164 NERVOUS CORRELATION 



the lower vertebrates, represent combinations of the two 

 kinds of organization. It is worth noting that when we 

 ascend to the very top of the scale, to man himself, we 

 find instincts, in the sense in which we usually speak of 

 them, almost wholly wanting, volitional acts taking 

 their place. 



Habit Formation. — It has been shown how, in ani- 

 mals that possess the property of associative memory, any 

 given act may be based upon the combined effect of im- 

 mediate sensory stimulation and such associated mem- 

 ories as are relevant. Assuming that the activity thus 

 set into motion proves useful, the recurrence of the same 

 combination of stimuli will naturally bring about the 

 same response. If the situation is one which recurs fre- 

 quently, there will be successive repetitions of the same 

 kind of activity. The result is to establish within the 

 nervous system exceptionally ready connections between 

 the sensory, association, and motor regions involved, 

 so that with the recurrence of the sensory stimulation the 

 performance of the particular act follows more and more 

 certainly. This is what we call habit formation. It will 

 be seen that it amounts to a suppression of the volitional 

 part of the control, thus making the process approach 

 the status of a pure reflex. No better illustration of 

 habit formation could be given than the driving of an 

 automobile. The beginner receives first a series of sen- 

 sory impressions in the form of instructions and observa- 

 tions of other drivers; from these he forms a series of 

 associated memories; and when he begins to drive, his 

 equipment for performing the act successfully consists of 

 these memories, together with the sensory stimuli which 

 come to him as the car moves along. The correlation 

 of these into correct muscular acts is at first extremely 

 fiifficult ; all the memories that have been acquired must 

 be railed into the mind every time a new movement on 

 the part of the driver becomes necessary. With the pas- 

 sage of time this necessity for recalling definitely one's 



