the tube-worms they thereupon draw back very quickly. Since their locomotor organs are not symmetrical, but are arranged in a peculiar unsymmetrical manner, they do not, after the next progressive movement return to the former direction of movement, but deviate sideways from it, and it is, therefore, easy to understand that such animals do not furnish the best material to demonstrate the laws of heliotropism, especially since they possess, moreover, only a slight photochemical sensitiveness. But Jennings has with special preference used observations on such organisms to argue against the theory of tropisms, and he has with these arguments caused much confusion in the minds of zoologists. One writer has, if I am not mistaken, asserted that the significance of tropisms is limited by the demonstration of the sense of difference. This writer overlooks the fact that it is a question of tracing psychical phenomena, and not merely tropisms, back to physico-chemical processes. Just as in muscles and nerves the action of a constant current is different from that of an intermittent current, so we find in the action of light an analogous case. If we wish to trace all animal reactions back to physico-chemical laws we must take into consideration besides the tropisms not only the facts of the sense of difference, but also all other facts which exert an influence upon the reactions. The influence of that mechanism which we call "associative memory" also belongs in this category, but we can not discuss that further here. Instead the reader is referred to my aforementioned book, as well as the newer work of Bohn, "La naissance de l'intelligence."[1] Let us bear in mind that "ideas" also can act, much as acids do for the heliotropism of certain animals, namely, to increase the sensitiveness to certain stimuli, and thus can lead to tropism-like movements or actions directed toward a goal.[2]
IX
Besides light and the electric current, the force of gravity also has an orienting influence upon a number of animals. The majority of such animals are forced to turn their heads away from the center of the earth and to creep upward. It was uncertain for a long time how the orientation of cells in relation to the center of gravity of the earth could influence the rate of the chemical reactions within, but it has been suggested that an enlargement or shifting of the reacting surfaces formed the essential connecting link. If it is assumed that in such geotropically sensitive cells two phases (for instance, two fluid substances which are not at all, or not easily, miscible, or one solid and one fluid substance) of different specific gravities are present, which react upon one another a reaction takes place at the surfaces of contact. Every
