Spatially Determined Reactions 197 



due to negative reactions given when the organism in its or- 

 dinary swimming movements, which usually involve turning 

 from side to side, either passes into a region of greater or 

 less illumination, or swings its anterior end "toward or 

 away from the source of light, so that it is shaded at one 

 moment and strongly illuminated at the next." Suppose, 

 that is, an animal makes in its locomotion slight random 

 movements of the head from side to side. Suppose that 

 one side of it is more brightly illuminated than the other. 

 If the animal is positive to light, it has the characteristic 

 of making a negative response whenever its head end is 

 suddenly darkened. This will happen when the head end 

 is accidentally turned away from the light; consequently 

 all such random movements will be checked, while random 

 movements of the head towards the light will not be 

 checked. Hence the animal will turn until its head points 

 towards the light: in this position random movements 

 towards either side will be equally checked because they will 

 equally tend to bring the head into a darker region ; and so 

 movement will take place in a line generally towards the 

 light, though still with balanced random movements to 

 either side. If the animal is negative, it has the charac- 

 teristic of making negative reactions when the illumina- 

 tion of the head is suddenly increased, and obviously this 

 will bring about orientation with the head end away from 

 the light. 



In Volvox (see page 136), orientation is held by Oltmanns 

 (528) and Mast (464) to occur after this fashion. The 

 reaction of a Volvox colony, which in moderate light is 

 positively phototropic, takes place in consequence of a 

 response by each individual in the colony given when, as 

 the colony rotates, that individual passes from a higher 

 to a lower intensity of light. 



