356 W. von BUDDENBROCK 



therefore the line AL lies in the plane of symmetry of the animal, 

 the animal can then assume different positions according to the 

 axis around which it has turned. Consequently, without dis- 

 regarding the conditions of the theory, it can move, according 

 to the direction of its long axis, along any line running from 

 A out into the plane of the paper, only one of which, AL, leads 

 to the light. There could be no clearer demonstration of the 

 inability of the tropism theory to furnish an explanation which 

 is clearly its function to give. That a direct movement toward 

 the light actually takes place is not to be explained by such 

 physico-chemical assumptions as Loeb makes, it shows rather, 

 that in the interior of the organism is a purposefully 'functioning 

 mechanism, which, in response to the light stimulus, sets the 

 locomotor organs into such activity that a general movement 

 toward the light must follow. The morphological-physiological 

 structure of the animal is consequently responsible for the occur- 

 rence of the tropism, and this structure must be investigated in 

 every separate case. 



THE DISAGREEMENT OF THE TROPISM THEORY WITH 

 THE PRESENCE OF A REFLEX ARC 



The question now arises of the relation of the tropism theory 

 to the special structure of the animal, or in other terms: if it 

 can be shown that the stimulus from the eye traverses a definite 

 path, a so-called reflex arc, how does the tropism theory apply 

 to this fact? 



The unprejudiced observer will certainly be inclined to con- 

 sider that the cause of the tropism is this reflex arc, which some- 

 what resembles the works of a clock where one wheel clutches 

 another until the hands move, and that any farther explanation 

 of this phenomenon is superfluous. 



On the other hand, in looking through Loeb's writings, we 

 are astonished to find that his opinion is quite different. This 

 is most clearly shown in the case of the crab's sidewise movement, 

 the facts of which we have already considered. Jennings having 

 previously made the criticism that the behavior of this animal 

 could not be harmonized with the theory, Loeb replied in the 

 following words, which have already been quoted on p. 347 : 

 ' I am rather inclined to draw another conclusion, namely, that 

 in the first place, in crabs an entirely different relation exist 



