CHAPTER II 



THE SYMMETRY RELATIONS OF THE ANIMAL 



BODY AS THE STARTING POINT FOR THE 



THEORY OF ANIMAL CONDUCT 



THE starting point for a scientific and quantitative 

 analysis of animal conduct is the symmetry relations of 

 the animal body. The existence of these symmetry rela- 

 tions reduces the analysis to a comparatively simple 

 problem. 



Organisms show two forms of symmetry, radial sym- 

 metry, for which jellyfish and the stems and roots of 

 most plants offer a well-known example, and lateral sym- 

 metry, such as exists in man and most animals. In radial 

 symmetry the peripheral elements are distributed equally 

 about an axis of symmetry, in the case of lateral sym- 

 metry the peripheral elements are distributed equally 

 to the right and left of the plane of symmetry (or the 

 median plane) by which the body is divided into a right 

 and left half. The importance of this symmetrical struc- 

 ture lies in the fact that the morphological plane of sym- 

 metry is also the dynamical plane of symmetry of the 

 organism. Symmetrical spots of the surface of an animal 

 are chemically identical, having the same chemical con- 

 stitution and also the same quantity of reacting masses. 

 Thus the two eyes are symmetrical organs, each contain- 

 ing the same photochemical substances in equal quantity. 

 In the eye itself each element is to be considered as 

 chemically identical with the symmetrical point in the 

 other eye. Hence, if the two eyes are illuminated equally, 



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