CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 255 



In all cases the permanent dentition is initiated by the appearance of the 

 first (permanent) molar. This tooth does not replace a deciduous tooth 

 but it emerges behind the most posterior element in that series. The 

 third molar tooth is the latest to emerge in the completion of the per- 

 manent dentition in Man, but in the Gorilla the canine teeth may be 

 delayed after the emergence of the third molars. Beyond this, little or no 

 information is available concerning the mode of replacement of the deciduous 

 teeth in the Primates other than the Hominidae. In the latter, the deciduous 

 teeth are functional from the second to the sixth year (approximately). This 

 is a long period in comparison with some other mammals. Leche has recently 

 pointed out (in an admirable memoir, Zool. Jahrbucher, Bd. xxvni. Heft 4, 1909, 

 p. 451) that the functional period varies greatly in duration even within a 

 single Order. Thus the Badger is distinguished by the extremely short dura- 

 tion of this period when compared with the Cat. The point is not without 

 importance for Leche is able to point to at least one character of the 

 deciduous teeth (viz. the absolute dimensions) which is related to the length 

 of the functional period. 



Apart from all this, the deciduous dentition is of importance in connection 

 with problems of phylogeny, since some authorities (e.g. Leche) hold that the 

 deciduous teeth present characters which at an earlier period were borne by 

 ancestral relatives. This point will demand further consideration in the sequel. 



The numbers and the general characters of the teeth having been reviewed, 

 enquiry is now to be made into the evolution of the teeth in the Primates. 

 This will be approached from several directions, and the following depart- 

 ments of research and study call for separate treatment : 



I. Variations of the teeth in the direction of increase in the number. 



II. Variations in form. 



III. Variations in position. 



IV. Variations in the direction of reduction in the number of teeth. 



V. Evolution of different dental types such as the incisors, premolars, etc 

 VI. Theories of cusp development and the origin and fate of cusps. 



I. Variations by way of increased numbers. 



Under this head, the teeth of the permanent series will be mainly con- 

 sidered. Retention of the deciduous teeth will naturally provide an apparently 

 increased permanent dentition. Thus Guttman (quoted by Kukenthal, 

 Jahresberieht der Anatomie, 1901), records the case of a youth (aet. 19) in 

 whom nearly all the deciduous teeth were retained in the presence of their 

 successors. Such examples are not relevant here, though one exception to 

 the exclusion of "retained milk-teeth" must be made. This is the case of 

 persistence of tin- second deciduous or "milk" molar tooth. But inasmuch 

 as the tooth that should normally replace it usually fails to emerge, no 

 numerical anomaly results. [This condition will lie discussed in the 

 sequel (V . | 



