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. 
Tip of tongue . - : . : . a SEA 
Volar surface of ungual phalanx of finger ‘i ‘ ‘ : s- 233 
Red surface of lip . i é c ‘ . ‘ » 45 
Volar face of second phalanx . Z ‘ 7 . é , © 45 
Dorsal face of third ake : ‘ A : é 3 C - 68 
Side of tongue ‘ ; F 2 : + 9.0 
Third line of tongue, 27 mm. Been Gp ‘ : e - ‘ - 9.0 
