ADAPTATION AND AGGREGATION 245 



e. Random movements and coming to rest in a given 

 place. — Probably all organisms that react to light at all 

 are affected in their movements by the action of light owing 

 to the absolute intensity. Such effects are not dependent 

 upon the time rate of change of intensity but upon the 

 actual amount of light energy present and the time of 

 exposure. Light tends to inhibit the movements of Plas- 

 modia, and some bacteria, while total darkness has the 

 same effect on purple bacteria, Volvox, Euglena, Chlamy- 

 domonas, Hydra, some planaria, earthworms and a number 

 of other organisms. In most of these organisms this action 

 of light is either insufficient to have any apparent effect on 

 aggregation or it is masked by other stronger reactions. 

 In planaria however the influence of relatively low light 

 intensity has a marked effect on the process of aggregation. 

 Some of these animals, as Loeb points out (1906, p. 136), 

 do not orient. When exposed in a dish in front of a window 

 they wander about aimlessly until they get into the darker 

 regions, where they come to rest and consequently remain. 

 Thus it is that they collect in the region of lower light 

 intensity. Loeb considers this reaction of planaria '' a 

 function of the quotient of the change of intensity over 

 time," i.e. time rate of change. I am however of the 

 opinion that it is a function of the absolute intensity, that 

 the action of light in these reactions is similar to the action 

 of heat, for active planarians respond by raising the head 

 and throwing it from side to side, both when the light is 

 suddenly decreased and when it is suddenly increased. 

 There is no indication of an immediate decrease in locomo- 

 tion under such conditions (Walter, 1907, p. 71), as would 

 be expected if the aggregation were due to a change of 

 intensity. And moreover, if the intensity is suddenly in- 

 creased after the planarians have come to rest, they do not 

 become active at once. Walter says (1907, p. 63) that the 

 interval between sudden increase in illumination and re- 

 sponse under such conditions "was often several minutes." 



