Yerkes, Modifahility of Behavior. 269 



forinatiou curves are not necessarily indicative of general differ- 

 ences in plasticity is rendered evident by the results of the sections 

 on sensitiveness, strength of stimulus, and difficultness of discrim- 

 ination. For in the light of these results we are able to name as 

 two important, and to a certain extent independently variable, con- 

 ditions upon which the acquisition of the visual habit depends, (a) 

 ability to sense the difference in illumination of the two boxes and 

 (b) ability to associate the darker box with the electric shock. 

 Evidently an individual which possesses highly developed white 

 light vision and is capable of distinguishing very slight differences 

 in illumination may, at the same time, possess little ability to asso- 

 ciate stimuli. The data of section V indicate that what we have 

 called age and sex differences in plasticity are in all probability, to be 

 referred to differences in visual discriminating ability and in '^asso- 

 ciative memory." Admitting that the results of the experiments 

 justify only tentative conclusions, we may say that the young dancer 

 seems to be somewhat superior to the old individual in ability to dis- 

 criminate on the basis of difference in illumination, whereas the 

 old individual seems to associate stimuli somewhat more rapidly, 

 if anything, than the young dancer. 



This suggestion, for it is scarcely more than that, of the way 

 in which a habit may break up into two relatively independent 

 factors is one of the most interesting results of the investigation. It 

 strongly emphasizes the importance of studying the various sense 

 factors separately and of attempting to discover upon what exter- 

 nal and internal conditions '^associative memory" depends. 



To sum up the results of the investigation point by point, it ap- 

 pears that — 



1. The dancer at one month of age acquires a particular white- 

 black visual discrimination habit more rapidly than do older indi- 

 viduals. From the first until the seventh month there is a steady 

 and marked decrease in rapidity of habit-formation; from the 

 seventh to the tenth month the direction of change is reversed. 

 These statements hold for both sexes. 



2. Young males acquire the habit more quickly than 3'oung fe- 

 males, but between the ages of four and ten months (at least) the 

 females acquire the habit the more quickly. 



