vi Preface 



Within these limits, the collection of references upon no topic 

 is as full as would be necessary for the bibliography of a spe- 

 cial 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 

 Bohn 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 Bent- 

 ley, 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. 



