7. This may be the place to dispel an error which has sometimes crept into the discussion of the tropistic reactions of animals. It has been stated occasionally that it is the energy gradient and not the automatic orientation of the animal by the light which makes the positively heliotropic animal move towards the source of light and the negatively heliotropic away from it. Thus the positively heliotropic animal would be compelled to move towards the source of light as a consequence of the fact that the intensity of the light increases the more the nearer the animal approaches the source of light. If the source of light be the reflected sky-light the difference of intensity at both ends of a microscopic organism is so slight that it is beneath the limit capable of influencing the motions. [Illustration: FIG. 47] A simple experiment published by the writer in 1889 suffices to dispel the idea that the energy gradient determines the direction of the motion of an animal in tropistic reactions. Let direct sunlight (_S_, Fig. 47) fall through the upper half of a window (_w w_) upon a table, and diffused daylight (_D_) through the lower half of the window on the same table. A test-tube _a c_ is placed on the table in such a way that its long axis is at right angles to the plane of the window; and one half _a b_ is in the direct sunlight, the other half in the shade. If at the beginning of the experiment the positively heliotropic animals are in the direct sunlight at _a_, they promptly move toward the window, gathering at the window end _c_ of the tube, although by so doing they go from the sunshine into the shade.[235] This experiment is in harmony with our idea that the effect of light consists in turning the head of the animal and subsequently its whole body toward the source of light. By going from the strong light into the shade the reaction velocity in both eyes is diminished equally and hence there is no reason for the animal to change its orientation, though its progressive motion may be stopped for an instant by the change. But at the boundary between sunlight and daylight a sudden change from strong to weak light occurs. If the energy gradient determined the direction of the positively heliotropic animal, the motion should stop at the boundary from strong to weak light, which may happen for an instant but which will not interfere with the progressive motion of the animal. [235] Loeb, J., _Dynamics of Living Matter_, p. 126.