in VARIATIONS: HUMAN SKULL. 193 



cumstances, so that the angle obtained is a complex 

 resultant of all these circumstances, and is not the 

 expression of any one definite organic relation of 

 the parts of the skull. 



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 be- 

 fore its sides and roof; it is converted into cartilage 

 earlier and more completely than the sides and 

 roof: and the cartilaginous base ossifies, and be- 

 comes 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 b) 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 li). 

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

 177 



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