158 



THE STRUCTURE OF MAN 



of the first series, are as a rule far less modified than the 

 permanent teeth ; and since, in view of this, it is found that 

 the former agree in Anthropoids and Man far more than the 

 latter, we are justified in concluding that the teeth of both 

 Man and the Apes point back to a common origin from some 

 more or less intermediate type. The dental formula of the 

 Anthropoid Apes appears to be comparatively fixed ; but the 



FIG. 97. HUMAN MOUTH, IN WHICH THE DEVELOPMENT OF THE UPPER OUTER 



INCISORS HAS BEEN SUPPRESSED. 



i'., inner incisors ; i"., outer incisors ; p.m., first premolar of the upper jaw ; c., upper 

 canines which, under the special conditions, come next in order to the upper inner 

 incisors. 



teeth of Man show indications of gradual reduction, especially 

 in the variations in the size of the molars and of the upper outer 

 incisors. 



The upper outer incisor shows every transition form between 

 a well-developed typical tooth and a short conical stump. In 

 many individuals, however, this tooth is altogether wanting (cf. 

 Fig. 97), and this dental variation may be hereditarily trans- 

 mitted through several generations. 



The recent researches of Eose have revealed reason for 

 believing that the upper molars of Man have been derived from 

 a four-cusped tooth type, and the lower from a five-cusped type, 

 and that the numerical reduction of these cusps has been due to 



