i2 4 THE POPULAR SCIENCE MONTHLY 



sides of the body usually act symmetrically. Consequently the helio- 

 tropic orientation, for instance, comes about as we have already de- 

 scribed. There are animals, however, which move sideways, for in- 

 stance, certain crabs, such as the fiddler crab. Holmes has found that 

 these crustaceans also go sideways toward the light. Jennings draws 

 from this fact the following conclusions : " The symmetrical position is 

 an incident of the reaction, not its essence." 



In other words, he uses these observations of Holmes to indicate that 

 the role ascribed to symmetry has no importance for the theory of 

 tropisms. I am, however, inclined to draw another conclusion, namely, 

 that in the fiddler crabs in the first place there is an entirely different 

 connection between the retina and the locomotor muscles from that in 

 other crustaceans and different animals, and that, secondly, there is a 

 special peculiarity in regard to the function of the two retinas whereby 

 they do not act like symmetrical surface elements. I believe that a new 

 discovery may be made here. 21 



XI 



These data may suffice to explain my point of view. To me it is a 

 question of making the facts of psychology accessible to analysis by 

 means of physical chemistry. In this way it is already possible to re- 

 duce a set of reactions, namely, the tropisms to simple rationalistic rela- 

 tions. Many animals, because their body structure is not only morpho- 

 logically, but, also chemically, symmetrical, are obliged to orient their 

 bodies in a certain way in relation to certain centers of force, as, for 

 instance, the source of light, an electric current, the center of gravity 

 of the earth or chemical substances. This orientation is automatically 

 regulated according to the law of mass action. The application of the 

 law of mass action to this set of reactions is thus made possible. I 

 consider it unnecessary to give up the term " comparative psychology," 

 but I am of the opinion that the contents of comparative psychology will 

 under the influence of the above-mentioned endeavors be different from 

 the contents of speculative psychology. But I believe also that the 

 further development of this subject will fall more to the lot of biolo- 

 gists trained in physical chemistry than to the specialists in psychology 

 or zoology, for it is in general hardly to be expected that zoologists and 

 psychologists who lack a physico-chemical training will feel attracted 

 to the subject of tropisms. 



In closing let me add a few remarks concerning the possible appli- 

 cation of the investigations of tropisms. 



21 From which I expect, furthermore, that they will only confirm still more 

 the laws of heliotropism. This expectation is based upon analogous relations in 

 the pleuronectids, which I can not, however, discuss further here. However, 

 probably no one will maintain that the existence of the pleuronectids invalidates 

 all laws in regard to the symmetrical body structure. 



