THE TEETH 443 



limit set by the destructive effects of such use, or by the recuperative 

 energy of nutrition. 



Of/ 



" 5. 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 corre- 

 sponding parts of the opposite jaw." 



"The Origin of Canine Teeth. The origin of canine, pseudo- 

 canine, and canine-like incisor teeth is due to the strains sustained bv 



/ 



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 

 position has been most used by Mammalia. The reason 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 receives. 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." 



Development of Molar Teeth. " In fishes and reptiles, where 

 teeth occasionally present very primitive conditions, the theory of 

 the origin of particular types of molar teeth is more simple than in 

 the case of Mammalia. The observations of Hiiter on the action of 

 osteoblasts under stimulus show that under moderate irritation osseous 

 tissue is deposited, while under severe pressure osseous tissue is 

 removed. Kolliker has shown that the action of these bodies is the 

 same in dentine as in true bone. Hence modifications of dental 

 structure must stand in close relation to the uses to which they are 



