THE SKELETON 53 
capacity of Man and that of the Anthropoid Apes, the latter 
ranging from about 427 cubic em. (Chimpanzee) to 557 (Gorilla), 
i.e. averaging less than half that of the human races mentioned 
above. As yet no human skull has been discovered which bridges 
over this gap. 
The cause of this great difference les largely in the fact that 
the brain of the Ape makes no marked progress after birth, and 
this no doubt applies not only to its size, but also to its micro- 
scopic anatomy, e.g. to the differentiation of its gray cortex. 
The Anthropoid skull is furnished with massive jaws con- 
trolled by powerful muscles and armed with formidable teeth. 
This extraordinary development of the facial portion of the skull 
which supports the entrance to the alimentary canal, is no doubt 
of compensatory value in the struggle for existence. We shall 
return to this subject in considering the dentition as a determin- 
ing factor in the modification of the jaws. _ 
The foregoing account of the changes undergone by the 
cranial skeleton has, I hope, shown that the human skull is 
subject to the same influences as that of the beasts, and that the 
two differ as divergent adaptive modifications of one and the 
same fundamental plan. This is not, however, an altogether 
satisfactory explanation, since the primary cause of this difference 
of modification (in Man in the psychic and brain-forming direc- 
tion, in the Anthropoids in the vegetative direction) remains 
unknown. 
That these divergent lines of modification from a common 
starting-point were entered upon very long ago is proved, not only 
by the sharply differentiated types of skull found both among 
Anthropoids and Men, but also by the fact that great and un- 
doubtedly atavistic deviations from the general normal type of 
human skull are comparatively rare. The type appears complete, 
well established, and sharply individualised. 
Exception must be made in the case of the dentition, to 
which the above is not applicable, and also in that of micro- 
cephalous and teratological conditions, although these are often 
enough utilised in building up the primitive history of the 
human skull. It is, however, possible, inasmuch as some of these 
cases certainly exhibit phenomena due to arrest of development, 
that an occasional indication of a former primitive condition may 
be revealed in them; but the pathological element is, as a rule, so 
strong that no certain morphological conclusions can be drawn— 
indeed, deceptive appearances may be expected at every step. 
