Spatially Determined Reactions 195 



sibility to difference," which Loeb recognizes as an indepen- 

 dent form of reaction. In support of his continuous action 

 theory Loeb lays great stress on the proof, by the botanist 

 Blauuw, that the "Bunsen-Roscoe Law/' that is, the law 

 that the effect of weak light acting a long time is equal 

 to that of strong light acting a short time, holds for plants ; 

 Loeb thinks it holds also for animals. 



The action of continuous light in producing a tropism has 

 been explained in two ways : (i) as the effect of the direction 

 of the light rays traversing the animal's body, and (2) as the 

 effect of having symmetrical points on the animal's body 

 stimulated with unequal degrees of intensity. In his 

 earliest discussion of the subject, Loeb (419) expressed 

 himself positively in favor of the former hypothesis. "The 

 orientation of animals to a source of light is, like that of 

 plants, conditioned by the direction in which the light rays 

 traverse the animal tissue, and not by the difference in the 

 light intensity on the different sides of the animal." Bohn, 

 in general the ardent follower of Loeb, urged as a "funda- 

 mental objection" to this that "the 'luminous rays' which 

 strike a living body have, save in wholly exceptional cases, 

 various directions, being reflected, diffused, and refracted 

 by neighboring bodies" (80). Moreover, the animal 

 bodies which are opaque could not be traversed by light 

 rays. Loeb seems later to have abandoned the " direction 

 theory" of the tropism. The "intensity theory" was first 

 proposed by Verworn (743). 



How can differences in the intensity of a stimulus falling 

 upon symmetrical and opposite points on an animal's body 

 bring about orientation? Let us call the two points a 

 and a', a being a point on the right side of the animal's 

 body and a! a symmetrically placed point on the left 

 side. Suppose the animal has a tendency to orient itself 



