No. 2. ] THE HARD PARTS OF THE MAMMALIA. 233 
for a very long time. We must then observe that the internal 
tubercle of the superior triangle has been pushed continually 
forwards and been reduced to a very small size. Why should 
this occur? Why should not the corresponding tubercles of the 
inner side of the lower crown have been pushed backwards, 
since action and reaction are equal? The reason is clear: The 
superior tubercle is supported by but one root, while the resist- 
ant portion of the inferior crown is supported by two, thus 
offering twice the resistance to the pressure that the superior 
does. But why should the anterior part of the inferior tooth 
move forwards? even if it be in the direction of least resist- 
ance? This is due to the 
regular increase in size of 
the teeth themselves, an in- 
crease which can be traced 
from the beginning to the 
end) of the ‘series: “And 
this increase is the usual 
result of use (Fig. 60). 
whe mechanics of” the 
above proposition I believe = 
to be correct, but I have Figure 59.— Cynodictis getsmarianus Cope; 
had occasion to modify the skull one-half natural size: a, right side; 4, left 
statement as to the initia- 
tory cause of the process. In many primitive Ungulata the 
canines have been as well developed as in the Carnivora, yet 
the forward pressure of the inferior molars on the superiors has 
not resulted, or has not been sufficient to produce sectorial 
molars in those types. In the Amblypoda, the lower molars 
even shear backwards on the upper ones. It seems then that 
this growth of the canines is not in all instances sufficient to 
cause a proterotome mastication. I suspect that the more usual 
cause is to be found in the voluntary effort of the primitive 
flesh-eater, to masticate flesh by the manipulation of his lower 
jaw and the body to be divided. The looseness of articulation 
in primitive Creodonta will permit a manipulation such as we 
observe in various Ungulates to-day. The formation of a habit 
of a proterotome mastication would result, and the structural re- 
sults would succeed as above pointed out. 
The excess of the forwards pressure of the inferior teeth 
side from below. 
