Yerkks, Modifability of Behavior. 247 



diminishes from the age of one month to between the seventh and 

 the tenth months. It seems then to increase slightly. Whereas 82 is 

 the index for the one-month individuals, that for the four-month 

 males is 128, and that for the seven-month males 192. For the ten- 

 month group the index 160 indicates increasing instead of diminish- 

 ing plasticity. 



The results indicate that the plasticity of the females does not 

 change greatly between the first and the fourth months ; that there- 

 after it decreases for a few months, and then again increases slightly. 

 The indices for the several ages, as they appear in Table 6, are 106, 

 106, 146, and 131. All of these except the first, indicate a degree of 

 plasticity higher than that of the males. 



Figures 3 and 4 represent graphically the principal results of the 

 experiments which have just been described. Figure 3 is based upon 

 the general averages of Table 5. The irregularly broken line is the 

 curve of the learning for the males of the dancer race ; the regularly 

 broken line, for the females of the race. The superiority of the fe- 

 males in the acquisition of this particular white-black discrimina- 

 tion habit is apparent. Figure 4 is based upon the data of the third, 

 fifth, and seventh columns of Table 6. The irregularly broken line 

 may be termed the plasticity curve of the male dancer (for a particu- 

 lar habit) between the ages of one month and ten months. The regu- 

 larly broken line may similarly be termed the plasticity curve of the 

 female dancer between the same age limits. The solid line, the 

 plasticity curve of the race. 



And now we are confronted by the question, Why the age differ- 

 ences in plasticity which are exhibited by our results ? In reply we 

 might say that preference determines rapidity of learning. For we 

 note that the females, apparently because of their strong preference 

 for the black box, make more errors of choice in the first training 

 series than do the males (general averages of Table 5), and that sub- 

 sequently they very rapidly learn to avoid the black box. It would 

 appear, then, that initial preference for the black box is a favorable 

 condition for habit-formation because it leads to a large number of 

 errors in the early training series and thus gives the animal that 

 experience which enables it to adjust itself to the situation. This 



