Significance of Tropisms for Psychology 51 



reactions of animals are determined by their needs or by natural 

 selection could hardly be foimd. 



One might be led to suppose that galvanotropism and heli- 

 otropism are not comparable. They are, however, as a matter 

 of fact, phenomena of the same category with the exception of 

 the aforementioned fact, that light acts generally only upon the 

 surface of the skin, while the electric current influences all the 

 cells of the body. As already mentioned, the disturbing com- 

 plications arising from this latter circumstance disappear for 

 the most part when we work with imicellular organisms, and we 

 should expect that galvanic and heliotropic reactions would 

 more nearly resemble one another in this case, provided that 

 we work with organisms possessing both forms of sensitiveness. 

 And this expectation is fulfilled. The algae of the species 

 Volvox show heliotropism and galvanotropism. The investi- 

 gations made by Holmes and myself upon heliotropism, as well 

 as those of Bancroft upon the galvanotropism of these organisms 

 indicate that the mechanism of these reactions in Volvox is 

 the same and the degree of determinism of the heliotropic 

 and galvanotropic reactions in Volvox is equally great. 



Claparede raises the objection that the galvanotropic 

 reactions are purely compulsory, while the heliotropic reactions 

 are governed by the "interest of the animal. "* Such a view, 

 however, is not supported by the facts. The reason why heli- 

 otropism may occasionally, as we have seen, be of use, while 

 galvanotropism has no biological significance, is because the 

 electric current does not exist in nature. It can, however, be 

 shown also that heliotropism is just as useless to many animals 

 as galvanotropism. For instance, I pointed out twenty years 

 ago that some varieties of animals which do not live in the hght 

 at all, for instance, the larvae of the goat moth, which live imder 

 the bark of trees, may show positive heliotropism. I found, 

 moreover, that the crab, Cuma Rathkii, which lives in the mud of 



1 ClaparSde, " Les tropismes devant la psychologie," Journ. /. Psyckologie und 

 Nemologie, XIII, 150, 1908. 



