Yerkes, Modifiahility of Behavior. 257 



dancers one month old and a pair twelve months old under the condi- 

 tions of discrimination used in the plasticity investigation, and sim- 

 ilarly with two pairs of dancers one of which was one month old 

 and the other eight months, under the conditions which I have 

 just characterized as easy. The results of these experiments, as 

 they are presented in condensed form in Table 9, are striking 

 indeed. As was the case in my first experiments, the old dancers^" 

 acquired their habit, under conditions of difficult discrimination, 

 much less rapidly than did the young individuals. The index of 

 plasticity for the twelve-month mice, Nos. 112 and 113 of the table, is 

 130; that for the one-month mice, Nos. 292 and 291, is 70. The 

 latter acquired the habit with few more than half as many training 

 tests as were necessary for the former. 



When we turn to the results of the experiments made under 

 conditions of easy discrimination, we find that the eight-month mice, 

 Kos. 204 and 121, learned with only 40 tests; whereas, the one- 

 month individuals, ]!^umbers 430 and 432, required 50 tests. 



In Table 10 are presented the results of additional experiments 

 like those just described. Two eight-month dancers, Nos. 136 and 

 166, acquired the habit on the basis of 40 and 20 tests, respectively. 

 Their index of plasticity is, therefore, 30. The index for two four- 

 month mice, which were subjected to the same training, was 80, and 

 that for two eight-month individuals, 75. 



The importance of the relation of age to difficultness of visual dis- 

 ci'imination is clearly exhibited by the indices of plasticity for 

 young and old dancers under conditions of easy and difficult dis- 

 , crimination in Table 11. 



Comparison of the data of Tables 9, 10 and 11 with those of 

 Tables 1 to 6 proves conclusively that the direction of the age dif- 

 ferences in plasticity which was revealed by the experiments described 

 early in this paper was determined, in part at least, by the condi- 

 tion of visual discrimination which happened to be chosen for the 



^°I shall use the terms of old and young in contrasting two groups of 

 dancers which differed in age by several months. As a matter of fact a 

 dancer at the age of eight, ten, or even twelve months is not, as a rule, 

 obviously senile. 



