RATE OF LEARNING IN THE CHICK 123 



sity of the stimulus is followed by decrease in learning rate, while, 

 for medium discrimination an optimal intensity of stimulus is 

 found, increase beyond which is followed by slower learning. 

 Thus far my results and those of Yerkes and Dodson in the case 

 of the dancing mouse seem to agree. In the case of the mouse 

 under the difficult condition of discrimination it was found that 

 the optimal stimulus approached much nearer the threshold 

 than with medium difference of illumination between the two 

 boxes. My results with chicks are in conflict with this unless, 

 as has been done, the cases of failure to learn to discriminate 

 are considered. Then it is found that, with the difficult condition 

 of discrimination and the weakest stimulus, none, with the next 

 greater strength of stimulus, one, and with the strongest stim- 

 ulus two chicks failed. With slight difference of brightness 

 between the two screens the strength of stimulus under whose 

 influence no chicks fail to learn to discriminate is nearer the 

 threshold than the optimal stimulus for the medium condition 

 of discrimination. Perhaps this is as close agreement of the 

 results for mice and for chicks as we should expect to find in 

 animals so unlike. The behavior of the chicks was, however, 

 the reverse of that of the mice. Yerkes writes :^ "The behavior of 

 the dancers varied with the strength of the stimulus to which they 

 were subjected. They chose no less quickly in the case of the 

 strong stimulus than in the case of the weak, but they were 

 less careful in the former case and chose with less deliberation 

 and certainty." My chicks, on the other hand, chose quickly 

 with weak stimuli, but only after long delay with strong stimuli. 

 A chick would sometimes require ten or fifteen minutes to make 

 a choice in the latter case. This difference might perhaps be 

 accounted for by the fact that, with the mouse, a moveable 

 cardboard partition was used by which the space in which the 

 animal could move was gradually restricted. Thus a choice of 

 one passageway or the other was finally necessary. This device 

 could not be used satisfactorily with chicks. 



The record of one chick, w^hich appeared to be perfectly normal 

 when I began experiments with it, but died before they were 

 completed, deserves notice. Its training series on successive days 

 were as follows: 



' Yerkes and Dodson, loc. cit., p. 476. 



