212 THE MAMMALIA. 



each other most; but to expect, as is generally 

 asked by those who believe in the invariability of 

 species, that we should be able to show the last 

 Palaeotherium, and his descendant the first Anchi- 

 therium, is to demand an impossibility. An origin- 

 ally normal characteristic sometimes begins not to 

 occur, then it becomes unimportant, i.e. is found 

 wanting as often as it occurs, then it appears rarely, 

 and finally disappears completely. Thus, for in- 

 stance, the small front premolar of Palseotherium 

 is smaller still in Anchitherium, but still occurs 

 regularly ; in the Hipparion it is met with as often 

 as it is found missing, and in our present Horses 

 it is extremely rare (as the "wolf's tooth").' This 

 very careful comparison of the differences in the 

 dentition has been further worked out by Kowa- 

 lewsky. 1 



We have now again to turn to America, to the 

 well-known fields of discovery to the right and left 

 of the Eocky Mountains, where to all appearances 

 a group of Odd-hoofed animals lies buried, much 

 more numerous in members than the group in the 

 Old World, showing no gaps, and terminating with 

 the horse. Compare the table on page 190. The 



1 W. Kowalewsky, ' Sur 1'Ancliitherium Aurelianense Cuv.,' 

 Mtm. de I' Academic imp. de St. Pelersbourg, 1873. 



