uniting in 

 one continuous chain of beings related to each other 

 by direct descent, of animal forms which no zoo- 

 logist accustomed to the study of actual types 

 would hesitate to consider as species, or even as 

 distinct genera. But how then can a natural di- 

 vision be effected in these continuous series, analo- 

 gous to the species and to the genera of zoological 

 nomenclature ? It must be owned that this task 

 is almost impossible when dealing with a branch 

 whose several series of mutations are complete and 

 without lacunae. In this case the demarcation of the 

 species and of the genus, phyletically considered, 

 becomes purely artificial and subordinate, it may 

 be said, to the personal feeling of the observer. 



To make this clear by an example, let us go back 

 to the table given above of the pedigree of the Pro- 

 boscidians, and in particular consider the branch of 

 the Mastodons with molars formed of conical mounds, 

 a branch of which the Mastodon arvernensis is the 

 last representative. Between this last form, which 

 characterizes the Pliocene as a whole, and the 



