Yerkes, Modifiability of Behavior. 



259 



factors it would be rash indeed to make general statements concern- 

 ing the relation of age to plasticity. We must limit ourselves care- 

 fully to particular statements, for what holds of one condition of 

 training may not hold at all of what appears to be a very similar 

 condition. 



TABLE 11. 



Indices of Plasticity for Dancers of Different Ages, Trained Under 



Conditions of Difficult or of Easy Visual Discrimination. 



2. Experiments luitli discrimination hex in darTc-room. The 

 results of the experiments which have just been described suggested 

 to me the idea that ability to acquire the white-black visual dis- 

 crimination habit depends largely upon two factors: capacity for 

 visual discrimination and associative memory. The facts of plas- 

 ticity thus far revealed might be accounted for, it would seem, by 

 the assumption that in the young dancer capacity for visual dis- 

 crimination was either greater at the outset or more readily devel- 

 oped than inj:he case of old individuals, whereas associative memory 

 is more highly developed in the old than in the young mice. This 

 hypothesis I immediately attempted to test experimentally. If it 

 be correct, young mice should develop the capacity to discriminate 

 slight differences in luminosity more quickly than old mice. To 

 test this matter I planned a series of training experiments with 

 the apparatus which I have previously described^ ^ as the Weber's 



"The Dancing Mouse, p. 118. 



