308 Animal Life and Intelligence. 



spatial quality, or local sign, dependent upon the part of 

 the body to which the stimulus is applied. These local 

 signs have, doubtless, in the long run, been established by 

 experience— if . under this term we may include a more or 

 less unconscious process, the outcome of evolution. But 

 they are so rapidly established in the individual, that we 

 are forced to conclude that we inherit very highly developed 

 aptitudes for localization. 



The refinement of localization is very different in the 

 different senses. In smell and taste there seems no more 

 than a general localization in the organ affected — the nose 

 or the mouth. In hearing there is not much more, unless 

 we regard the discrimination of pitch as a mode of localiza- 

 tion. In touch (and temperature) the refinement is much 

 higher, but it varies with the part of the body affected. 



If the back be touched by two points less than two 

 inches and a third apart, the sensation will be that of a 

 single point ; the finger-tips, however, can distinguish two 

 points separated by less than one-tenth of an inch ; and 

 the tip of the tongue is still more refined in its power of 

 discrimination, distinguishing as two, points separated by 

 less than the twenty-fifth part of an inch. So that the 

 tongue is about sixty times as refined in its discrimination 

 as the skin of the back. Moreover, the delicacy of localiza- 

 tion may be cultivated, so that in some cases the refine- 

 ment may, by practice, be doubled. 



When we come to sight, the refinement of localization 

 reaches its maximum, the local signs in the retina showing 

 the highest stage of differentiation, the distance on the 

 retina between two points distinguishable by local signs 

 being, according to Helmholtz, not much more than -^^ 

 of an inch ("0044 millimetre), which nearly corresponds 

 with the space between two cone's in the yellow spot. 



We must remember that the presentations of sense are 

 in all cases given in a stippled form, that is, by the stimu- 

 lation of a number of separate and distinct points. In 

 vision the stippling is very fine, owing to the minute size 

 and close setting of the retinal cones. In the case of 



