No. 2.]} THE HARD PARTS OF THE MAMMALIA. 219 
away from the direction of greatest, and in the direction of least 
resistance. 
III. It follows, from their greater flexibility, that crests of 
crowns of teeth yield to strains more readily than do the cusps. 
IV. The increase in the length of crests and cusps in all 
directions, and therefore the plications of the same, is directly 
as the irritation from use to which their apices and edges are 
subjected, to the limit set by the destructive effects of such 
use, or by the recuperative energy of nutrition. 
V. The direction of growth of the branches of a V, or of the 
horns of a crescent, will be the direction of movement of the 
corresponding parts of the opposite jaw. 
1. THE ORIGIN OF CANINE TEETH. 
The origin of canine, pseudocanine, and canine-like incisor 
teeth is due to the strains sustained by them on account of 
their position in the jaws at points which are naturally utilized 
in the seizing of prey, or the fighting of enemies. In some 
reptiles (Dimetrodon) the end of the muzzle has been utilized ; 
in Crocodiles the side of the jaw; while the intermediate posi- 
tion has been most used by Mammalia. The reason why the 
canine instead of the incisor teeth have been selected by car- 
nivorous Mammalia for prehensile purposes is not at present 
clear to me. In accordance with Rule [, its increased size has 
been due to the especial and energetic strains to which it has 
been subjected while in use as a prehensile or offensive weapon, 
when buried in the body Of its prey or enemy. The superior 
canine would acquire larger size earlier in time than the inferior 
canine, since it bears the greater part of such strain, as attached 
to the more fixed head and body of its possessor. The anterior 
teeth of the lower jaw would be less available for use, since 
they offer weaker and less fixed resistance to the opposing 
body. That the first tooth behind the canine was not generally 
enlarged is (under I.) due to the fact that its posterior position 
prevents it from having the same amount of use, and experi- 
encing the strain that a tooth more anteriorly placed necessarily 
receives. It is excluded from considerable use by the project- 
ing muzzle above and.in front of it. 
That the increased size of canine teeth is due to strains is 
