daphnia positively heliotropic, this may often be accomplished by simultaneously reducing the temperature. From the physico-chemical standpoint we must assume that likewise in the dark, at the ordinary temperature, the photosensitive substance is destroyed so rapidly that its active mass is generally rather too low to cause a heliotropic reaction. By reducing the temperature the rate of decomposition of the photosensitive substance is decreased more than the rate of its formations.
This illustration may suffice, under the limitations of the space allowed us, to indicate how the facts in this field might be correlated when viewed from the standpoint of physical chemistry.
IV
The animals which are strongly positive heliotropic and those animals which do not react at all to light offer the observer no difficulties. Nevertheless, some zoologists, apparently not very familiar with the laws of physical chemistry, seem to have found difficulty in explaining the behavior of those animals which come between the two extremes. For instance, one writer has asserted that with greater intensity of light the laws of heliotropic orientation hold good, while with a lessened light-intensity the animals react to light by the method of "trial and error." From a chemical standpoint the behavior of animals at low intensity is easily to be understood. If a positively heliotropic animal is illuminated from one side a compulsory turning of the head toward the source of light occurs only when the difference in the rate of certain photochemical reactions in the two eyes reaches a certain value. If the intensity of the light is sufficient and the active mass of photochemical substance in the animal great enough, it is only a short time, for instance, the fraction of a second, before the difference in the mass of the reaction products formed on the two sides of the animal reaches the value necessary for the compulsory turning of the head toward the source of light. In this case the animal is a slave of the light; in other words, it has hardly time to deviate from the direction of the light rays; for if it turns the head even for the fraction of a second from the direction of the light rays, the difference in the photochemical reaction-products in the two retinas becomes so great that the head is at once automatically turned back toward the source of light. But if the intensity of the light is lessened (or the photosensitiveness of the animal lessened) the animal may deviate for a longer period from the direction of the light rays. Such animals do eventually reach the lighted side of the vessel, but they no longer go straight toward it, but move instead in zig-zag lines or very irregularly. It is, therefore not a case of a qualitative, but of a quantitative, difference in the behavior of heliotropic animals under greater or lesser illumination, and it is therefore erroneous to assert that heliotropism determines the movement of animals
