302 TOWARDS THE HUMAN FORM 



disappearance once achieved in the Chevrotains, it would be 

 preserved by heredity in other ruminants, such as the Cervidae, 

 where the males still have a canine. 



The molars, being employed in the trituration of food, 

 naturally become modified according to the use that the animal 

 makes of them, and end by being more specially adapted to the 

 consistency of the food than either the incisors or the canines. 

 The primitive number of seven — four premolars and three molars 

 — may be reduced, but they never disappear in animals which 

 masticate their food. Arising from the union of several teeth, 

 such as the purely prehensile teeth of the Reptile, in the first 

 place they naturally had a crown with a broad and 

 mamillated surface, especially as this crown might already 

 present, in Reptiles, certain surface complications, such as we 

 noticed in Theriodontia. These surface bosses or tubercles 

 may be joined in such a way as to form ridges transversal to 

 the direction of the jaw (Mastodonts, Tapirs, etc.), or 

 longitudinal crests (Carnivora). On the basis of these data, 

 a first approximation of the essential facts may be condensed 

 into some such formula as this :— 



As generation succeeds generation, the modifications in the teeth 

 occur as though the adults transmitted to their descendants the 

 forms that have been acquired in the course of their lifetime 

 through use and attrition. 



According to the degree in which a Mammal adopts an 

 increasingly carnivorous diet, the molars of the lower jaw meet 

 those of the upper jaw scissor-fashion, and become sharpened 

 by the shearing away of the upper edge of their crown. Thus 

 we see the transition from the tuberculate molars of the Bear 

 to the exclusively scissor-edged molars of the Cat. On the other 

 hand, when Mammals live on vegetable food, generally hard, 

 the crowns of the opposing molars in the two jaws become 

 planed down and present a large grinding surface striped with 

 bands of enamel. It is in this fashion that the molars of 

 Mastodons, with their protruding transverse ridges, became the 

 flat-crowned teeth of the Elephants, in which the enamel is 

 arranged in lozenge-form (Loxodon or African Elephant), or 

 in flattened ellipses (Elephas or the Asiatic Elephant). In the 

 same way the tuberculate bossed teeth of Palceotherium and 

 Anchitherium are replaced by the flat-surfaced teeth of Horses 

 in which the enamel bands with an apparently capricious 



