182 THE MECHANISM OF LIFE 



Memory and Habit. 



The simplest kinds of organic responses, such as the helio- 

 tropic movements of a plant or infusorian or insect larva, or 

 the responses of a muscle-nerve preparation to artificial stimuli, 

 are therefore physically determined, or are nearly so. Reflexes 

 and instinctive actions are also largely determined, but we find 

 that there are often variations from the usual kind of acting. 

 Spontaneous, intelligent, and deliberate actions are very different 

 from the former categories, and we are quite unable to explain 

 them in the way we describe a reflex. What, then, is the 

 difference ? Plainly that these higher forms of acting involve 

 experience ; the response that an animal makes to a stimulus 

 depends not only upon the physical nature of the stimulus, but 

 also upon the effects of former stimuli, and the responses made 

 thereto under analogous conditions to those in which the present 

 stimulus occurs. Instances illustrating this can quite easily be 

 given, but we leave the reader to find them for himself by reflect- 

 ing upon his own behaviour. A cat, says Mark Twain, which 

 sits down on a hot stove never sits on one again, but neither 

 does she sit down on a cold stove ! 



What, then, is experience in the physical sense, and how is it 

 stored in an animal ? It is stored in two ways : first as motor 

 habits, and next as pure memories. 



The whole contents of Chapters VI., VII., and VIII. of this 

 book deal with the mechanism of motor habits; the receptor 

 organ, afferent nerve, intracerebral centres and nervous tracts, 

 efferent nerve and effector organ — such a chain of structures is 

 the unit mechanism that underlies a motor habit. Any skilled 

 technique — say the ability quickly and neatly to deal a pack of 

 cards — involves a preparation, in the course of which such action 

 was practised somewhat laboriously and clumsily. But with 

 repetition experience accumulates — that is, certain afferent 

 nervous impulses become directed easily along certain cerebral 

 tracts, and pass out from the brain into certain definite efierent 

 nerves and muscles, properly timed and co-ordinated, and so 

 produce the required muscular movements. That is to say, 

 definite nervous paths become established or laid down. Every 

 time the same stimuli, in the same conditions, recur, these paths 

 are used the more easily. This is the formation of a motor habit, 

 and all education or training which leads to a technique of any 

 kind establishes such paths and habits. 



