SIZE AND FORM PERCEPTION 99 



partment. In this respect the conditions were exactly reversed 



from those of all former series, but the chicks had, in earlier 



series, become accustomed to reversal of brightness. It was 



under these conditions that the records of chicks 15, 18, 20, 



and 21, as presented in table 3, were made. It is incredible 



that the discriminations occurred on any basis other than size 



difference. 



IV. FORM PERCEPTION 



1. Literature 



On account of the contrast between the outcome of my study 

 of the chick's discrimination of form differences and the results 

 reported by earlier experimenters in the same field, it is desir- 

 able to preface the account of my experiment with a brief review 

 of these preceding studies on form perception. I shall limit 

 this review to a consideration of two papers; one by Katz and 

 Revesz ' and the other by Breed. 10 In both papers positive 

 results have been reported. 



In its simpler form, the " Klebmethode " of Katz and Revesz 

 consists in pasting on a cardboard, kernels of grain at which 

 the chicken's pecking response is to become inhibited. Among 

 these "glued" kernels is scattered a different kind of grain 

 which the bird is allowed to pick up. To illustrate, it was found 

 by the experimenters that the birds preferred rice to wheat. 

 Twenty kernels of rice were glued to the background and 10 

 grains of wheat were scattered among them. A series was 

 recorded every time the chicken picked up the wheat. The 

 bird was classed as "Fehlerfrei" when it had picked up the 

 wheat without pecking at the rice. The observers had two 

 quantitative measurements for the rapidity of learning: (1) The 

 number of series necessary for a perfect reaction ; (2) the number 

 of reactions to rice. To make sure that the "errorless" birds 

 did not avoid the rice by means of the glue which might be 

 visible on the pasted kernels, the rice was scattered loosely 

 upon the cardboard in the same manner as the wheat. The 

 wheat was again eaten by the "errorless" birds and the rice 

 was left. The discrimination, the authors concluded, was be- 

 tween wheat and rice. 



9 Katz, D. and Revesz, G. Experimentell-psychologische Untersuchungen mit 

 Huhnern, Zeit. f. Psych, u. Physiol, d. Sinnesorgane, 1909, Bd. 50, P. 93. 



10 Breed, Frederick S. Reactions of chicks to optical stimuli, Jour. Animal Be- 

 havior, 1912, vol. 2, pp. 280-295. 



