34 ROBERT M. YERKES 



or not, and it is indicated that in a few series of observations 

 the conditions of illumination were mixed, that is, for some of 

 the reactions general illumination was employed, whereas in 

 others it was lacking. Throughout the regular experiments the 

 electric stimulus was employed. On April 17th, as is indicated, 

 the intensity of the visual stimulus was lessened, thus dimin- 

 ishing the difference in the stimuli to be distinguished. 



Table 3 presents the comparable results for doves number 1 

 and number 2. The chief difference in the conditions for these 

 results and those obtained with doves numbers 3 and 4 is the 

 absence of the electric stimulus in the case of the former. With 

 the exception of one week, March 23rd to March 28th, Mr. 

 Eisenberg trained number 1 and number 2 to achromatic dis- 

 crimination on the basis of food as a reward without the use 

 of the electric shock as punishment for mistakes. His results, 

 therefore, may be compared with those of the writer, with a 

 view to discovering the value of punishment as contrasted with 

 reward in this experiment with ring-doves. 



Such comparison indicates, in the first place, that it is pos- 

 sible to make a larger number of observations per series with 

 punishment than without it. Thus, the writer by the aid of 

 the electric stimulus was able to make ten, fifteen or even twenty 

 observations per series. Whereas, Mr. Eisenberg, without the 

 electric stimulus, could not satisfactorily make more than ten 

 observations, and during a considerable portion of the training 

 he made only five. Second, the time required for the work 

 varied much more widely when punishment was not used than 

 when it was used. As appears from tables 2 and 3, all of the 

 doves acquired the ability to discriminate with a reasonable 

 degree of certainty, and to react appropriately. The course of 

 habit formation in case of each of the four subjects is surprising. 

 Instead of being steady, regular, and fairly rapid, as the writer 

 had anticipated, it proved to be irregular and extremely slow. 

 One day the experimenter would feel confident that his subjects 

 were acquiring the habit, and the next day he would find them 

 utterly unable to react properly. 



In table 4 the choices are presented by groups of fifty, and 

 the course of habit formation is indicated with the daily varia- 

 tions eliminated. This table shows that as the result of three 

 hundred trials, no one of the four doves had acquired the ability 



