124 THE GENESIS OF SPECIES. [CHAP. 



other characters are found to go with this obvious one. 

 Even the very earliest Ungulata show this distinction, 

 which is completely developed and marked even in the 

 Eocene palaeotherium and anoplotherium found in Paris by 

 Cuvier. The former of these has the toes odd (perissodac- 

 tyl), the other has them even (artiodactyl). 



Now, the macrauchenia, from the first relics of it which 

 were found, was thought to belong, as has been said, to 

 the even-toed division. Subsequent discoveries, however, 

 seemed to give it an equal claim to rank among the pe- 

 rissodactyl forms. Others, again, inclined the balance of 

 probability toward the artiodactyl. Finally, it appears that 

 this very recently extinct beast presents a highly-generalized 

 type of structure, uniting in one organic form both artio- 

 dactyl and perissodactyl characters, and that in a manner 

 not similarly found in any other known creature living, or 

 fossil. At the same time the differentiation of artiodactyl 

 and perissodactyl forms existed as long ago as in the 

 period of the Eocene ungulata, and that in a marked de- 

 gree, as has been before observed. 



Again, no armadillo now living presents nearly so re- 

 markable a specialty of structure as was possessed by the 

 extinct glyptodon. In that singular animal the spinal col- 

 umn had most of its joints fused together, forming a rigid 

 cylindrical rod, a modification, as far as yet known, abso- 

 lutely peculiar to it. 



In a similar way the extinct machairodus, or sabre- 

 toothed tiger, is characterized by a more highly differ- 

 entiated and specially carnivorous dentition than is shown 

 by any predacious beast of the present day. The special- 

 ization 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 prede- 

 cessors (or milk-teeth), and any which are in front of such, 

 i. e., between such and the canine tooth. The true molars 



