Yerkes, Modifahility of Behavior. 241 



it entered the white one and passed through to the nest box. In the 

 tables appear the number of errors per series made day after day by 

 the various individuals. At the outset of the experiments each mouse 

 was given two series of what may be called "preference tests." In 

 connection with these tests no electric shock was given and the mouse 

 was permitted to enter and j)ass through either the white or the black 

 box, for it was the sole purpose of the experimenter to discover, by 

 means of these series, any initial preference that the subject might 

 have for either the white or the black box. 



A habit of discriminating between the boxes, and of uniformly 

 choosing to enter the white one, was considered perfect when the 

 mouse made no errors in three successive daily series. As a measure 

 of the rapidity of habit-formation we may use the number of tests 

 between the beginning of the first training series (following pref- 

 erence series B) and the end of the series which preceded the three 

 perfect series. This measure of rapidity of learning, which I have 

 named the index of plasticity, proves to be extremely useful for pur- 

 poses of comparison. 



To ascertain age differences in rapidity of white-black habit forma- 

 tion I used groups of individuals which, so far as I could tell, differed 

 from one another constantly only in age. Five males and five females 

 constituted each group, and four such groups were used. During 

 their lives all of the animals were kept under the same conditions. 

 They were paired at the age of twenty-five days, and thereafter a male 

 and a female were kept in a separate cage and were placed in the ex- 

 periment box for their daily training at the same time and given 

 their tests alternately. 



We may now examine the results of the experiments. Table 1 con- 

 tains records of the number of errors of choice made by each of the in- 

 dividuals of the one-month-old group in each daily series. The num- 

 bers at the top of the columns refer to the mice. Even numhers 

 always designate males; odd numbers, females. The two preference 

 series are indicated by the letters A and B. ITo. 210, it will be noted, 

 made six erroneous choices in each of the preference series and also in 

 the first training series ; that is, he attempted to enter the black box 

 instead of the white box six times in ten. In subsequent training ae- 



