HISTORICAL REVIEW 29 



that If there are several sources of Hght of unequal inten- 

 sity, the Hght with the strongest intensity determines the 

 orientation and direction of motion of the animal. Other 

 possible complications 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 helio- 

 tropic 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 demon- 

 strated 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." 



In these papers it is clear that the important factors in 

 orientation to light are considered to be: (i) symmetry of 

 the body; (2) the angle between the rays and the sensitive 

 surface on opposite sides; and (3) constant intensity, 

 functioning as it does in case of the electric current. Orien- 

 tation in light is supposed to be controlled by the direct 

 action of the external agent, on the locomotor tissue or 

 through a direct reflex arc. It is controlled unequivocally 

 by the external agent, which acts constantly as a directive 

 stimulus similar to the action of a constant electric current. 



At this time Loeb evidently still placed much dependence 

 upon the assumed effect of the angle which the rays make 

 with the sensitive surface (ray direction), for if he con- 

 sidered merely intensity difference on opposite sides it 

 would be impossible for him to say as he does that when 

 organisms are exposed to light from " several sources . . . 

 of unequal intensity, the light with the strongest intensity 

 determines the orientation and direction of motion of the 

 animal." In a more recent discussion however he uses 

 the following expression (1906, p. 130)1 " We started w^th 



