3 a6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



ments in two groups of Mammalia so widely separated 

 as these two is strong evidence in favor of the belief 

 that the two facts stand in the relation of cause and 

 effect. 



I now present an example of the effect of strain, as 

 shown by the direction of the inferior incisors of the 

 lemurine Quadrumana. These teeth project horizon- 

 tally from the extremity of the mandible, so as not to 

 oppose the superior incisors, in consequence of which 

 they are useless as organs of prehension. But they 



Fig- 95- Lemur collaris, dentition from below and above ; natural size ; 

 original. 



are used by their possessors as a comb for the fur, 

 drawing them from below upwards when thus employ- 

 ing them. The strain is always in one direction, and 

 must have resulted in developing the procumbent posi- 

 tion which they now display (Fig. 95). This is a di- 

 rect deduction from the fact that the incisor teeth are 

 similarly displaced by the pressure of the tongue in 

 cases of the abnormal enlargement of that organ in 

 man. 



I now describe the general character of mammalian 

 dentition, with the view of pointing out how strong, in 



