250 E. C. MACDOWELL AND E. M. VICARI 



learning, while in later trials the difference is reduced. Figure 

 8 gives the distributions of the averages of the individual rats for 

 the six groups of trials. There appears a strong tendency for 

 the test rats to lie further up the scale (to the right) than the 

 controls. This is less marked in the second half of training; 

 and in retention there is very little difference between the distri- 

 butions of the tests and controls. The averages of the distance 

 for each rat for each of the groups of trials from which the pre- 

 ceding summaries have been made are given in table B in the 

 appendix. 



b. Distance: averages for each day. The next step in the aver- 

 ages of the distance data is the comparison of the tests and con- 

 trols on each of the eight days of training and on the four daj'S 

 of retention. In table 9 the averages are arranged in the same 

 combination of sexes and strains as before, and then compared 

 by their ratios; 'plus' ratios indicate that the tests covered more 

 distance than the controls. The averages for each day, when 

 the sexes and strains are put together, are shown by the curves 

 in figure 9; in figure 10 are represented graphically the ratios 

 of the tests vs. controls for the males and females together and 

 the strains separately as well as together. Obviously, the tests 

 covered more ground in the great majority of cases. The females 

 in strain A show the reverse, but there can be no doubt that this 

 is not a general sex difference nor a strain difference, since the 

 females in the other two strains and the males in all three strains 

 do show that the controls covered shorter distances on the 

 average than did the tests. There is a tendency for the first 

 day of retention to show the tests covering less distance, but it 

 will be noticed that this is true of two of the strains, not of the 

 third (L). The real interest lies in the table (10) where the sig- 

 nificance of the differences may be found. This table is based on 

 all the males and females in all strains together. It appears 

 that on the first, third, and fourth days of training and on the 

 fourth day of retention the differences between the averages of 

 the tests and controls are more than three times their probable 

 errors; on the second and sixth days of training the differences 

 are only very slightly below three times their probable errors 



