342 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Canidae, and wanting in some Mustelidae, and all Fe- 

 lidae. The heel of the same tooth has had a similar 

 history. With the diminution in size of the first supe- 

 rior tubercular, with which it comes in opposition in 

 mastication, its functional stimulus also diminished ; 

 and it disappeared sometimes a little sooner (Felidae) 

 and sometimes a little later (Hyaenidae) than that tooth. 

 The specialization of one tooth to the exclusion of 

 others as a sectorial, appears to be due to the follow- 

 ing causes. It is to be observed in the first place that 

 when a carnivore devours a carcass, it cuts off masses 

 with its sectorials, using them as shears. In so doing 



* 3 i. 3 



Fig. 104. Aehirodon scevus Leidy ; diagram representing coadaptation of 

 crowns of superior and inferior molars in mastication ; lines and lettering as 

 in Fig. 102. 



it brings the part to be divided to the angle or canthus 

 of the soft walls of the mouth, which is at the front of 

 the masseter muscle. At this point the greatest amount 

 of force is gained, since the weight is thus brought 

 immediately to the power, which would not be the case 

 were the sectorial situated much in front of the mas- 

 seter. On the other hand, the sectorial could not be 

 situated farther back, since it would then be inacces- 

 sible to a carcass or mass too large to be taken into 

 the mouth. 



The position of the sectorial tooth being thus shown 

 to be dependent on that of the masseter muscle, it re- 

 mains to ascertain a probable cause for the relation of 



