Yerkes, Modifiahility of Behavior. 253 



(3j That when visual discrimination is moderately difficult, rap- 

 idity of habit-formation increases as strength of stimulus is in- 

 creased up to a certain point, and with further increase in the 

 stimulus it rapidly decreases. A moderate strength of stimulus is 

 most favorable for habit-formation under this condition of discrim- 

 ination. 



(4) That when visual discrimination is very difficult, rapidity of 

 habit-formation increases as strength of stimulus is increased for 

 a time, but not nearly so long as in the case of the medium con- 

 dition of discrimination, and then with further increase in the 

 stimulus it rapidly decreases. A low intensity of stimulus is most 

 favorable for habit-formation under this condition of discrimina- 

 tion. 



The law which is indicated by these facts may be formulated 

 thus. As difficuUness of visual discrimination increases that strength 

 of electrical stimulus luhich is most favorable to hahit- formation 

 approaches the threshold. The easier the hahit the stronger that 

 stimidus ivhich most quickly forces its acquisition; the more diffi- 

 cult the hahit the wcalxcr that stimidus ivhich rnost qiiichly forces 

 its acquisition. 



From these facts it is evident that the value of a given strength 

 of electric stimulus, for the training of a dancer whose sensitive- 

 ness is accurately known, can be stated only if the degree of difficult- 

 ness of discrimination for the individual also be known. A degree 

 of difference between the white and the black boxes which renders 

 discrimination moderately easy for one dancer may render it 

 (^xtremely easy for another. Male and female, or old and young, 

 or even two individuals of the same sex and age, may differ, 

 both in discriminating ability and in sensitiveness. 



This consideration makes apparent the incomparability of the 

 results of the plasticity experiments. Instead of uniformity and 

 simplicity of conditions, we have variability and complexity. It is 

 evident that before a given individual can be used to advantage in any 

 such training experiments as these, or rather before we can in- 

 terpret the results, we mu,st know accurately the relations of the 

 conditions of experimentation to the individual. 



