292 LIGHT AND THE BEHAVIOR OF ORGANISMS 



after they had been exposed five minutes. Many similar 

 instances have been cited in the preceding pages, notably 

 those with reference to reactions to shadows. 



It will thus be seen that there is a tendency in organisms 

 toward adaptation to environmental conditions. Exposure 

 to low intensity tends to lower the optimum and increase 

 the sensitiveness, w^hile exposure to high intensity tends 

 to produce the opposite effect. But momentary exposure 

 to high intensity, as we have seen in Volvox, may actually 

 lower the optimum. The reaction of an organism depends 

 not only upon the rate of change in illumination and the 

 intensity, but also upon the time it is exposed. 



Among the most interesting and conclusive observations 

 on variation and modification in reactions to light are 

 those of Mrs. Yerkes (1906) and Professor Hargitt (1906) 

 on the annelid, Hydroides dianthus. Hydroides, as pre- 

 viously stated, ordinarily jerks rapidly back into its tube 

 when the light intensity is suddenly decreased, but it does 

 not respond when the intensity is increased. This is 

 clearly a reaction to a sign. The decrease of intensity, 

 the shadow, is of no direct consequence to these creatures, 

 but what ordinarily follows the shadow, an attack of an 

 enemy, may be. 



Mrs. Yerkes was primarily interested in modification of 

 behavior. She selected two specimens, one of which did 

 not respond at all to a given reduction of light intensity 

 and the other responded only once. Both however re- 

 acted definitely when lightly touched. For ten days 

 these two specimens were subjected to a series of stimu- 

 lations consisting of shadows followed by light tactile 

 stimuli. The first day one responded to the shadow, 

 alone, three times in forty trials, and the other only once. 

 In the former there was a great increase in the number of 

 responses to shadow from the fourth to the eighth day, 

 then a slight falling off. In the latter the increase was not 

 so great, but still it was definite, especially from the second 

 to the fifth day. It thus appears that these creatures 



