88 ARBOREAL MAN 



the terrestrial Insectivora. In the arboreal Tree Shrews 

 (Tupaiadce) it has become reduced to thirty-eight by the 

 loss of one upper incisor and an upper and lower pre- 

 mclar upon each side. In the Lemurs only thirty-six 

 teeth remain, for a corresponding lower incisor has been 

 lost. In the Old- World Monkeys, the Anthropoid Apes, 

 and Man, one more premolar is lost in each jaw upon 

 either side, and the dentition is reduced to a set of thirty- 

 two teeth. In Man there are signs of a further reduction. 

 The reduction of the tooth series and the shortening of 

 the jaw in the arboreal stock go on step by step together, 

 and for the same reason, and yet to a certain extent the 

 two developments are independent of each other. The 

 tooth series may diminish and may even disappear, yet 

 if food is still reached for and is seized by the mouth the 

 snout region will remain elongated. A toothless animal 

 may still be a long-jawed animal if long jaws are needed 

 to take food which when taken requires no teeth for its 

 killing or for its mastication. With hand-feeding the 

 recession of the snout may outstrip the reduction of the 

 tooth series, and this process is evidenced in the stock 

 we are considering. Starting with a full mammalian 

 series of forty-four teeth, the snout region may yet be so 

 long that gaps exist between the teeth, and different 

 groups of teeth may be widely separated from each other. 

 Reduction in the tooth series in this stock does not in- 

 crease the gaps, but the gaps diminish faster than the 

 tooth series is reduced. The ultimate result of this 

 process is that Man, with a reduced number of teeth, 

 has the most crowded dentition. Man is the only living 

 f\ Primate that has its teeth arranged in a continuous series, 

 ., and it is one of his distinctions that there are no gaps 

 v^ between them. The process of the shortening of the 

 snout, outstripping the process of reduction of the dental 

 series, gives rise to one of the great problems of modern 

 dentistry the proper treatment of the many evils arising 

 from overcrowded jaws. To this subject we will turn 



