12 HAROLD C. BINGHAM 



For testing size discrimination, two circular areas — one 5 cm. 

 in diameter, the other 8 cm. — were presented to the chick. 

 Evidence of a perfect habit appeared at approximately two 

 hundred trials. Controls were introduced throughout the 

 training to eliminate "difference in brightness of the lighted 

 areas" and "difference in brightness of the right and left sides 

 of the experiment box." In spite of ambiguity in the use of 

 "brightness" Breed's conception is fairly obvious. 



In the study of form perception. Breed used three chicks, one 

 of which, he declares, "learned to discriminate two optical 

 stimuli on the basis of difference of form." The forms were a 

 circle and a square equal in area. Because his studies of the 

 reactions of other chicks to similar stimuli yielded negative re- 

 sults, he attributes the positive reactions of this particular 

 chick to fortune in the choice of animal. In this connection, it 

 is significant that the chick was credited with a perfect record 

 in the fifth training series or after only forty trials. A perfect 

 habit is clearly evident after one hundred trials. For space 

 stimuli differing in form alone this rate of habit formation ap- 

 pears, in the light of later results, unusually rapid and suggests 

 that the successful chick may have discovered some easy cue 

 other than the form difference. This possibility suggests it- 

 self rather forcefully when one considers that Breed was using 

 a mechanism only partially standardized; it rises to the level 

 of probability w^hen one notes that he does not report controls 

 for varying the relative positions of the stimuli within the slid- 

 ing frame.^ 



It is unfortunate that Breed also failed to make control tests 

 to determine whether the distribution of light on the chick's 

 retina was the basis of discrimination. Such a control is easily 

 made in the case of a triangle by inverting the stimulus. The 

 inversion of a square would cause no change in the distribution 

 of light, but such a change might be produced by turning the 

 square through 45 degrees. Even had there been no possibility 

 of an unsuspected cue furnishing the chick with a discriminable 

 factor, there remained this second possibility which Breed did 

 not consider. 



2 A discussion of controls in which this possibiUty was found to be a reality 

 appears in the Journal of Animal Behavior, 1913, vol. 3, pp. 86-90. 



