150 yournal of Comparative Neurology and Psychology. 



speed more rapid. After travelling over a few spaces (centi- 

 meters), however, their speed became remarkably uniform until 

 the end of the trough was reached." Unfortunately, Holmes does 

 not give the length of the trough, but he says the distance over 

 which there is a marked increase of speed is considerably less than 

 the space over which the speed is nearly uniform. 



Holmes concludes from these results that the increase in rate of 

 speed is due to increase in light intensity and consequently that 

 orientation cannot be due to difference in intensity on opposite 

 sides of the organism, because if it were, the backward stroke of 

 the flagella would have to be more effective on the side in the 

 higher light intensity than on the side in the lower, and this would 

 cause the organism to turn from the source of light instead of 

 toward it. Are these conclusions correct .^ 



If the increase in rate of speed is due primarily to increase in 

 light intensity, one would certainly not expect the rate to become 

 uniform after the colonies have traveled a few centimeters in the 

 trough, nor would one expect it to increase if the colonies are 

 exposed to light of a given intensity for some time. But Holmes 

 states that the rate does become uniform, and I frequently observed 

 that if relatively quiet colonies in an aquarium containing water 

 a few millimeters deep, are illuminated from above, they gradually 

 become more active. Since, under these conditions, they cannot 

 move toward the source of light, it is evident that this increase in 

 activity is not due to increase in light intensity- It is very probable 

 then, that the increase in rate of movement is more dependent 

 upon the time of exposure to light than upon the increase in inten- 

 sity. Moreover, Holmes states that orientation is more exact 

 after the colonies have traveled some little distance, i. e., after the 

 rate has become nearly uniform. It must, therefore, be least 

 exact when the increase in rate of speed is greatest. If this be 

 true, it follows that the factors which regulate rate of speed are 

 quite different from those which regulate orientation. We have 

 demonstrated that difference in light intensity on opposite sides 

 of the colonies modifies the direction of movement. And since 

 the factors which regulate the direction of motion and those which 

 regulate the activity of the colonies are different, we may conclude, 

 from this point of view, as well as from what has gone before, that 

 the increase in the rate of speed is not primarily due to increase 

 in light intensity. Such being the case, the argument of Holmes 



