DARWIN AND PALEONTOLOGY 245 



(different from that of the tapirs, rhinoceroses, 

 or horses) apparently tincture and condition 

 many things which happen in the evolution of 

 this group. There are forty-four grinding teeth 

 altogether; twelve of these teeth (the molars) 

 early attain their final form, but are destined 

 through family kinship to lose certain characters 

 and to change their proportions through generic 

 kinship; twelve others (the premolars) have not 

 attained their final form, but gradually do so 

 through the origin of from forty to forty-eight 

 new characters, each of which appears to arise 

 through an unknown law of hereditary predispo- 

 sition, which operates alike, through ordinal kin- 

 ship, not only in the titanotheres but in all other 

 odd-toed or perissodactyl mammals to which the 

 titanotheres are related. Changes of proportion 

 in the skull, whether toward breadth (brachy- 

 cephaly), or toward length (dohchocephaly), af- 

 fect the form of the grinders as a whole and thus 

 the birth-form of each of these new cusps. The 

 immediate cause of changes of proportion is not 

 interpreted as due to hereditary predisposition, 

 because in teeth, in skull, in foot and limb, and 

 even in horns each generic branch or phylum from 

 the original stem forms of titanotheres acquires 

 its own proportions. Thus changes of proportion 

 are interpreted rather as immediately affected by 

 ontogeny, by the mechanics of use and disuse, by 

 an environment which favors some rather than 

 other proportions, but especially by the selection 



