124 THE GENESIS OF SPECIES. [CHAP. 



the even-toed division. Subsequent discoveries, however, 

 seemed to give it an equal claim to rank amongst the 

 perissodactyle forms. Others again inclined the balance 

 of probability towards the artiodactyle. Finally, it appears 

 that this very recently extinct beast presents a highly 

 generalized type of structure, uniting in one organic form 

 both artiodactyle and perissodactyle characters, and that 

 in a manner not similarly found in any other known 

 creature living, or fossil. At the same time the differen- 

 tiation of artiodactyle and perissodactyle forms existed as 

 long ago as in the period of the Eocene ungulata, and even 

 in a marked degree, as has been before observed. 



Again, no armadillo now living presents nearly so re- 

 markable a speciality of structure as was possessed by the 

 extinct glyptodon. In that singular animal the spinal 

 column had most of its joints fused together, forming a 

 rigid cylindrical rod, a modification, as far as yet known, 

 absolutely peculiar to it. 



In a similar way the extinct machairodus, or sabre-toothed 

 tiger, is characterized by a more highly differentiated and 

 specially carnivorous dentition than is shown by any pre- 

 dacious beast of the present day. The specialization is of 

 this kind. The grinding teeth (or molars) of beasts are 

 divided into premolars and true molars. The premolars 

 are molars which have deciduous vertical predecessors (or 

 milk teeth), and any which are in front of such, i.e. between 

 such and the canine tooth. The true molars are those, 

 placed behind the molars having deciduous vertical pre- 

 decessors. Now, as a dentition becomes more distinctly 

 carnivorous, so the hindermost molars and the foremost 

 premolars disappear. In the existing cats this process is 

 carried so far that in the upper jaw only one true molar is 



