n8 THE POPULAR SCIENCE MONTHLY 



instance, the larvae of the goat moth, which live under the bark of trees, 

 may show positive heliotropism. I found, moreover, that the crab, 

 Cuma Batlikii, which lives in the mud of the harbor of Kiel, when 

 brought into the light and removed from the mud shows positive helio- 

 tropism. 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 conse- 

 quences of Mendel's theory of heredity, according to which the ani- 

 mal is to be looked upon as an aggregate of independent hereditary 

 qualities. 



VII 



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 larvae of Chrysorrhcea or copepods, which have been made helio- 

 tropic 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 posi- 

 tively 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. 16 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 re- 

 action 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 larvae 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 helio- 

 tropic larvae soon become negatively heliotropic. For these experiments 



16 Quite often without even stopping for a moment. In animals sensitive to 

 differences (see next chapter) a stopping occurs in this experiment in the pass- 

 ing 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 iiber die Dynamik der Lebenserschein- 

 ungen. ' ' 



