SENSATION 17 



Sensitiveness and Responsiveness. Animals and 

 plants do not live in self-contained independence : 

 to exist they must draw food from their environ- 

 ment and avoid being eaten by others. They 

 must, then, possess some means of communica- 

 tion with their surroundings, and this is provided 

 by their senses. By sight, touch, smell, taste, and 

 hearing, animals can discover their food material 

 and perceive their enemies. Plants obviously 

 possess some sensory powers : they are affected 

 by light and colour : some of them are very 

 sensitive to touch : in absorbing food material 

 their roots appear to exercise some measure of 

 selection. Our senses give us only symbolic im- 

 pressions of the things around us, and leave us in 

 ignorance of their real nature. We have, as it 

 were, to imagine the machinery of a musical box 

 from the tones and intervals of the music it plays 

 to us. But our symbolic impressions suffice for 

 our animal needs, although they fail altogether 

 to satisfy our philosophic curiosity. 



To be of practical utility sensation must be 

 followed by muscular reaction : the sight of food 

 must involve its seizure. Experiment seems to 

 have established that every sensation excites 

 instinctive action, and is followed by some move- 

 ment too slight it may be to be noticed by 

 consciousness, but capable of being detected by 

 elaborate appliances for measurement. Where 

 conduct is governed simply and uniformly by 

 directive instinct, movements that respond to 

 sensation ordinarily develop into definite action : 

 the mouth secretes its saliva immediately it is 

 touched by food ; the behaviour of insects 

 approaches the automatic. In the external 

 conduct of the higher animals, as we ascend the 

 scale of animal life, directive instinct gradually 

 surrenders some of its authority to inference from 



