Mast, Light Reactions in Loiver Organisms. 149 



amount practically to the same thing: the diminished intensity 

 of light on the shaded side of the body may act as a stimulus to 

 the backward p>ase of the stroke, or decrease the efficiency of the 

 forward phase of the stroke of the flagella; or the light on the 

 brighter side of the body may inhibit the backward phase or 

 increase the forward phase of the stroke of the flagella; In any 

 case, if the organism is passing into regions of ever-increasing 

 intensity of light, we should expect its rate of speed would be 

 lowered. If the orientation is affected by a shading of the side away 

 from the light it would follow that in a region in which the shading 

 were less the speed of the travelling body would be diminished. 

 If the parts of the body which are most shaded are the parts where 

 the effective beat of the flagella is the strongest, then, as the organ- 

 ism passes to a point where the illumination on both sides of its 

 body is increased, its rate of transit would be diminished. If we 

 suppose that the forward stroke is most stimulated, or the back- 

 ward stroke most inhibited on the brightest side of the body we 

 should expect that with more illumination the more inhibition 

 there would be, or the more the backward phase of the stroke 

 would be increased, and the rate of locomotion would likewise be 

 reduced. If we imagine a machine in the form of a Volvox colony 

 and provided on all sides with small movable paddles so adjusted 

 that when they come into regions of diminished light as the 

 machine rolled through the water their effective beat would be 

 increased, it is clear that such a machine might orient itself to the 

 direction of the rays and travel towards the source of illumination, 

 but its rate of locomotion would be diminished the brighter the 

 light into which it passed. We may conceive the light to increase 

 or decrease the backward or forward stroke of the paddles in any 

 way we please and we cannot explain how such a machine can 

 orient itself and go towards the light and at the same time move 

 through the water more rapidly as it comes into regions of greater 

 illumination." 



It is evident that the crux of this whole argument is the relation 

 between rate of movement and light intensity. This relation was 

 w^orked out in detail by Holmes ('03, p. 323) with the following 

 results: "It was found that, as the Volvox travelled towards 

 the light, their movement was at first slow, their orientation not 

 precise, and their course crooked. Gradually their path became 

 straighter, the orientation to the light rays more exact and their 



