HISTORICAL REVIEW 55 



the body will be different. The consequence will be in a 

 positively heliotropic animal a stronger tension or tendency 

 to contract in the muscles connected with the photosensitive 

 points of the one side of the body than in those connected 

 with the opposite side." This view is affirmed in a recent 

 address (1909). 



(4) It is ordinarily assumed that Verworn"- considers 

 orientation in the lower forms to be due to the direct effect ' 

 of the external agent on the locomotor appendages. If, 

 e.g.t one side is more highly illuminated than the other the 

 cilia beat more or less effectively on that side and thus 

 produce orientation. This process is termed heliotropism 

 or phototaxis. 



(5) " Two kinds of effects are produced by light " accord- 

 ing to Davenport (1907, pp. 210, 211), " one by the direc- 

 tion of the rays . . . either through difference of intensity 

 on the two sides of the organism, or by the course the rays 

 take through the organism — phototactic; the other by 

 the difference in illumination of parts of the organism — 

 photopathic." 



(6) Yerkes says (1903, p. 361), "All those reactions in 

 which the direction of movement is determined by an 

 orientation of the organism which is brought about by the 

 light are phototactic; and all those reactions in which the 

 movement, although due to the stimulation of light, is not 

 definitely directed through the orientation of the organism 

 are photopathic." 



(7) To Radl (1903) heliotropism means orientation due 

 to difference in light pressure on unequally illuminated 

 symipetrically located surfaces. 



(8) Holmes (1905) calls orientation by selection of ran- 

 dom movements phototaxis (heliotropism). 



(9) Barrows (1907, p. 530) and Walter (1907, p. 149) 

 suggest "asymmetrical response to asymmetrical stimula- 

 tion" as a criterion of tropisms; and because the organisms 

 worked on respond thus they conclude that their reactions 

 are tropic. According to this criterion it is of course evident 



