68 THE BEHAVIOR OF LOWER ORGANISMS. 



source of light, into the region of less illumination. But is not this 

 exactly what we must expect? His former experiment showed us that 

 under the prism the change from light to darkness was so gradual that it 

 produced no effect on the organisms. Hence the direction from which 

 the rays come is left to produce its effect alone, and it produces the 

 usual effect. The organism reacts in the usual " trial and error" way 

 until the anterior end is directed toward the light ; then it moves in that 

 direction. Incidentally it comes into a region of less intensity of light, 

 though the decrease is so slight as to produce no effect on the organism. 



Parallel considerations hold for the negative organism. Under 

 similar circumstances, if the variation in illumination is very gradual, 

 it directs its sensitive anterior end away from the source of light (by the 

 method of " trial and error") and swims to the opposite side of the 

 drop, incidentally moving into a region of slightly greater (but 

 " unperceived ") intensity of illumination. Under similar conditions, 

 as we have seen in the experiment described on p. 39, if the decrease 

 in illumination is marked, the animal swims back into the shadow, 

 though in so doing it passes toward the source of light. 



Thus in Strasburger's experiments with the prism the difference in 

 the intensity of light between neighboring regions has been made so 

 slight that they are unmarked by the organism and have no effect upon 

 it. We need not be surprised, therefore, that it reacts as if these differ- 

 ences did not exist ; for the organism they do not exist. 



The reaction is in this case just what it would be in a higher organ- 

 ism under similar conditions. Let us suppose that the light stimulates 

 strongly the sensitive anterior end, the eyes, of a higher animal or man ; 

 it causes pain in the case of man. There will be a tendency (i) to 

 move into less illuminated regions ; (2) to turn the eyes away from the 

 light. Suppose that the man is enclosed in a space into which the 

 sun shines obliquely from above, and that the end from which it shines 

 is a little less illuminated than the opposite end, owing to causes similar 

 to those in Strasburger's experiment on the swarm-spores. Suppose that 

 the man is at the end next the sun. He cannot know that the other 

 end is more illuminated, for the only way this would be possible would 

 be for the greater number of rays of light to meet his eye coming from 

 that direction, while by hypothesis all, or a much larger number, of 

 the rays are coming from the opposite direction. He will, therefore, 

 turn his eyes away from the sun, and if he moves will move toward 

 the end away from the sun. After having traversed some distance he 

 may observe, if he is very discriminating, that he is as a matter of fact 

 getting into a region of somewhat greater illumination, and may perhaps 

 reason that the best thing he can do under the circumstances is to keep 

 his eyes turned away from the source of light and move backward to the 



