342 W. von BUDDENBROCK 



bed of the theory or else so superficially studied that at first 

 one is hardly aware of the discrepancy which lies deeper. 



The crowning example of a theory which exerts such tyranny 

 over facts is the so-called tropism theory, whose principal sup- 

 porter is J. Loeb. To the criticism of this theory and, if pos- 

 sible, its refutation, the following pages are devoted. 



The tropism theory has already been criticized by various 

 writers (Jennings, Radl, etc.). They, however, always content 

 themselves with pointing out that certain isolated cases do not 

 harmonize with the theory. The most important general argu- 

 ments have not yet been presented. Besides this, in his last 

 publication in the Handbuch der vergleichenden Physiologie of 

 Winterstein, 1913, Loeb shows that these criticisms have not 

 affected him in the slightest degree, for he still reiterates the 

 same opinions. For this reason a further investigation of the 

 subject seems justified. 



The word tropism denotes only a simple fact of observation. 

 Many lower animals have the peculiarity of either creeping in 

 a straight line away from or toward a source of energy, or a 

 point from which light, heat, chemical energy, etc., radiates, 

 or of choosing a path which is at right angles to the energy 

 rays. These movements of orientation, which are also present 

 in the lower plants, are termed tropisms, and are further known 

 as photo-, chemo-, thermo- tropism, according to the kind of 

 energy. Such lower animals can be drawn in a definite direc- 

 tion just as surely as a physicist can deflect a magnetic needle; 

 and their behavior, giving so strongly the impression of being 

 purely physical, has led the fathers of the tropism theory to 

 regard the whole phenomenon as something very simple, whose 

 solution may be found without any regard to the complicated 

 structure of the organisms themselves. 



To show how such an attempt has been made, the positive 

 heliotropism of the winged aphid is cited, using Loeb's own 

 words. He writes: "Two factors determine the progressive 

 movement of animals under these conditions. One is the sym- 

 metrical structure of the animal and the second is the photo- 

 chemical action of the light. * * * 



The symmetrical structure of the animal is expressed anat- 

 omically in that, as is well known, the right and left body halves 

 are symmetrical. But in my opinion, such symmetry exists not 



