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 sensitiveness 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, Rádl 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.
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 doctrine 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 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
- Rádl, "Der Phototropismus der Tiere," Leipzig, 1903.
- Or where the movement of the ions within the cell is retarded.