PROBLEMS OF HUMAN DEXTITIOX 97 



wit: the loss of its succeeding tooth (the third premohir of the 

 lower forms), and secondly, the fact that an originally decidu- 

 ous tooth became a persisting element. It is clear that these 

 phenomena stand in a close relation to each other, for a milk 

 tooth cannot acquire the character of a persisting tooth, so 

 long as the evolution of its successor is not suppressed. The 

 two events must have happened simultaneoush'. As to the 

 question whether the evolution of the milk tooth or the re- 

 gression of the permanent element was the leading factor in 

 this process, I incline to the first of these two possibilities, on 

 the following ground: I consider that the evolution of the dental 

 structure of the catarrhine Primates commenced with the reduc- 

 tion and final loss of the hindmost molar of an ancestor with a 

 platyrrhine dentition, perhaps in consequence of the shortening 

 of the jaws. By this reduction the grinding surface of the set of 

 teeth underwent a shortening. This circumstance (without im- 

 portance in small animals such as the Hapalidae, who live princi- 

 pally on soft food or insects) became disadvantageous as the 

 species grew taller and the nature of the food required a larger 

 grinding surface. It is very important that the third milk 

 molar of the platyrrhine monkeys is a larger tooth, with a greatei- 

 surface and more cusps than its successor, the third premolar. 

 Especially in Hapalidae is the difference notable. And so it 

 was advantageous to the grinding function of the dental arch of 

 the historically succeeding larger forms of monkeys, that the 

 third milk molar with its four or five cusps was not replaced by a 

 tooth with two cusps only. Thus the grinding surface of the 

 dental arch regained at its anterior end what it had lost in an 

 earlier period of evolution at its posterior end. 



These considerations lead me to the supposition that in the proc- 

 ess of evolution, the alteration of the character of the third milk 

 molar occurred first, the loss of the third premolar being a 

 necessary consequence of it. Because the third premolar was 

 leduced, the third milk molar became a persisting tooth; but 

 because the permanence of this milk molar brought a functional 

 advantage, the evolution of the third premolar was suppressed. 



