Yerkes, Modifability of Behavior. 255 



of electric stimulus to rapidity of habit-formation it becomes per- 

 tinent to inquire, Is utter hunger as favorable a condition for the 

 discovering of a certain method of obtaining food as moderate hun- 

 ger ? Is extreme eagerness to escape from confinement as favorable 

 as a moderate desire ? What we really should know before we 

 undertake to study the intelligence of a particular animal is the 

 value for it of the several factors which constitute the chief ex- 

 perimentally controlled conditions of activity. So long as we 

 continue to use external conditions as incentives to habit-formation, 

 without definite knowledge of their values for the individual, we 

 shall work blindly. Food supply — the internal aspect of which is 

 hunger — as a condition of habit-formation, may be studied experi- 

 mentally; and the same is true of every other so-called motive upon 

 which the experimenter depends. It is high time that we made 

 serious efforts to discover the values of our stimuli instead of sloth- 

 fully assuming that they will answer our purposes. 



That there are a number of important laws of habit-formation 

 to be discovered no student of animal behavior can doubt. These 

 laws, of which the one offered above may serve as an example, 

 should rapidly replace what is too much talked of as "the law of 

 habit-formation." 



V. RELATION OP DIFFICULTNESS OF DISCRIMINATION TO 

 RAPIDITY OF HABIT-FORMATION AT DIFFERENT AGES. 



Among the important results of the investigation of relation of 

 strength of stimulus to rapidity of learning was the demonstration 

 of the fact that differences in plasticity depend upon the con- 

 dition of visual discrimination as well as upon the strength of the 

 electric stimulus. What holds with respect to rapidity of acquisi- 

 tion of the white-black discrimination habit in young and old 

 dancers, under conditions which render discrimination difficult, does 

 not necessarily hold under conditions of easy discrimination. This 

 I have demonstrated, and thrown further light upon, by three dif- 

 ferent methods, the results of which will now be presented in 

 turn. 



1. Experinienis with cardboards in discrimination hex furnished 



