244 



Evolution and Adaptation 



the body is the result of individual experience ? Have we 

 learned to discriminate in those parts of the body that are 

 most often brought into contact with surrounding objects ? 

 Even the power of discrimination in the tips of the fingers 

 can be improved, as Spencer himself has shown, in the case 

 of the blind, and of skilled compositors. Can we account in 

 this way for the power of discrimination in various parts of 

 the body ? In other words, if, beginning in infancy, the middle 

 of the back constantly came into contact with surrounding 

 objects, would this region become as sensitive as the tips of 

 the fingers ? The experiment has not, of course, been carried 

 out, but it is not probable that it would succeed. I venture 

 this opinion on the ground of the relative number of the 

 nerves and of the organs of touch on the back, as compared 

 with those of the finger-tips. But, it will be asked, will not 

 the number of the sense-organs become greater if a part is 

 continually used by the individual ? It is improbable that 

 much improvement could be brought about in this way. The 

 improvement that takes place through experience is probably 

 not so much the result of the development of more sense- 

 organs, as of better discrimination in the sensation, because 

 the increased power can be very quickly acquired. 



An examination of the relative abundance of touch-spots in 

 the skin shows that they are much more numerous in regions 

 of greater sensitiveness. The following table, taken from 

 Sherrington's account of sense-organs in Schaefer's "Text- 

 book of Physiology," gives the smallest distance that two 

 points, simultaneously applied, can be recognized as such (and 

 not simply as one impression) in different regions. 



Mm. 

 I.I 



2-3 



4-5 

 4-5 

 6.8 

 9.0 

 9.0 



Tip of tongue ..... 



Volar surface of ungual phalanx of finger 



Red surface of lip . 



Volar face of second phalanx . 



Dorsal face of third phalanx . 



Side of tongue .... 



Third line of tongue, 27 mm. from tip 



