x PREFACE TO SECOND EDITION 



any case, the revolution which has overtaken biological 

 theory during the same period is profound. Its signifi- 

 cance is as yet imperfectly grasped, but it will, I believe, 

 be found, as time goes on, to have invested the constitu- 

 tion of the living being as against the environing con- 

 ditions with a new importance, and in this constitution the 

 fundamental fact everywhere is that the living being is 

 not passive but active, not mechanical in its reaction to 

 things, but assertive, plastic, and, in a measure propor- 

 tioned to its development, self-determining. If this is so, 

 psychology will in the future have a larger part to play 

 than has hitherto been supposed in the study of the rise 

 and decay of forms of life. 



My obligations to many contemporary writers will, I 

 think, appear clearly enough in the text, but I should 

 like, in particular, to thank Professor Yerkes for informa- 

 tion kindly supplied in correspondence ; Mr. Bullough 

 for a valuable note on the Elberfeld Horses, and the 

 Society for Psychical Research for putting much literature 

 on this subject at my disposal at a time when it was 

 otherwise difficult to come by. In general psychology I, 

 like everyone else, have gained many new suggestions 

 from the work of Professor Stout, Professor Graham 

 Wallas and Dr. McDougall. 



L. T. H. 



HlGHGATE, 



September, 1915. 



