By substituting for nature's means of defence and self-preservation a variety of implements, weapons, and tools made with their own hands, the subsequent well-being of these novel bipeds became dependent on their ability to interpret and utilise the laws and forces of nature. As time progressed they began to recognise the value of the faculty of reasoning as the source of inventive skill; and hence a premium was put on useful inventions. In this way, stimulants to the production of new ideas and mechanical discoveries were constantly coming within the scope of their daily avocations, the outcome of which was a steady increase of human intelligence, and consequently of brain substance. Now, according to the well-established doctrine of the localisation of brain function, the additional brain molecules and cells thus acquired had their seat of growth somewhere in the cerebral hemispheres, 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 upset its finely poised position on the top of the spinal column. But as any interference with the free and easy rotary movements of the head would be disadvantageous to the individual in the struggle for existence, it became necessary to counteract the influence of this disturbing element by some other concurrent morphological process which would not be prejudicial to the general well-being of the human economy. This object was partly secured by a retrocession, or contraction, of the facial bones, especially of the jawbones, 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 progressed pari passu with these cranial modifications, we have in the facial angle of Camper, a rough but ready mechanical means of estimating the progress of mental development during the successive stages of man's existence since he attained the erect attitude.
Cranial Development.
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
