148 ^Journal of Comparative Neurology and Psychology. 



Volvox be exposed to parallel rays of light so that there is a differ- 

 ence in intensity on opposite sides of the organisms when the 

 longitudinal axis is parallel with the rays, they do not move 

 directly toward the source of light but deflect toward the side 

 most highly illuminated. In accordance with Holmes' theory we 

 should expect them to move parallel with the rays under these 

 conditions. 



2. Holmes states that the condition for orientation, according 

 to his theory, would be fulfilled "if the rays diverge from a spot in 

 front of the colony in line with its long axis, " If this be true, we 

 should certainly expect the conditions for orientation also to be 

 fulfilled, if the rays converge from two luminous points in front of 

 the organism and if "the eye-spots are most sensitive to light 

 striking them at a certain angle" we should expect the organisms 

 to move tow^ard a point nearly, if not exactly, midway between 

 the two sources of light regardless of their relative intensity. But 

 it has been demonstrated (p. 133) that if Volvox colonies be exposed 

 to light from two sources of unequal intensity, they orient and 

 swim toward a point nearer the more intense source. It is, there- 

 fore, evident that the explanation of orientation in Volvox, sug- 

 gested by Holmes, is not consistent with the experimental results 

 which I have presented. 



I have demonstrated beyond a reasonable doubt that the dif- 

 ference in intensity on opposite sides of Volvox modifies its direc- 

 tion of motion regardless of the direction of the light rays, and 

 since the direction of motion is changed by diflerence in the 

 effective stroke of the flagella on opposite sides, it must be differ- 

 ence in intensity which influences the stroke of the flagella. But 

 Holmes, as stated above, concluded that the reaction of Volvox 

 cannot be explained upon the assumption that difference in inten- 

 sity on opposite sides of the body causes the flagella to beat with 

 unequal vigor. Upon what does he base this conclusion and 

 wherein lies the fallacy of his argument ? 



I can present his line of thought best by quoting verbatim 

 ('03, p. 321-322): "Let us consider a Volvox in a region of sub- 

 optimal stimulation and lying obliquely to the rays of light. If it 

 orients itself to the light the backward stroke of the flagella, i. e. 

 the stroke that is effective in propelling the body forward must be 

 more effective on the shaded side than on the brio-hter side. This 

 may conceivably occur in the following ways, which, however, 



