256 'Journal of Comparative Neurology and Psychology. 



the first indication of the great importance of condition of discrim- 

 ination. With other points of method the same as in the plas- 

 ticity exj^eriments, I so arranged the black and white cardboards 

 of the discrimination box that the amount by which the white 

 box differed in illumination from the black box was very much 

 greater than it had been in the earlier experiments. Whereas, 

 formerly, discrimination had been rather difficult, it was now made 



TABLE 9. 



Relation of Age to Rapidity of Habit-formatiox Under Conditions of 



Difficult and of Easy Discrimination 



White-black Discrimination 



* At this point condition of discrimination was changed from " easy " to " difficult." 



easy. The plasticity experiments, it is to be remembered, showed 

 that the young dancers acquired the habit much more rapidly than 

 the old individuals. Just the reverse proved to be true under the 

 conditions of easy discrimination : the old mice learned more quickly 

 than the young individuals. 



In order to make the results perfectly conclusive, I carried out 

 series of training experiments at the same time with a pair of 



