322 S. J. HOLMES. 



of light. If it orients itself to the light the backward stroke of 

 the flagella, /. c. the stroke that is effective in propelling the body 

 forward must be more effective on the shaded side than on the 

 brighter side. This may conceivably occur in the following ways, 

 which, however, 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 phase, 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-increas- 

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

 lowered. If the orientation is effected 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 di- 

 minished. 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 organism passes to a point where the illumination on 

 both sides of its body is increased, its rate of transit would be dimin- 

 ished. If we suppose that the forward stroke is most stimulated, 

 or the backward 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 pad- 

 dles so adjusted that when they came 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. 



Does Volvox react to light as the theory above mentioned 



