16 TROPISMS 



ture of their body was, therefore, designated as the 

 tropism theory of animal conduct. 



We started with symmetrical animals since in their 

 case the analysis of conduct is comparatively simple; 

 the results obtained in the study of these symmetrical 

 organisms allow us also to understand the conduct of 

 asymmetrical animals. We shall see that the principles 

 underlying their conduct are the same as in the case of 

 symmetrical animals, the asymmetry of the body altering 

 only the geometrical character of the path in which the 

 animal is compelled to move, not, however, the mechanism 

 of conduct. While a perfectly symmetrical organism, 

 possessed of positive heliotropism, moves in a straight 

 line to the source of light, the path deviates from the 

 straight line in the case of an asymmetrical organism 

 and may in some cases, as, e.g., in Euglena, be a spiral 

 around the straight line as an axis. Some authors have 

 tried to use asymmetrical organisms as a starting point 

 for the analysis of conduct, but since it is impossible 

 to understand the conduct of the asymmetrical organisms 

 unless it is based upon that of the symmetrical animals, 

 these authors have been led to anthropomorphic specula- 

 tions, such as "selection of random movements" which, 

 as far as the writer can see, cannot even be expressed in 

 the language of the physicist. 



Although the tropism theory of animal conduct was 

 offered thirty years ago 285 > 286 > 287 its acceptance was 

 delayed by various circumstances. In the first place, the 

 majority of the older generation of biologists did not 

 realize that not only the methods of the physicist are 

 needed but also the physicist's general viewpoint con- 

 cerning the nature of scientific explanation. In many 

 cases the problem of animal conduct is treated in a way 



