192 HUMAN FOSSILS 



III 



I have arrived at the conviction that no com- 

 parison of crania is worth very much that is not 

 founded upon the establishment of a relatively 

 fixed base line, to which the measurements, in all 

 cases, must be referred. Nor do I think it is a 

 very difficult matter to decide what that base line 

 should be. The parts of the skull, like those of 

 the rest of the animal framework, are developed 

 in succession : the base of the skull is formed 

 before its sides and roof; it is converted into 

 cartilage earlier and more completely than the 

 sides and roof: and the cartilaginous base ossifies, 

 and becomes soldered into one piece long before 

 the roof. I conceive then that the base of the 

 skull may be demonstrated developmentally to be 

 its relatively fixed part, the roof and sides being 

 relatively movable. 



The same truth is exemplified by the study of 

 the modifications which the skull undergoes in 

 ascending from the lower animals up to man. 



In such a mammal as a Beaver (Fig. 29), a line 

 (a 1} drawn through the bones, termed basiocci- 

 pital, basisphenoid, and presphenoid, is very long 

 in proportion to the extreme length of the cavity 

 which contains the cerebral hemispheres (g h). 

 The plane of the occipital foramen (h c) forms a 

 slightly acute angle with this " basicranial axis," 

 while the plane of the tentorium (i T) is inclined 

 at rather more than 90 to the " basicranial axis " ; 

 and so is the plane of the perforated plate (a d), 

 by which the filaments of the olfactory nerve 



