34 o PRIMARY FACTORS OF ORGANIC EVOLUTION. 



suit between the anterior external edge of the lower 

 triangle and the posterior internal edge of the superior 

 triangle. Now the characters of the true sectorial teeth 

 consist in the enormous extension of these same edges 

 in a fore and aft direction, the inferior shutting inside 

 of the superior. To account for the development of 

 these blades we must understand that the oblique pres- 

 sure of the front edge of the lower tooth, on the hind 

 edge of the superior tooth, has been continued for a 

 very long time. We must then observe that the inter- 

 nal 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 on the internal apex of the trigon, and is 

 supported by but one root, while the resistant portion 

 of the inferior crown is the base of the trigonid, and 

 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 resistance? This 

 is due to the regular increase in size of the teeth them- 

 selves, an increase which can be traced from the be- 

 ginning to the end of the series. And this increase is 

 the usual result of use (Fig. 102). 



The mechanics of the above proposition I believe 

 to be correct, but I have had occasion to modify the 

 statement as to the initiatory 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 sec- 



