174 FOSSIL KEMAINS OF MAN. 



becomes more acute by the bending down, as it were, of 

 the facial axis upon the cranial axis. At the same time, 

 the roof of the cranium becomes more and more arched, 

 to allow of the increasing height of the cerebral hemi- 

 spheres, which is eminently characteristic of man, as well 

 as of that backward extension, beyond the cerebellum, 

 which reaches its maximum in the South American mon- 

 keys. So that, at last, in the human skull (Fig. 30), the 

 cerebral length is between twice and thrice as great as 

 the length of the basicranial axis ; the olfactory plane is 

 20° or 30° on the %mder side of that axis ; the occipital 

 angle, instead of being less than 90°, is as much as 150° 

 or 160° ; the cranio-facial angle may be 90° or less, and 

 the vertical height of the skull may have a large propor- 

 tion to its length. 



It will be obvious, from an inspection of the diagrams, 

 that the basicranial axis is, in the ascending series of 

 Mammalia, a relatively fixed line, on which the bones of 

 the sides and roof of the cranial cavity, and of the face, 

 may be said to revolve downwards and forwards or back- 

 wards, according to their position. The arc described by 

 any one bone or plane, however, is not by any means al- 

 ways in proportion to the arc described by another. 



N^ow comes the important question, can we discern, 

 between the lowest and the highest forms of the human 

 cranium, anything answering, in however slight a degree, 

 to this revolution of the side and roof bones of the skull 

 upon the basicranial axis observed upon so great a scale 

 in the mammalian series ? l^umerous observations lead 

 me to believe that we must answer this question in the 

 affirmative. 



The diagrams in figure 30 are reduced from very care- 

 fully made diagrams of sections of four skulls, two round 

 and orthognathous, two long and prognathous, taken Ion- 



