Page:Popular Science Monthly Volume 79.djvu/121

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117
THE SIGNIFICANCE OF TROPISMS

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 instance, 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 galvanotropism, no animal has ever had the chance to come under the influence of an electric current. And yet galvanotropism is a remarkably common reaction among animals. A more direct contradiction 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 disappear 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 expectation is fulfilled. The colonial algæ of the species Volvox show heliotropism and galvanotropism. The investigations 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.

Claparède raises the objection that the galvanotropic reactions are purely compulsory, while the heliotropic reactions are governed by the "interest of the animal."[1] 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 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 light at all, for

  1. Claparède, "Les tropismes devant la Psychologie," Journ. f. Psychologie und Neurologie, Bd. 13, S. 150, 1908.