62 MAN AN ADAPTIVE MECHANISM 



just beyond, encounters this resistance to its passage, 

 which varies in degree for every impulse, as well as for 

 the same impulse from day to day, from year to year, 

 from experience to experience. When resistance to 

 the incoming impulse is great, the threshold is said to 

 be "high" ; when it is small, the threshold is said to 

 be "low/' Repeated response to any stimulus lowers 

 the threshold to that stimulus. The conditions which 

 govern the threshold are of material importance to the 

 individual. On the state of his thresholds to various 

 stimuli depend a man's capacity for education, his 

 habits and conduct. 



The threshold is responsible for the fact that no 

 more than the required energy is discharged in the 

 consummation of a given act, as it is also for the 

 condition which makes it possible for an individual to 

 profit as well as to suffer by past experience. 



Summation 



The result of any given stimulus depends upon 

 both the height of the threshold and the intensity of 

 the stimulus. Response may also be hastened and 

 intensified by repeating the stimuli at sufficiently 

 short intervals of time to produce a cumulative effect. 

 This is summation. In the conversion of energy for 

 adaptive purposes the principle of summation plays an 

 important role. If drops of water are allowed to fall 

 upon the skin at such a rate that the effect of the 

 stimulus of one drop has not passed before another 

 drop falls in precisely the same spot, there will be a 

 gradually increasing sensation in that spot, rising finally 

 to an unbearable degree of pain. The threshold to 



