190 THE TRANSFORMATIONS OP THE ANIMAL WORLD 



simply due to some of the intermediate mutations 

 being still unknown to us ; and when we shall 

 have discovered the representatives of this same 

 branch in the middle and upper Oligocene, it will 

 doubtless be no longer possible to determine where 

 the Palceomastodon ends and the real Mastodon 

 begins. 



These inextricable difficulties in the division of 

 the phyletic branches into genera and species 

 susceptible of an exact diagnosis are due, in my 

 opinion, to the fact that palaeontologists persist in 

 designating by the same name things entirely differ- 

 ent from each other. On the one hand, the species 

 and the genus at a specified epoch ; on the other, the 

 species and the genus in time. To help the reader 

 to understand and appreciate the importance of 

 this distinction, I will draw up the following brief 

 table : 



Let there be A, A 1 , A, 2 three species of an existing 

 genus ; B, B 1 , two species of a second genus ; and 

 one species of a third genus, all alike existing. 

 According to the laws we have recognized regarding 

 variation in living nature, these genera and these 

 species are easy to characterize, since there is gener- 

 ally no transition between them. Let us establish 

 elsewhere the phyletic branches of each of these 

 species by utilizing the representative forms of 

 each branch throughout a series of geological 

 periods. We shall thus have a sort of rectangular 

 construction in which the horizontal lines will 

 represent the fauna of the same epoch and the 

 vertical lines the successive forms of the same 

 branch throughout time. 



