THE SIGNIFICANCE OF TROPISMS 117 



all the cells and nerves of the animal. Thus, in general, the action of 

 the current upon the skin becomes complicated and modified by its 

 simultaneous effect upon the nerve branches and upon the central 

 nervous system. The result is thus much more complicated than that 

 of the action of light where essentially only the effect upon the skin 

 and retina is involved. For this reason, a distinct galvanotropism is 

 found more often in organisms with simple structure, as, for in- 

 stance, in one-celled organisms, than in vertebrates, although it is also 

 demonstrable in the latter. 



Galvanotropism is, however, purely a laboratory product. With 

 the exception of a few individuals, which have in recent years fallen 

 into the hands of physiologists who happened to be working on gal- 

 vanotropism, no animal has ever had the chance to come under the 

 influence of an electric current. And yet galvanotropism is a re- 

 markably common reaction among animals. A more direct contradic- 

 tion of the view that the reactions of animals are determined by their 

 needs or by natural selection could hardly be found. 



One might be led to suppose that galvanotropism and heliotropism 

 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 influence.: all the cells of the body. As already mentioned, the 

 disturbing complications arising from this latter circumstance disap- 

 pear for the most part when we work with one-celled 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 which possess both forms of sensitiveness. And this expec- 

 tation is fulfilled. The colonial alg« of the species Volvox show helio- 

 tropism and galvanotropism. The investigations made by Holmes and 

 myself upon heliotropism, as well as those of Bancroft upon the gal- 

 vanotropism 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." 15 Such a view, however, is not supported 

 by the facts. The reason that heliotropism may occasionally, as we 

 have seen, be of use, while galvanotropism has no biological signifi- 

 cance, is because the electric current does not exist in nature. It can, 

 however, be shown also that heliotropism is just as useless to many ani- 

 mals as galvanotropism. For instance, I pointed out twenty years ago 

 that some varieties of animals which do not live in the light at all, for 



15 Claparede, "Les tropismes devant la Psychologie, " Journ. f. Psychologie 

 una Neurologie, Bd. 13, S. 150, 1908. 



