288 ADA W. YERKES 



cult conditions than the stock rats, it is to be expected that the 

 total number of trials would be greater, but the comparison of 

 figures under conditions 1 and 2 shows that a large part of the 

 total difference is due to differences under the earlier condi- 

 tions. After the rats had learned to discriminate under the 

 simpler conditions, very few mistakes were made until discrim- 

 ination became difficult. 



The time records (average of 5 trials) of series 1 indicate 

 marked individual differences in the effects of punishment on 

 the different individuals. The two inbred females of group 2 

 (table 8) made choices very quickly in spite of mistakes and 

 shocks. Their choices were made more slowly on the days 

 following those on which mistakes were made, though never 

 as slowly as were the choices made by some of the other rats. 

 Three of the stock rats and one inbred male were very much 

 delayed in making their decisions by being punished for mis- 

 takes. The fourth inbred rat was rendered so excited that 

 after resisting vigorously reentering the apparatus, he dashed 

 wildly in and chose at random, making quick choices and many 

 errors. These facts will be discussed further in the following 



section. 



3. GENERAL BEHAVIOR 



In both the maze and the discrimination experiments certain 

 of the differences in behavior of the stock and inbred rats have 

 been indicated in the preceding sections. Notes made each day 

 concerning the general behavior of individuals throw additional 

 light on the differences and help us to interpret them more 

 correctly. To anyone who is accustomed to working with 

 animals nothing is more striking than the fact that no two 

 animals are alike in behavior. Even among white rats the 

 individual peculiarities are so great that the animals can readily 

 be distinguished from one another by behavior alone, by one 

 familiar with their habits. 



We will consider first those animals which w r ere tested in both 

 the short maze and the discrimination apparatus, since they 

 form a homogeneous and readily comparable group. From the 

 beginning of the experiments these young rats showed a marked 

 difference in timidity between stock and inbred rats. When 

 the cage door was opened for the stock rats to be removed to 

 the experiment box they would crowd up to the opening, climb- 



