262 'Journal of Comparative Neurology and Psychology. 



the old ones. But we must remember that the couditioiis of this 

 experiment are markedly different from those in which cardboards 

 were used to render the two boxes visually distinguishable. 



ISText the amount of difference in luminosity was reduced to one- 

 half, and the experiment continued. Again the young individuals ac- 

 quired the habit more quickly than the old ones. The index of 

 plasticity for the old is 250, for the young it is 1G5. 



With a difference in luminosity of only one-fourth, the training 

 was now continued for several days, but as no one of the four mice 

 succeeded in acquiring a perfect habit it was changed finally to 

 one-third. It is noteworthy that in the thirty-four series (340 

 tests) that were given to the mice with the difference one-fourth, 

 the old individuals did not succeed in making a correct series, 

 whereas both of the young mice did. With the difference one- 

 third, No. 294 quickly acquired a perfect habit, and No. 293 came 

 very near to doing so, but failed in twelve series. At the con- 

 clusion of the twelfth series, neither of the old individuals had 

 learned to choose correctly, with the difference one-third. 



Although the results of this experiment are not as convincing 

 as they might be, they do indicate that young dancers can ac- 

 quire the ability to discriminate slight differences in luminosity 

 more readily than can old individuals. It is conceivable, then, 

 although by no means demonstrated as true, that the young indi- 

 viduals in the plasticity experiments acquired the white-black habit 

 more quickly than the old individuals did because they could dis- 

 criminate better or acquired discriminating ability more rapidly 

 and not because they acquired an association more readily. In 

 this event, our experiment measures differences in visual discrim- 

 ination, and in changes which it undergoes with training instead 

 of associative plasticity. 



I have already sho\\ai^- that the dancer is capable, as the result 

 of prolonged training, of developing the power to discriminate be- 

 tween boxes which differ from one another in illumination by 

 less than one-tenth. This fact becomes important at this point, for 



"The Dancing Mouse, pp. 127, 128. 



