94 Proceedings of Royal Society of Edinburgh. [sess. 
additional brain molecules and cells thus acquired had their seat 
of growth for the most part somewhere in the cerebral hemi- 
spheres which lie well within the anterior portion of the brain- 
casing. The mere mechanical effect of this increment to the 
physical organ of thought would be to increase the weight of the 
anterior half of the head, and so to upset its finely equipoised 
position on the top of the spinal column. But as any interfer- 
ence with the free and easy rotatory movements of the head 
would manifestly be disadvantageous to the individual in the 
struggle of life, it became necessary to counteract the influence 
of this disturbing element by some other concurrent morpho- 
logical process, which would not be prejudicial to the general 
well-being of the human economy. This object was partly 
attained by a retrocession or contraction of the facial bones, 
especially the jaw bones, towards the central axis of the spinal 
column, and partly by a backward shifting of the cerebrum over 
the cerebellum. As the gradual filling up of the cranial cavity pro- 
gressed necessarily pari passu with these cerebral modifications, 
w e have, in the facial angle of Camper, a rough mechanical means 
of estimating the progress of mental development during the 
period of man’s existence as a human being, i.e. since he 
attained the erect attitude. 
One of the results of this retrocession of the facial bones was 
the gradual contraction of the alveolar borders of the jaws, thereby 
diminishing the space allotted to the teeth, — a fact which plausibly 
accounts for some of the peculiarities which differentiate the older 
fossil jaws from modern specimens. Thus, in the dentition of the 
former, the third or last molar is the largest, whereas in the latter 
it is the smallest. Not only so, but among Neolithic and some 
European races of to-day these four molar teeth (wisdom) make 
their appearance at a later date in the individual’s life than for- 
merly, so that they seem to be on the highway to become vestigial 
organs. It is interesting to note that the shortening of the dental 
portion of the human jaw attracted the attention of Mr Darwin, 
who, however, attributed it to “civilised men habitually feeding 
on soft, cooked food, and thus using their jaws less.” 
Another peculiarity of civilised races is the greater prominence 
of the chin, a peculiarity which may also be due to the contraction 
