n6 TEE POPULAR SCIENCE MONTHLY 



bees were ready to swarm out of the opening of the box used for the 

 experiment when he suddenly removed the dark covering of the box 

 so that the light now entered it from above. The heliotropic sensitive- 

 ness of the animals was so great that they crept upward within the 

 box, following the direction of the light rays and were not able to make 

 the nuptial flight. Thus, according to these observations the bees at 

 the time of the nuptial flight are positively heliotropic machines. 



These observations may serve as examples of the way in which 

 analyses of the vital phenomena of certain animals show tropisms to be 

 elements of these phenomena. Many observations of a similar nature 

 are found in the papers of George Bohn, Parker, Badl 13 and myself. 

 What appear to us upon incomplete analysis as acts of will or instinct 

 prove upon more careful analysis, in a series of cases, to be tropisms, the 

 theories of which we have explained in the foregoing pages. 



VI 



Under the influence of the theory of natural selection the view has 

 been accepted by many zoologists and psychologists that everything 

 which an animal does is for its best interest. But now the exact doc- 

 trine of heredity, founded by Mendel and advanced to the position of 

 a systematic science in 1900, reduces this false idea to its proper 

 value. It is only true that species possessing tropisms which would 

 make reproduction and preservation of the species impossible must die 

 out. The opposite view, however, namely, that every reaction or every 

 tropism which an animal possesses is for its interest, or of great benefit 

 to it, is just as incorrect as the view that every structural characteristic 

 of a species must be useful to it. 



Galvanotropism illustrates this in a striking manner. If a galvanic 

 current is passed through a trough filled with water, and if animals are 

 placed in this trough it can be observed that an orientation in relation 

 to the direction of the current takes place in many animals and that 

 the organisms move in the direction either of the positive or of the 

 negative current. In this case we speak of galvanotropism. 

 In galvanotropism the current lines or the current curves play the 

 same role as the light rays in heliotropism. The explanation is that at 

 those points where the current curves enter the cells 14 a collection of 

 ions takes place which influences the chemical reactions. The number 

 of organisms which show typical galvanotropic reactions is not so large 

 as the number of those which show typical heliotropism. According to 

 my opinion this difference is the result of the physical difference in 

 the action of light 'and of the electric current. Light acts essentially 

 upon the free surface of the animal, while the electric current affects 



"Eadl, "Der Phototropismus der Tiere," Leipzig, 1903. 



14 Or where the movement of the ions within the cell is retarded. 



