FORCED MOVEMENTS, 



TROPISMS, AND ANIMAL 



CONDUCT 



CHAPTER I 



INTRODUCTION 



THE analysis of the mechanism of voluntary and 

 instinctive actions of animals which we propose to under- 

 take in this volume is based on the assumption that all 

 these motions are determined by internal or external 

 forces. Our task is facilitated by the fact that the over- 

 whelming majority of organisms have a bilaterally sym- 

 metrical structure, i.e., their body is like our own, divided 

 into a right and left half. 



The significance of this symmetrical structure lies 

 in the fact that the morphological plane of symmetry 

 of an animal is also its plane of symmetry in physiological 

 or dynamical respect, inasmuch as under normal con- 

 ditions the tension in symmetrical muscles is the same, 

 and inasmuch as the chemical constitution and the velocity 

 of chemical reactions are the same for symmetrical ele- 

 ments of the surface of the body, e.g., the sense organs. 



Normally the processes inducing locomotion are equal 

 in both halves of the central nervous system, and the ten- 

 sion of the symmetrical muscles being equal, the animal 

 moves in as straight a line as the imperfections of its 



13 



