180 THE ROYAL SOCIETY OF CANADA 



One will now be able to appreciate what these facts all signify. 

 They indicate very emphatically that the dentition is the chief factor 

 in determining the configuration and the modelling of the jaws. 

 Thus it comes to pass that in lower animals and in the lower races of 

 modern mankind who possess a macrodont dentition the jaws are 

 markedly prognathous. Further, in the white races who possess, on 

 the other hand, a microdont dentition, the jaws become retracted and 

 orthognathous, the anterior limb of the spheno-maxillary angle (see 

 Figs. 12 and 13) meanwhile having been swinging steadily backwards 

 pendulum-like from a measurement of 150° or so in certain of the 

 anthropoid apes to 75°, a reduction of exactly one half. A further 

 swing backwards is found in microcephaly with its associated deficiency 

 of the dentition, thus still further retracting the jaws and reducing the 

 size of the spheno-maxillary angle. For this latter condition I pro- 

 pose to coin a new word namely, "retrognathism" which is legitimate 

 as it accords, and falls into line, with the two previous terms. I 

 cannot find any reference to the existence of this word in the American 

 Illustrated Medical Dictionary, and must therefore offer some apology 

 for inflicting one more word on the English language. 



Critics may ask, "If the skull in microcephaly represents a 

 reversion to the stage of Java-man, why do not the jaws also con- 

 sistently exhibit a 'throwback' to the same type by becoming 

 prognathous, as his jaws probably were?" The most satisfactory 

 answer to that is secured by again studying the question of cause 

 and effect. In the case of the microcephalic skull the brain was the 

 primary cause of the condition, the effect being shown by a concomitant 

 stoppage in the growth of the investing cranial wall. The same 

 principle holds good in the case of the jaws in microcephaly. The 

 superior and inferior maxillae, in the early stages of their ossification, 

 it may be recollected, are fragile bony shells enclosing the dental 

 germs. For example, the lower jaw at birth is simply a thin trough 

 of bone enclosing the developing teeth. The cause in this case is a 

 deficiency or actual total failure of development of the dental germs, 

 the effect being that the investing jaws likewise fail to execute their 

 normal growth and evolution. 



VI. Summary and Main Conclusions. 



1. The minimum post-frontal diameter of the cranium of modern 

 white races shows on the average an increase of about 23%, while the 

 maximum parietal breadth exhibits an increase of merely 11% above 

 the corresponding dimensions of the skull of the Java man-ape. 



2. The evolution of the minimum post-frontal diameter and the 

 maximum parietal breadth has been practically stationary since the 



