THE TEETH 453 



* 



as well as cases of supernumerary teeth, because the doubling of a 

 tooth appears to be dependent upon some general law manifested in 

 all parts of the body. Inasmuch as we have no record of variations 

 in the incisors and canines, we must confine ourselves to variations in 

 the cheek teeth. 



I believe that these variations tend to establish the following 

 points : 



(1.) The variations in structure occur almost exclusively in those 

 teeth which are placed in the jaw at the points of greatest functional 

 activity. 



(2.) The variations are directly dependent upon use. 



(3.) They occasionally occur as reversions to a less specialized 

 type. 



(4.) The reduction in the number of teeth begins at the ends of 

 the series ; that is, in the regions of the jaw which are functionally 

 less important. 



(5.) The evolution of the third upper premolar has taken place 

 in the manner described by Scott, by the addition of an inner cusp or 

 deuterocone to the specialized protocone and tritocone. 



(6.) The lower molar is developed, as Cope has pointed out, from 

 the tuberculo-sectorial type by the loss of the metaconid and the heel. 



I have observed no case of absence of any of the cheek teeth 

 except of the first upper premolar and the upper molar. That the 

 former tooth is gradually disappearing is evident from its rudimentary 

 condition and from the fact that it was entirely absent in almost twenty 

 per cent, of the maxillaries studied. The molar is functionally of 

 more importance, because it still shears on the lower molar ; hence it 



A 



is less frequently absent. 



These two teeth are absent in a group of highly specialized extinct 

 cats, the Machserodonts, illustrated by the Sabre-toothed Tiger, in 



,31 2 



which the dental formula is reduced to i ~, c -> pm - c > m -- = 24 



2 I 2 or 1 1 



or 26. Here, however, the reduction was associated with an enormous 

 development in size of the upper canines, probably to accommodate 

 the animal to special conditions of life, the removal of which led to 

 its extinction. 



It is interesting to note that of all the cheek teeth the second 

 upper premolar is the most variable, and, further, that the variations 



