PREFACE 



The observations on which the experimental portion of the 

 following pages is based were made upon four different groups 

 of Plymouth Rock chicks. Work with the first three groups 

 was conducted in the Harvard Psychological Laboratory during 

 1911-1912 when the visual factors in space perception described 

 in Chapters V and VI were studied. The data on flicker per- 

 ception reported in Chapter VII and the greater portion of the 

 material on problem learning, presented in Chapter VIII, were 

 gathered in the same laboratory during 1915-1916. The results 

 on relativity and specificity of reactions reported in Chapter 

 IX were obtained quite incidentally during the two years of 

 research. 



In this Monograph there is presented an account of the re- 

 sults as written in 1916. A preliminary report had been pub- 

 lished in 1913 in the Journal of Animal Behavior, following 

 which there were several published comments relative to certain 

 questions raised in the paper. Cognizance of these discussions 

 and the related literature was taken at the time the manuscript 

 was prepared. Rather than isolate the unpublished portion, 

 it now seems advisable to present the entire study in the form 

 it had taken before the United States became a participant in 

 The World War. A revision of the manuscript to make it 

 strictly current is not deemed worth while. 



The problem in spatial perception was proposed by Professor 

 Robert M. Yerkes, under whose guidance the entire study was 

 made. During the vear preceding: my initial work on this 

 problem, he completed in the Harvard Psychological Laboratory 

 the construction of the apparatus which has been described in 

 a Monograph^ on "Methods of studying vision in animals." 

 For the use of this apparatus, for his numerous suggestions in 

 method, for his precise formulation of problems, and above all 

 for his continued and sympathetic encouragement, I wish to 



^ Behavior Monographs, Vol. 1, No. 2. 



