Significance of Tropisms for Psychology 53 



arrangement of the physical conditions of the exjxTiiiH'nt, I 

 have never found a single indication of such an adaptation. 

 In every case it has been sho\Mi that positively hcliotropic 

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

 Thus winged plant lice or wingless larvae of Chrysorrhoca or 

 copepods, which have been made heliotroi)ic 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 retjuired 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 iih(jt(j- 

 chemical action of light may inhibit the positive h('liotroi)isin. 

 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 heliotroi)ic. 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 

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

 Polygordius. But it would be wrong in this case to sjM-ak of 

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



1 Quite often without even stopping for a moment. In animals sonsltivr to 

 differences (see next chapter) a stopping occurs in ihis cxp.rinn-nt in tlir pavsinn 

 from the light into the shadow, but they go. nevertheless, ininu-dlately on in the 

 direction of the source of light. The reader will And a furtlu-r account of this 

 experiment in my book on The Dynamics of Livinu MntUr. 



