REACTIONS TO LIGHT IN THE EARTHWORM. 21 



straight ahead. Any swerving from this path leads to an increase 

 of stimulation and hence is corrected. Holmes regards none of 

 the movements as forced by light. All are random in direction 

 but certain favorable ones are followed up and unfavorable ones 

 checked by the increase of stimulation resulting from them. 



Holmes proposes his theory of the " selection of random move- 

 ments " only as one factor in phototaxis, not wishing to exclude 

 the possibility of a slight amount of directive influence in the 

 light. His reason for so doing is based on the observation of 

 himself and the other experimenters alluded to that there is an 

 excess of negative turnings over positive ones. Of course if the 

 movements of the animal are random there should be an equal 

 number of movements in the positive direction as in the negative, 

 when one considers only the first movements occurring after 

 stimulation. Holmes counted a number of first movements and 

 found them about as equally divided between the positive and 

 negative side as could perhaps be expected (23 : 27). Parker 

 and Arkin found an excess of negative movements over positive 

 of 26 per cent. Miss Smith (on the same basis of reckoning) 

 found an excess of 39 per cent, and Adams, using different inten- 

 sities of light, found that the excess was greater with an increase 

 in the intensity. If the observers did not count only the first 

 movements after stimulation but also many subsequent move- 

 ments, the excess of negative movements is not against the sup- 

 position of their random character. It may be well for clearness 

 to suppose a case. Of one hundred first movements after stimu- 

 lation (when the worms are placed at right angles to the light) 

 there should be an equal number of positive and negative, if they 

 are purely random. But according to the theory, negative move- 

 ments tend to be continued while the positive ones are checked 

 and may be followed by negative movements. This would give 

 rise to an excess of negative movements in any large number that 

 were counted. Holmes says that the excess of negative move- 

 ments may be due to one of three causes accident, failure to. 

 count many of the slight positive movements which are easily 

 overlooked, or to a slight orienting tendency of the light. 

 Holmes undoubtedly has in mind first movements only, when he 

 assumes that an excess of negative movements is against the 

 supposition of their random character. 



