142 Journal of Comparative Neurology and Psychology. 



field where the Hght intensity is low than in those where it is high. 

 As is clear from Table IV this was found to be true. But since we 

 have demonstrated (p. 121) that deflection in light of uniform 

 intensity can be increased either by decreasing or increasing the 

 intensity, it may be maintained that the difference in deflection 

 recorded in Table IV is due to the diflerence in light intensity in 

 the field regardless of diff^erence in intensity on opposite sides of 

 the organisms. It must be remembered, however, that deflection 

 in a field uniformly illuminated, is increased only if the intensity 

 is decreased to a point near the threshold or increased to a point 

 near the optimum. In the experiments just referred to, the inten- 

 sity, in all probability, was far 



^F 



below the optimum and above 

 the threshold, so that it is not 

 likely that mere reduction in 

 illumination afl'ected the deflec- 

 tion to, any considerable extent. 

 The diff"erence between the de- 

 gree of deflection in 142 candle 

 meters and 380 candle meters 

 of light, graded in intensity, 

 must, therefore, have been due 

 to the greater efi^ect of the differ- 

 ences in light intensity on oppo- 

 site sides of the organism when 

 exposed to weak light than 

 when exposed to strong. The 

 experimental results recorded in Table IV therefore support our 

 previous conclusion, that the direction of motion in Volvox is 

 regulated by the relative intensity of light on opposite sides of the 

 colonies. 



LoEB, however, as is well known, asserts that orientation is 

 caused by the direction of the rays regardless of the diff^erence in 

 light intensity. He bases his assertion largely on the results in the 

 three following experiments on Porthesia larvae (Loeb '05, p. 25-28). 

 ^^ Experiment J. — The test tube is placed perpendicular to the 

 plane F of the window, and at the beginning of the experiment the 

 animals are collected at the window side jB of the test tube." Now 

 if the half near the window is covered, the animals soon collect at 

 A. "As soon as they emerge from the box K into A they turn about. 



Fig. 9. After Loee, 1405, p. 25, Fig. i. 



