324 STUDIES, SCIENTIFIC AND SOCIAL chap. 



There is also a general argument, in the fact of so many 

 special types of teeth having been developed, which cannot 

 have been produced by the corresponding use. Such are 

 the arrangement of enamel and dentine in the incisor 

 teeth of rodents, so that they preserve a continual chisel- 

 like cutting edge, and, unlike the teeth of most other 

 mammals, keep on growing at the root so that they are 

 pushed up as fast as they wear away ; and the remarkable 

 molar teeth of the elephants, which come forward in 

 succession, and by the arrangement of the folds of hard 

 and soft material always keep a grinding surface, while the 

 enormous tusks grow on continuously during life. These 

 and many other singular modifications of teeth can 

 certainly not be traced to coi responding diversities of use 

 as directly producing them, while they are easily explained 

 by the great variability of all complex organic structures, 

 furnishing material for endless modifications according to 

 the various needs of the widely different mammalian 

 types. 



Argument from Tactual Discriminativeness. 



We have now to consider Mr. Herbert Spencer's objec- 

 tions in the articles already referred to, which bear the 

 formidable title, " The Inadequacy of Natural Selection." 

 The first of these objections is founded on Weber's experi- 

 ments on the sense of touch, showing that the power of 

 distinguishing the sensations produced by two points 

 rather close together varies greatly in different parts of 

 the body, the tips of the fingers being able to distinguish 

 the points of compasses when the twelfth of an inch apart, 

 while on the middle of the back they have to be opened 

 more than two inches in order that the pressure of two 

 points may be distinguished from that of one. Between 

 these extremes tactual discrimination varies in different 

 parts of the body, apparently without much relation to 

 utility, except of course in the case of the fingers ; and 

 after detailing these at some length Mr. Spencer asks how 

 these divergences can possibly be explained by natural 

 selection. " Why," he asks, " should the thigh near the 

 knee be twice as perceptive as the middle of the thigh ? " 



