vi Preface to the First Edition 



no topic is as full as would be necessary for the bibliography 

 of a special research upon that topic. Doubtless there are 

 omissions for which no excuse can be found. In one or two 

 cases, where the literature upon a single point is very large, 

 as for example, in the case of the function of the semicircular 

 canals, only a few of the more important references have 

 been given. 



One further comment may be made. The book through- 

 out deals with comparative rather than with genetic psy- 

 chology. 



I gratefully acknowledge help from a number of sources. 

 To Professor Titchener I owe, not only my share of that 

 genuine psychological spirit which he so successfully imparts 

 to his pupils according to their ability, but various helpful 

 criticisms upon the present work, about half of which he 

 has read in manuscript. Dr. Yerkes has given me much 

 invaluable aid in securing access to material, and has very 

 kindly permitted me to see the proofs of his book on "The 

 Dancing Mouse." As editor of the series he has reviewed 

 my manuscript to its great advantage. Professors Georges 

 Bonn and George H. Parker have showed especial courtesy 

 in making their work accessible to me. Professor Jennings 

 has kindly allowed the use of a number of illustrations from 

 his book on "The Behavior of the Lower Organisms." My 

 colleague Professor Aaron L. Tread well has generously 

 helped me in ways too numerous to specify. But perhaps 

 my heaviest single obligation is to Professor I. Madison 

 Bentley, who has read the manuscript of the entire book, 

 and whose advice and criticism have been of the utmost 



benefit to every part of it. 



M. F. W. 



VASSAR COLLEGE, POUGHKEEPSIE, N.Y. 

 October i, 1907. 



