56 LIGHT AND THE BEHAVIOR OF ORGANISMS 



that every diflferentlal response to a localized stimulation 

 even in a human being may be a tropic response. 



(10) To Bohn forced orientation constitutes a tropism; 

 (1908, p. 78), " L' orientation est directe; I'animal est attire 

 sans qu'il pitisse resister: il y a la un ' tropisme' au sens de 

 Loeb "; (p. 80), " On n'a pas besoin de nier la 'volonte' de 

 Tanimal; on peut dire que ces impulsions sont plus fortes 

 qu'elle. On ne peut nier les tropismes." 



(11) Parker apparently considers any reaction which 

 carries an animal toward or away from the source of stimu- 

 lation as tropic; he says (1908, p. 426), " Since amphioxus 

 swims away from a source of light, it is negatively photo- 

 tropic." Minkiewicz (1907, p. 47), uses the term tropism 

 in much the same sense, as does also Hadley, who defines 

 it and photopathy as follows (1908, p. 201) : " A phototactic 

 reaction [is] one in w^hich the organism tends to place the 

 longitudinal axis of the body parallel to the direction of 

 the rays and to approach or recede from the source of those 

 rays. ... A photopathic reaction is one in w^hich an or- 

 ganism, without previous assumption of a body-orientation, 

 ^selects' regions of optimal light-intensity." 



(12) Washburn (1908, p. 57) refers to tropisms as "the 

 direct motor response of an animal to an external stimulus," 

 and Torrey defines the term similarly but somewhat more 

 definitely. He says (1907, p. 319): *' In heliotropism as 

 well as in galvanotropism, the oriented organism is in a 

 condition of physiological stimulation, and . . . the re- 

 sponse to stimulation is local." This definition is in all 

 essentials like those of Verworn and Loeb. 



(13) Driesch (1908, p. 11) says, "A tropism ... is a 

 directed movement of a growing part of a plant or hydroid 

 determined by the direction of a directed agent." 



(14) Wheeler (1910, p. 515) considers reactions which 

 *' involve an adaptive orientation" as tropic. 



(15) Jennings (1909, p. i) suggests the following defini- 

 tion: " The tropism includes those reactions in which the 

 organism takes and maintains a definite orientation — places 



