352 W. von BUDDEN BROCK 



be shown that one of these movements has nothing to do with 

 the tropism theory, we shall come to the conclusion that the 

 other, the turning around the vertical axis, is also only appar- 

 ently connected with the tropism theory, but in reality is caused 

 by the purposeful adaptive structure of the organism. 



THE IMPOSSIBILITY OF EXPLAINING THE REACTION WHICH 

 RESULTS FROM THE REVERSED SYMMETRICAL POSITION 



In the above section we found for the first time an argu- 

 ment that is applicable to all tropisms. The turning around 

 the horizontal axis furnished convincing evidence entirely con- 

 tradictory to Loeb's theory, but it is different with respect to 

 the turning around the vertical axis. In case that the evidence 

 against the tropism theory already presented should be con- 

 sidered insufficient upon this point, (resting as it does, so far 

 as this point is concerned, on reasoning from an analogy) we 

 will now especially examine these movements, which were appar- 

 ently the only ones Loeb had in mind. We shall show that 

 even for these movements (of turning round the vertical axis) 

 Loeb's assumption leads to consequences which contradict the 

 facts; this will be shown first for a particular case, — that of the 

 reversed symmetrical position. 



If I take any bilaterally symmetrical negatively heliotropic 

 organism and place it as exactly as possible so that the anterior 

 end is turned to the light, the facts of the experiment are as 

 follows: The animal turns itself very quickly through 180°, 

 and swims, flies, or creeps away from the light. Now I ask you 

 to consider that the whole principle of the tropism theory is 

 founded upon the dissimilar action of energy upon both sym- 

 metrical body halves. In the present case there is no such 

 dissimilar action, since both sides of the animal are stimulated 

 by the light in exactly the same degree. In consequence, if the 

 relations were actually what the tropism theory assumed, then 

 a reaction would be either entirely lacking, or else would take 

 place very gradually, after the animal had lost its symmetrical 

 position through accidental, small movements. In other words: 

 According to the tropism theory, the position with head turned 

 away from the light for positively heliotropic animals, and that 

 with head turned toward it for negative forms, must be a so- 

 called "dead center" or point of no motion. An indisputable 



