14 SMELL, TASTE, ALLIED SENSES 



environmental alterations. Thus the chemical changes of 

 the surroundings affect the organs of smell and of taste, 

 the pressure changes those of touch and hearing, and al- 

 terations in the radiant energy those of sight. These 

 natural groups of environmental changes have been des- 

 ignated as homologous, or, better, adequate stimuli for 

 the sense organ that they activate. Such organs are ordi- 

 narily arranged under five heads each with an adequate 

 stimulus and productive of a special sensation ; they are 

 the organs of smell, taste, touch, hearing, and sight. 



Experience has also shoAvn that when in a given per- 

 son a sense organ exhibits complete congenital incapacity, 

 such an individual lacks certain mental elements that can 

 never in reality be made good to him by the activity of the 

 remaining parts. A state of this kind implies a certain 

 mental deficiency in the given individual. If a person has 

 been blind from birth, no amount of description cjin 

 supply to him the sensations of the wealth of color that the 

 external world holds for the normal man. Where blind- 

 ness is an acquired defect, the rememberance of the 

 former color sensations as compared with the present 

 deprivation, makes the state of deficiency still more pro- 

 nounced. And in those rare cases where there is a 

 unilateral defect in color vision with sight otherwise unim- 

 paired, the subject can contrast most vividly the state of 

 deficiency with that of normal completeness. Such con- 

 ditions, which are known to occur not only in sight but in 

 the other senses as well, have had a most profound influ- 

 ence on the interpretations that naturalists have placed 

 upon the states presented by the lower animals. 



It has been commonly assumed, and with no small 

 show of reason, that where an animal is found to possess 



