Sensory Discrimination: Vision 151 



light. But if a blue screen is interposed between the 

 light and the animal, in spite of the fact that the intensity 

 is thereby diminished, the Daphnias avoid it; if yellow 

 Ught is added to white light, in spite of the fact that the 

 intensity of the Ught is thereby increased, the Daphnias 

 seek it. These results were obtained by Von Frisch (248), 

 who is as determined to find color vision in invertebrates 

 as Hess is to disprove it. Ewald (213) reports that of the 

 Daphnias under his observation one group sought the hght, 

 which was in this case most effective in the green-yellow 

 regions, but that another group avoided Ught, and for these 

 the most effective region was the blue-violet, so the effect 

 of colored rays was independent of intensity. He reports, 

 however, that certain colored rays could be replaced by 

 colorless rays without affecting the responses of the Daph- 

 nias ; these rays were the red and green ones. He there- 

 fore concludes that Daphnia is not totally color-bUnd, but 

 red-green blind. This, as we shall see, is Von Frisch's 

 beUef with regard to certain other invertebrates. Ewald 

 thinks he has also evidence in the case of Daphnia of si- 

 multaneous contrast and successive contrast, such as human 

 vision shows. The successive effect (negative after-images) 

 occurs for both color and brightness stimuU, and is shown 

 by the fact that the animals reverse their reaction to the 

 same white Ught according as they have been exposed pre- 

 viously to white (or blue) Ught, or to darkness (or yellow 

 Ught). Simultaneous contrast Ewald concludes from the 

 observation that when the region surrounding a constant 

 stimulus light is brightened, the reaction of the animals 

 tends to become positive, that is, they move towards the 

 Ught; darkening the surroundings makes them move 

 towards the same Ught. This effect, it is argued, is due to 

 the stimulation of the side regions of the eye : now, since 



