REACTIONS TO LIGHT 255 



classes, — phototropism and photopathy. All cases of ori- 

 entation belong to the former, and all cases where organisms 

 aggregate without orientation, to the latter. Davenport 

 mentions Planaria torva, Daphnia and Volvox as examples 

 of organisms which are photopathic. Both he and Loeb 

 affirm that there are some organisms which are both photo- 

 tropic and photopathic. The one mentions Daphnia as an 

 example, the other Spirographis. 



Yerkes (1903, p. 361) refers to this problem as follows: 

 " The motor reactions of organisms to light, so far as known 

 at present, are of two kinds: phototactic and photopathic. 

 In both intensity of the light, not the direction of the rays, 

 is the determining factor. 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 photo- 

 pathic. . . . /\n organism which selects a particular in- 

 tensity of light and confines its movements to the region 

 illuminated with that intensity is photopathic." Thus it is 

 seen that Yerkes, like Loeb and Davenport, divides reac- 

 tions into two classes and ascribes both kinds of reactions 

 to a given individual in certain cases. All three authors 

 agree in designating orienting reactions as photopathic or 

 phototropic, but they do not agree in their explanation of 

 the process of orientation. Loeb claims it is due to the 

 effect of constant intensity; Davenport thinks in some cases 

 it is due to the direction in which the rays pass through 

 the organism, and in others to the effect of difference of 

 intensity on opposite sides; and Yerkes maintains that it is 

 due to difference of intensity in all cases. But if the light 

 intensity on the surface of an organism is not uniform 

 almost every movement of the organism produces changes 

 of intensity on some part of it. It is therefore evident that 

 orientation (phototropism), according to Loeb, Davenport 

 and Yerkes, may be due respectively to the effect of con- 



