AUTHOR'S PREFACE 



ANIMAL conduct is known to many through the roman- 

 tic tales of popularizers, through the descriptive work 

 of biological observers, or through the attempts of vital- 

 ists to show the inadequacy of physical laws for the 

 explanation of life. Since none of these contributions 

 are based upon quantitative experiments, they have led 

 only to speculations, which are generally of an anthropo- 

 morphic or of a purely verbalistic character. It is the 

 aim of this monograph to show that the subject of animal 

 conduct can be treated by the quantitative methods of 

 the physicist, and that these methods lead to the forced 

 movement or tropism theory of animal conduct, which 

 was proposed by the writer thirty years ago, but which 

 has only recently been carried to some degree of com- 

 pletion. Many of the statements, especially those con- 

 tained in the first four chapters of the book, are familiar 

 to those who have read the writer's former publications, 

 but so much progress has been made in the last few years 

 that a new and full presentation of the subject seemed 

 desirable. Chapters V to XIII and Chapter XVI are 

 partly or entirely based on new experiments. 



Only that part of the literature has been considered 

 which contributes to or prepares the way for quantitative 

 experiments. 



