66 THE BEHAVIOR OF LOWER ORGANISMS. 



The results of this method of reacting may be stated correctly, though 

 not completely, as follows : In a negative organism light falling upon 

 the sensitive anterior end causes a reaction by which the anterior end 

 is pointed in many different directions ; the reaction ceases as soon as 

 a direction is reached in which the anterior end is pointed away from 

 the light. In a positive organism the shading of the sensitive anterior 

 end produces the reaction by which the anterior end is pointed in many 

 different directions ; the reaction ceases as soon as the anterior end is 

 no longer shaded.* The reaction is thus by the method of trial and 

 error ; when stimulated the organism tries many different positions, 

 till one is found in which there is no further stimulation. 



Consideration will show, I think, that the factors producing reaction 

 to light in these lowest organisms are essentially the same as in higher 

 ones, if man may be taken as a type of the latter. The factors are, as 

 we have seen, variations in intensity of illumination, and, indirectly, 

 the direction from which the light comes. It is possible that in man 

 the latter factor works more directly than in the infusoria ; leaving this 

 question out of consideration, the two factors are present in both cases. 

 Consider a human being who reacts to light as a purely physical agent, 

 not with regard to the associations which it brings up. In a dark space 

 a gleam of light is pleasant and induces movement toward it. There 

 is then a positive reaction with orientation, but the orientation is not 

 due to the difference in intensity of light on different parts of the body, 

 nor to its direct effect on the motor organs. The orientation is such as 

 to keep the light shining on the more sensitive part of the body, the 

 eyes. An excessively powerful light is unpleasant and induces a nega- 

 tive reaction just as happens in Euglena ; the orientation is then such 

 as to keep the more sensitive part of the body, the eyes, away from the 

 light. Further, man is sensitive to a sudden change in illumination. 

 A strong light bursting from the darkness, or sudden darkness in the 

 midst of bright light, induces a marked motor reaction, and less striking 

 differences may produce a response. Both in man and in Euglena the 

 reaction likewise depends upon color ; but with this phase of the matter 

 we are not at present concerned. 



When the factors above set forth are taken into consideration certain 

 peculiar experimental results that have given rise to much discussion 

 become clearly intelligible. I refer particularly to the experiments in 

 which the direction of the light and the decrease in intensity of illumi- 

 nation do not show the usual relations. Under ordinary conditions 

 movement away from a source of light is movement into a region of less 



* This statement is incomplete in that it does not bring out the fact that it is 

 a change from light to shade or vice versa that induces the reaction; if this be 

 understood, the statement is correct. 



