The few young leaves on top of a twig are quickly eaten by the caterpillar. The light, which saved its life by making it creep upward where it finds food, would cause it to starve could it not free itself from the bondage of positive heliotropism. The animal, after having eaten, is no longer a slave of the light, but can and does creep downward. It can be shown that a caterpillar, after having been fed, loses its positive heliotropism almost completely and permanently. If we submit unfed and fed caterpillars of the same nest contained in two different test-tubes to the same artificial or natural source of light, the unfed will creep to the light and stay there until they die, while those that have eaten will pay little or no attention to the light. Their sensitiveness to light has disappeared; after having eaten they become independent of light and can creep in any direction. The restlessness which accompanies the condition of hunger makes the animal creep downward--which is the only direction open to it--where it finds new young leaves on which it can feed. The wonderful hereditary instinct, upon which the life of the animal depends, is its positive heliotropism in the unfed condition and its loss of this heliotropism after having eaten. The latter phenomenon is in harmony with the experiments which show that the heliotropism of certain species of _Daphnia_ disappears when the water becomes neutral. And finally it may be pointed out that the majority of green plants could not exist if their stems were not positively, their roots negatively, heliotropic. It is the positive heliotropism which makes the top grow toward the light, which enables the leaves to get the light necessary for assimilation, and the roots to grow into the soil where they find the water and nutritive salts. 10. While we do not wish to deal here with the different tropisms it should be stated that aside from heliotropism, chemotropism as well as stereotropism play the most essential rôle in the so-called instinctive actions of animals. It is a problem of orientation by the diffusion of molecules from a centre when a male butterfly is deviated from its flight and alights on the wooden box in which is enclosed a female of the same species. We have already alluded to certain phenomena of chemotropism in Chapter IV. Certain organisms have a tendency to bring their bodies as much as possible on all sides in contact with solid bodies; thus the butterfly _Amphipyra_, which is a fast runner, will come to rest under a glass plate when the plate is put high enough above the ground so that it touches the back of the butterfly. The animals which live under stones or underground or in caves are as a rule both negatively heliotropic and positively stereotropic. Their tropisms predestine or force them into the life they lead.