FORMS OF JOINTS AND TEETH 115 



not our experience. Pressure and friction have an 

 unfortunate way of wearing a hole in the tooth, rather 

 than causing it to grow an elevation. As a matter of fact 

 we know that the shape of teeth is predetermined, long 

 before they are cut, in the soft dental matrix beneath the 

 gum. It is not a question of the transmission of acquired 

 characters, but the supposed transmission of a character 

 which the parent cannot by any means acquire. Teeth, 

 so far as they react to pressure or friction, can only react 

 by wearing away. 



With regard to the joint, we are told by some La- 

 marckian writers that pressure and friction produce the 

 reverse effect and wear away cavities rather than stimulate 

 growth. I was reading the other day a most interesting 

 paper by Dr. Wortman of New York, in which the 

 author attempted to explain the occurrence of a furrow 

 in a joint owing to the pressure of a corresponding ridge. 

 The pressure of the ridge, he said, produces a furrow in 

 the opposite side of the joint. It seems to me that in this 

 we are going a little beyond what physiology and histology 

 teach us and making a blind appeal to mechanical forces 

 unsupported by any adequate investigation of the tissues 

 concerned. Is it likely that a bone would react to 

 intermittent pressure by producing a furrow ? It is far 

 more probable that the reverse effect would tend to be 

 caused. 



I will only ask one more question with regard to this 

 subject of use and disuse, and that is, why, if you are 

 going to explain any of these parts by pressure and 

 friction, should the process be stopped when a useful 

 level is reached ? If the pressure does cause such effects 

 and they are hereditary, how are they prevented from 

 increasing beyond all bounds in the course of generations ? 

 Why^houjdjpressure on teeth cease to produce further 

 growth, when the tubercle has reached its proper height ? 

 The_Jact that all these shapes of bones and teeth just 

 reach and stay at an adaptive level is the strongest 

 evidence that they are not produced by the operation of 

 mechanical forces, but by Natural Selection. 



We now pass to the consideration of indirect evidence : 



I 2 



