94 TROPISMS 



an animal by chance gets its median plane too far out 

 of the direction of the rays of light (we assume them to 

 be parallel), the rate of photochemical reaction will be- 

 come different in both eyes. As soon as the difference 

 between the photochemical reaction products in both eyes 

 exceeds a certain limit the animal will automatically put 

 its plane of symmetry again into the direction of the rays 

 of light. The weaker the light and the less sensitive the 

 animal, the longer it will take until this happens, and the 

 greater the freedom of the animal to deviate from the 

 straight line. 



