Significance of Tropisms foe Psychology 53 



arrangement of the physical conditions of the experiment, I 

 have never fomid a single indication of such an adaptation. 

 In every case it has been shown that positively heliotropic 

 animals are positive to any intensity of light above the threshold. 

 Thus winged plant lice or wingless larvae of Chrysorrhoea or 

 copepods, which have been made heliotropic by acids, go toward 

 the light 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 the intensity of light required for the 

 reaction is exceeded. 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.^ I have never observed a 

 "selection" of a suitable intensity of light. 



What probably lies behind these interpretations of the 

 "selection of a suitable intensity of light" is the fact that under 

 certain conditions reaction products formed by the photo- 

 chemical 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 

 heliotropic larvae soon become negatively heliotropic. For 

 these experiments the larvae should be placed only in a very 

 shallow depth of sea-water. 



Even in a strong light which is not so rich in ultra-violet 

 rays as the light of the mercury lamp, it is sometimes possible 

 to cause positively heliotropic animals to become negatively 

 heliotropic. This is the case, for instance, with the larvae of 

 Polygordius. But it would be wrong in this case to speak of 

 an adaptation of the animal to a certain intensity of light. 



1 Quite olten without even stopping for a moment. In animals sensitive to 

 diflerences (see next chapter) a stopping occurs in this experiment in the passing 

 from the Ught 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 book on The Dynamics of Living Matter, 



