22 E. H. HARPER. 



Holmes's theory of the "selection of random movements as a 

 factor in phototaxis " is thus based upon observational evidence 

 which is easy to verify. It is" easy to observe that the move- 

 ments toward the light are apt to be checked and the movements 

 away to be more prolonged. It is less easy to note in weak light, 

 as the final result of orientation takes longer in that case. 



ABSENCE OF RANDOM MOVEMENTS IN NEGATIVE PHOTOTAXIS 



IN VERY STRONG LIGHT. 



All of the experimenters referred to used artificial light except 

 Miss Smith, who used diffuse daylight. Since all of them but 

 Holmes took for granted the direct orienting power of light, they 

 did not care to put the matter to a crucial test. It would seem 

 that a test of the orienting power of light would require the use 

 of lights of various strength, and especially of very strong inten- 

 sity, since the perceptive power for light is so poorly developed 

 in the earthworm. A test of the orienting power of direct sun- 

 light is a very easy thing to make. Place the earthworm upon a 

 sheet of wet paper in a beam of direct sunlight from a window. 

 The light may be passed through a water chamber. The results 

 are sufficiently obvious as to leave no doubt of their general 

 nature. Pericliceta is oriented directly away from the light, when 

 placed at right angles to the rays. The first effect is a turning 

 of the anterior end away from the light and by a series of turns 

 the worm gets into the oriented position and crawls directly 

 away. Usually the result is produced without a false move- 

 ment. It is immaterial whether heat effects are excluded by 

 passing the light through water or not. A species of Lmnbricus 

 was experimented with and behaved in the same manner. 



If the sheet of paper is turned as the worm turns, so as to keep 

 it at right angles to the rays the worm will travel in a circle con- 

 tinuously. To show the difference between the orienting effect 

 of sunlight and that of an ordinary artificial light the following 

 experiment was tried. By using a sort of searchlight consisting 

 of a tube of asbestos paper surrounding a 32 c.p. incandescent 

 light and narrowed to a small aperture, the light was so manipu- 

 lated by the hands as to keep it constantly directed upon the 

 anterior end of the worm, with the worm at right angles to the 



