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.

FIG. 26.--Side and front views of the round and orthognathous skull of a
Calmuck, after Von Baer. One-third the natural size.

I have arrived at the conviction that no comparison 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 moveable.

Fig. 27.--Oblong and prognathous skull of a Negro; side and front views.
One-third of the natural size.

The same truth is exemplified by the study of the modifications which
the skull undergoes in ascending from the lower animals up to man.

FIG. 28.--Longitudinal and vertical sections of the skulls of a Beaver
('Castor Canadensis'), a Lemur ('L. Catia'), and a Baboon
('Cynocephalus Papio'), 'a b', the basicranial axis; 'b c', the
occipital plane; 'i T', the tentorial plane; 'a d', the olfactory plane;