instance, the larvæ of the goat moth, which live under the bark of trees, may show positive heliotropism. I found, moreover, that the crab, Cuma Rathkii, which lives in the mud of the harbor of Kiel, when brought into the light and removed from the mud shows positive heliotropism. It is, therefore, just as incorrect to assert that the heliotropic reactions are governed by the biological interests of the animal as that this is true for galvanotropism. We must therefore free ourselves at once from the overvaluation of natural selection and accept the consequences of Mendel's theory of heredity, according to which the animal is to be looked upon as an aggregate of independent hereditary qualities.
The attempt has been made to prove that organisms are attuned to a certain intensity of light and so regulate their heliotropism that they invariably reach that intensity of light which is best suited to their well-being. I believe that this is also a case of a suggestion forced upon the investigators by the extreme application of the natural selection theory. I have made experiments upon a large number of animals, but, with a clear arrangement of the physical conditions of the experiment, I have never found a single indication of such an adaptation. In every case it has been shown that positively heliotropic animals are positive with any intensity of light above the threshold. Thus winged plant lice or wingless larvæ of Chrysorrhœa or copepods, which have been made heliotropic by acids, go toward the light regardless of whether the source of light is the direct sunlight or reflected light from the sky or weak lamp light, provided that the (threshold) value of intensity of light required for the reaction is passed. Indeed, I have been able to show that positively heliotropic animals also move toward the source of light even if the arrangement is such that by so doing they go from the light into the shadow. A "selection" of a suitable light intensity I have never observed.
What probably lies behind these interpretations of the "selection of suitable light intensity" is the fact that under certain conditions reaction products formed by the photochemical action of light may inhibit the positive heliotropism. I found a very clear instance of this sort in the newly hatched larvæ of Balanus perforatus, which are positively heliotropic. If they are placed in the light of a quartz mercury lamp (of Heraus) which is very rich in ultra-violet rays, the positively heliotropic larvæ soon become negatively heliotropic. For these experiments
- Quite often without even stopping for a moment. In animals sensitive to differences (see next chapter) a stopping occurs in this experiment in the passing from the light into the shadow, but they go, nevertheless, immediately on in the direction of the source of light. The reader will find a further account of this experiment in my "Vorlesungen über die Dynamik der Lebenserscheinungen."