364 W, vox BUDDENBROCK 



SUMMARY AND CONCLUSIONS 



The tropism theory is not applicable to all tropisms for the 

 following reasons: 



1. In many cases the conditions premised by the theory are 

 absent, and yet true tropisms, that is, involuntary ^movements 

 regulated by a source of energy, take place. Example: The 

 action of energy in geotropism is lacking in so far as it is con- 

 nected with the presence of statocysts. The movement is the 

 result of a mechanical stimulus caused by the statoliths. 



2. In many cases, although the conditions premised by the 

 theory are present, the tropisms take place in a manner that 

 contradicts the theory. Examples: (a) Sidewise movement of 

 crabs; (b) changeable heliotropism of certain marine animals; 

 (c) turning around the horizontal cross axis not in the plane of 

 symmetry, to be observed in all tropic movements. 



There remain now, as possibly to be explained by the theory, 

 only those cases in which symmetrical animals manifest a turning 

 around the vertical axis. But here also the theory is unsatis- 

 factory, because: 



3. The theory cannot explain the reaction which results from 

 the position of reversed symmetry. 



4. The theory cannot explain the coordinated working together 

 of the two sides of the body. 



.S. The theory cannot tell why the turning always takes place 

 around the axis which is perpendicular to the line joining animal 

 and source of energy; in other words, it especially cannot explain 

 how the animal finds its way to the source of energy. 



6. The presence of a reflex arc in many cases renders the 

 theory entirely superfluous and without meaning, for the move- 

 ment is already satisfactorily explained by the reflex arc itself. 



7. The theory cannot explain satisfactorily the undeniably 

 purposeful nature of many tropisms. 



On the other hand, one positive result of our investigation is 

 that wherever tropisms occur a purposeful adaptive mechanism 

 for movement is found which has the task of guiding the animal 

 to the source of energy, and that during the movement the 

 organism always acts as a whole. We shall therefore adhere to 

 our old opinion that tropisms, like all other reflexes, were origi- 

 nally individual actions, which, in the course of time, have 

 become mechanical and involuntary. This view will not con- 



