28 GARDNER CHENEY BASSET 



allowed to work thirty minutes and then, if unsuccessful, he was 

 taken out, the food box door was opened, and he was then re- 

 turned to feed for five minutes and used no more that day. 

 When they began to succeed within the half hour, each rat was 

 required to open and enter the food box five times daily. At 

 the end of the fifth trial it was allowed to feed for five minutes, 

 but was permitted no more food until the completion of the 

 next day's experiment. Each rat was used daily for- twenty 

 days, making one hundred trials. As a time limit had been 

 placed, no criterion of perfect learning was established for this 

 experiment. At the conclusion of the learning experiment the 

 rats were fed in the runway, which has already been described, 

 for sixty days. At the end of this period they were tested for 

 absolute retention .and relearning, and were worked for five 

 days, twenty-five trials, in order to ascertain the effects of the 

 previous training. 



In Table IV is a comparative summary consisting of the daily 

 averages of the entire inbred group and, directly beneath, the 

 corresponding daily averages of the entire normal control group. 



From the eleventh day the daily time averages of the control 

 rats are less than those of the inbreds. The absolute retention 

 of the control rats is superior to that of the inbreds. In the five 

 days allotted to testing the effects of previous training, the average 

 time of the control rats is less each day than that of the inbreds. 



In these criteria of ability the rats of the normal control series 

 are shown to be, on the average, superior to those of the inbred 

 series. 



Body length of the inbred rats used in the preliminary inclined 

 plane is, on the average, slightly greater than is the case with the 

 control; body weight, however, is a trifle less. The average 

 actual brain weight of the inbreds is less than that of the control. 

 The relative brain weight (in reference to body length) of the 

 inbreds is 11.61% less than that of the control. The relative 

 brain weight (in reference to body weight) of the inbreds is 1 1.65% 

 less than that of the control. Although killed at a later age, the 

 percentage of water in brain and cord of the inbreds is greater 

 than is the case with the control. 



The preliminary inclined plane figures presented in this table 

 (IV) support the hypothesis that a less than normal average 

 brain weight in a strain of rats is accompanied by a lesser ability 

 to form habits. 



