298 LIGHT AND THE BEHAVIOR OF ORGANISMS 



General Summary of Part III 



(i) Reactions to light are in general adaptive. There 

 are, however, certain reactions which are clearly injurious 

 and often fatal; .as, for example, the flying of insects into a 

 flame and the positive reactions of organisms which live in 

 darkness. But the positive reactions of insects are ordi- 

 narily advantageous. It is only under artificial condi- 

 tions that they prove fatal, and the ancestors of many 

 animals which now live in darkness lived in the light. 

 Positive reactions were probably advantageous to them, 

 and the power to respond thus was probably inherited by 

 the offspring, in which it is useless. 



(2) Organisms ordinarily collect in light conditions 

 which facilitate life processes. They get into an optimum 

 light condition either by orienting and moving directly 

 toward it or by random movements; and they remain 

 either because they come to rest there or because they 

 respond with the avoiding reaction when they reach the 

 limit of the region of optimum light. All of the reactions 

 involved in aggregation are responses to changes of inten- 

 sity, with the probable exception of those in which the 

 organisms come to rest at the optimum. In these, aggre- 

 gation is no doubt due to the effect of continued illumina- 

 tion or constant intensity. 



(3) Many organisms react to light without orienting 

 or aggregating. In nearly all cases these reactions are 

 sudden contractions or changes in direction of movement 

 caused by sudden changes in light intensity, as, for ex- 

 ample, the jerking into its tube of Hydroides when a 

 shadow passes over it. Most of these reactions are highly 

 protective against the attack of enemies, but some serve to 

 indicate the presence of food, as in the case of Clepsine. 

 These are clearly reactions to signs. It is not the shadow 

 in which these organisms are interested, but what ordi- 

 narily follows. There are, however, organisms which re- 

 spond to the effect of light more directly, as, for example, 



