266 JOHN D. DODSON 



in choosing than any other group used throughout the experi- 

 ments is, also, to be noticed. During the retraining these sub- 

 jects showed great eagerness for food. These facts go to sub- 

 stantiate the indications in the latter part of the training series: 

 viz., That rats as they grow older are better able to undergo 

 long starving periods and that the period of eagerness for food is 

 extended; and that the organism may tend to adapt itself to its 

 conditions. While the first of these is sufficient to account for 

 the facts it is very probable that the second entered into the 

 situation. 







VALUES OF DIFFERENT STRENGTHS OF ELECTRICAL STIMULI 



The experimenter determined roughly the minimum stimulus 

 to which the white rat would respond with the control box used 

 in this experiment. Six males and six females of the same age as 

 the subjects for the experiment were used for this purpose. With 

 eighteen units these rats gave no observable signs of response but 

 with twenty-five units all animals tested gave slight movements 

 indicative of feeling the shock. Thus it is fairly certain that 

 with well developed rats the threshold of sensation for subjects 

 of seventy-eight days of age is between eighteen and twenty-five 

 units with the apparatus used in this experiment. This is from 

 ten to fifteen units below the threshold for the experimenter. 



As may be seen from table 13, subjects trained with an electric 

 shock of sixty units perfected the learning process on an average 

 of 73.33 trials; subjects trained with seventy-five units on an 

 average of 38.89 trials; subjects trained with one hundred and 

 fifteen units on an average of 54 trials; and those trained with 

 one hundred and fifty units perfected the process on an average 

 of 57.77 trials. Thus seventy-five units proved to be the most 

 favorable strengths of stimulus for the learning process in rats 

 of seventy-eight days of age. This is also a very favorable 

 strength of stimulus for animals of fifty-six days of age. 



Individual differences were very marked with the two weaker 

 strengths of stimuli. All subj ects reacted to a stimulus of seventy- 

 five units very vigorously with the exceptions of numbers 105 and 



