INTRODUCTION 15 



certain parts of the brain are injured, but are compelled 

 to deviate constantly toward one side, which is (accord- 

 ing to the species and the location of the injury in the 

 brain) either the side of the injury or the opposite side. 

 The explanation of these forced movements is that on 

 account of the one-sided injury of the brain the tension 

 of the symmetrical muscles is no longer the same. As a 

 consequence, the impulses' for locomotion which are equal 

 for symmetrical muscles will cause greater contraction 

 in certain muscles of one side of the body than in the 

 symmetrical muscles of the other side, and the animal will 

 no longer move in a straight line. The only difference 

 between the forced movements induced by unequal illu- 

 mination of the two eyes and by injury to the brain is 

 that in the latter case the forced movements may last 

 for days or throughout the whole life, while in the former 

 case they last only as long as the illumination on the two 

 sides of the body is unequal. If we bring about a per- 

 manent difference in illumination in the eyes, e.g., by 

 blackening one eye in certain insects, we can also bring 

 about permanent circus motions. This shows that animal 

 conduct may be justly designated as consisting of forced 

 movements. 



The idea that the morphological and physiological 

 symmetry conditions in an animal are the key to the 

 understanding of animal conduct demanded that the same 

 principle should explain the conduct of plants, since plants 

 also possess a symmetrical structure. The writer was 

 able to show that sessile animals behave toward light 

 exactly as do sessile plants ; and motile animals like motile 

 plants. The forced orientations of plants by outside 

 sources of energy had been called tropisms; and the 

 theory of animal conduct based on the symmetrical struc- 



