94 PKOF. DIl. L. BOLK 



the usually accepted hypothesis is further strengthened by the 

 consideration that the diminution from the primiti-s'e number of 

 four premolars to three, which happened in the eocene Primates, 

 can be followed step by step in the different well-known genera 

 of their group of the common ancestors of all recent Primates. 



The following objection may also l)e advanced. If really the 

 first post-canine tooth in both dentitions should be reduced and 

 lost, there is very strong ground to expect that in the embryo- 

 logical evolution of the dentition of man, apes or catarrhine 

 monkeys, the anlage of this tooth or occasionally the tooth 

 itself should be found in a rudimentary form and size. With- 

 out doubt the teeth are reduced and lost during the last phase 

 of development in all mammals. Although the development of 

 the human dentition in human embryos has been examined by 

 a great number of investigators, there has ne\'er been found a 

 single vestige of the anlage of a rudimentary milk molar imme- 

 diately behind the canine. 



I shall not consider in a detailed manner the current hypothe- 

 ses, being of a wholly different opinion with regard to the rela- 

 tion between the dentition of American monkeys and that of 

 Old World monkeys. This opinion may be briefly expressed as 

 follows: The dentition of the catarrhine Primates (including 

 man) with its two premolars is derived from an ancestral form 

 with three premolars in two phases. The first phase was char- 

 acterized by the reduction and final loss of the third molar of 

 that ancestor. By this process a form resulted with three pre- 

 molars and only two molars, just as we actually find in the group 

 of the recent Hapalidae. The second phase was of an entirely 

 different nature: during the same, the third milk molar devel- 

 oped into a ])ermanent tooth, whilst the development of its 

 successor (the third premolar) was suppressed, in consequence 

 of which the number of permanent molars increased to three, 

 as in the primitive form. 



By this hypothesis the Hapalidae, with regard totheirdentition, 

 are placed on a higher level in the phylogenetical system than 

 they occupy in the common systems of Primates. One is accus- 

 tomed to consider the Marmosets the most })]-imitive recent 



