176 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



the Ancodus branch, with very lengthy muzzle and 

 cranium, and molars with very long denticles in half- 

 crescents (type hypsoselenodont). The third branch, 

 that of the Ancodus, is very short, and limited, as 

 far as we know, to the later part of the Oligocene 

 epoch. The other two branches, on the contrary, are 

 both very long, and, while remaining distinct, persist 

 from their simultaneous appearance in the middle 

 Eocene or Lutetian stage. One of them, the Anihra- 

 cotherium, continues down to the end of the Oligocene 

 or Aquitanian stage ; the other, the Brachyodus, 

 as far as the lower Miocene or Burdigalian stage. 

 I have personally, and with care, studied the phy- 

 letic series of the Brachyodus of the middle Eocene 

 down to the lower Miocene, the lengthy space of 

 seven great geological stages, and have established 

 the very gradual series of mutations in the Table 

 on page 177. 



The variation of the series of the Brachyodus 

 consists : (1) in a gradual increase in size from an 

 animal no larger than a Chevrotain to one as large as 

 our Rhinoceros ; (2) in the progressive reduction of 

 the pre-molars which are long and a continuous 

 series in the early forms, but become shorter and 

 reduced in proportion in recent types, while, at the 

 same time, the first, and sometimes the second, are 

 separated by intervals from the other pre-molars, as 

 well as from the canine ; (3) by the rapid increase 

 in size of the upper canine, which becomes a kind 

 of dagger with crenellated edges in the Brachyodus 

 borbonicus, and a long triangular tusk in the Brachy- 

 odus onoidens. But, notwithstanding these differ- 

 ences, of great importance if we consider the ex- 



