Preface ix 



Gratefully, and with this expression of my thanks, I ac- 

 knowledge my indebtedness to Professor Hugo Miinsterberg 

 for placing at my command the resources of the Harvard 

 Psychological Laboratory and for advice and encouragement 

 throughout my investigation; to Professor Edwin B. Holt 

 for valuable assistance in more ways than I can mention ; 

 to Professor Wallace C. Sabine for generous aid in connec- 

 tion with the experiments on hearing ; to Professor Theobald 

 Smith for the examination of pathological dancers ; to Miss 

 Mary C. Dickerson for the photographs of dancing mice 

 which are reproduced in the frontispiece ; to Mr. Frank Ash- 

 more for additional photographs which I have been unable 

 to use in this volume ; to Mr. C. H. Toll for the drawings 

 for Figures 14 and 20; to Doctors H. W. Rand and C. S. 

 Berry for valuable suggestions on the basis of a critical read- 

 ing of the proof sheets; and to my wife, Ada Watterson 

 Yerkes, for constant aid throughout the experimental work 

 and in the preparation of this volume. 



R. M. Y. 



CAMBRIDGE, MASSACHUSETTS, 

 August, 1907. 



