[CAMERON] THE HUMAN SKULL 171 



that this increased extent of the dental arch would have the important 

 effect of making the upper and lower jaws very prominent, thus 

 producing a marked degree of prognathism, and consequently a low 

 type of skull. This is a very significant and indeed fundamental 

 fact, for the great feature of the more highly evolved forms of the 

 human skull is the relatively greater degree of development of its 

 cranial portion when compared with the facial portion. In lower 

 animals on the other hand the jaws are thrust forward in a prominent 

 manner, the result being that the facial portion dominates the cranial 

 portion to a marked degree. On elaborating this idea a little further 

 it becomes evident that the shrinkage of the dental arcades has been 

 of the utmost importance, has in fact been one of the great factors in 

 producing- the human type of skull. What proof have we that this 

 shrinkage is going on ? The reply is that there are two. In the first 

 place the segment of the dental arch occupied by the canines and 

 incisors is relatively much greater in the jaw of the ape, where there 

 are, in addition, distinct gaps especially on each side of the canines. 

 These spaces have become greatly reduced in the modern human skull 

 when compared even with prehistoric skulls such as for example the 

 Piltdown type. In the latter Smith Woodward^ calculated the length 

 of the alveolar arch in front of the molars to be 60 mm., as compared 

 with an extent of 30 or 40 mm. in the modern human jaw, representing 

 a diminution in the case of the latter to the remarkable extent of one- 

 third or even one-half. Moreover, the extent of the canine-incisor 

 segment is less in the higher types of modern hominidae than in those 

 more lowly. This crowding together of the teeth has been one of the 

 strongest influences in guiding and controlling the evolution of the 

 chin, as illustrated in Fig. 10, for it is obvious what the effect of the 

 contraction of the dental arch would be if the lower border of the jaw, 

 were to remain relatively very slightly altered in length. The result 

 would naturally be to cause a retraction of the alveolar border of the 

 jaw, causing the teeth to become closely huddled together, and indeed 

 tend to crowd out the molar series. Now the latter effect is just 

 exactly what is happening at the present day; for it is granted on all 

 sides that the third molars are gradually undergoing a process of 

 devolution, and it has been prophesied that in future generations it 

 will have vanished utterly. Three signs of this suppression are already 

 in evidence. Firstly, a reduction in its size in the higher races of 

 modern hominidae when compared with the lower races, secondly, 

 the very frequent reduction in the number of its cusps, and thirdly, 

 the not uncommon fact that it may fail to develop or erupt altogether, 

 even in the lowest races of modern mankind. In fact I recently 



^ op. cit. 



