Yerkes, Modifiability of Behavior. 267 



often heretofore we have discovered sex differences to be of import- 

 ance for the interpretation of our results. Comparing the young 

 dancers with the okl, we note that the males of one to two months 

 of age acquire both the labyrinth D and the labyrinth C habits 

 considerably less quickly, as measured by the number of tests, than 

 the ten-month individuals. In the case of the two groups of 

 females there is practically no difference in rapidity of learning. 

 The general averages likewise show that the old dancers are some- 

 what superior to the young in ability to learn these labyrinth 

 paths. 



What evidence we have, favors the conclusion that the associative 

 memory of the dancer improves somewhat during the first year 

 of life. Possibly change in the "apperceptive mass" is responsible. 

 It is only fair to admit, in concluding the presentation of experi- 

 mental data, that I consider this problem unsolved, for I have 

 presented insufficient evidence to convince the critical observer that 

 associative memory improves with age. 



VII. CONCLUSIONS AND SUMMARY. 

 This attempt to discover the relation of age and sex of the danc- 

 ing mouse to plasticity, or rapidity of habit formation, makes 

 possible interesting and important, albeit not altogether favorable, 

 comments upon the methods of the investigation. As the work 

 progressed it became increasingly clear that the use of cardboards 

 as means of producing different degrees of illumination of the two 

 boxes between which the mouse was forced to discriminate was 

 unsatisfactory. Chief among the objections to this method may be 

 mentioned the practical impossibility of keeping the difference 

 in illumination constant throughout even a single series; the im- 

 possibility of determining accurately, except by very elaborate and 

 time-consuming methods, either the relative or the absolute illu- 

 mination of the white and black boxes ;^^ the impossibility of chang- 



"Photometric determinations of the amount of light reflected by the white 

 and the black cardboards which were used throughout these experiments 

 indicate that the white cardboard reflected about 10.5 times as much light as 

 the black cardboard. For a careful measurement of these values of the card- 

 boards I am indebted to Trofessor J. W. Baird. 



