KINETOGENESIS. 327 



the light of the facts already cited, is the evidence of 

 their origin through mechanical strains and impacts. 



a. The Origin of Canine Teeth. 



The origin of canine, pseudo-canine, 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 (Dimetro- 

 don) the end of the muzzle has been utilized ; in croco- 

 diles, the side of the jaw ; while the intermediate 

 position has been most used by Mammalia. The rea- 

 son why the canine instead of the incisor teeth have 

 been selected by carnivorous Mammalia for prehensile 

 purposes is not at present clear to me. In accordance 

 with Rule I., 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 experiencing the strain 

 that a tooth more anteriorly placed necessarily re- 

 ceives. It is excluded from considerable use by the 

 projecting muzzle above and in front of it. That it 

 was not drawn out into a horizontal position was due 

 to the presence of teeth anterior to it. 



