88 LIGHT AND THE BEHAVIOR OF ORGANISMS 



several sources of light of unequal intensity, the light with 

 the strongest intensity determines the orientation and 

 direction of motion of the animal. Other possible compli- 

 cations are covered by the unequivocal statement, made 

 and emphasized in this and the following papers on the 

 same subject, that the main feature in all phenomena of 

 heliotropism is the fact that symmetrical points of the 

 photosensitive surface of the animal must be struck by 

 the rays of light at the same angle. It is in full harmony 

 with this fact that if two sources of light of equal intensity 

 and distance act simultaneously upon a heliotropic animal, 

 the animal puts its median plane at right angles to the line 

 connecting the two sources of light. This fact was not 

 only known to me but had been demonstrated by me on 

 the larvae of flies as early as 1887, in Wiirzburg, and often 

 enough since. These facts seem to have escaped several 

 of my critics; " (p. 61), " When the diffuse daylight which 

 struck the [Musca] larvae came from two window^s, the 

 planes of which were at an angle of 90° with each other, the 

 paths taken by the larvae lay diagonally between the two 

 planes. . . . This experiment was recently published by an 

 American physiologist as a new discovery to prove that I 

 had overlooked the importance of the intensity of light!" 

 (p. 82), '' The direction of the median plane or the direction 

 of the progressive movements of an animal coincides with 

 the direction of the rays of light ... if there is only a 

 single source of light. If there are two sources of light of 

 different intensities, the animal is oriented by the stronger 

 of the two lights. If their intensities be equal, the animal 

 is oriented in such a w^ay as to have symmetrical points of 

 its body struck by the rays at the same angle;" (p. 268), 

 " Attention need scarcely be called to the fact that if rays 

 of light strike the animal [larvae of Limulus polyphemus] 

 simultaneously from various directions, and the animal is 

 able to move freely in all directions, the more intense rays 

 will determine the direction of the progressive movements." 

 Note that this animal is in the list mentioned above (p. 87). 



