RELATIVE PHOTOTROPISM. 



357 



by shaking. The end of the tube was stopped by a cork covered 

 with black paper, and the tube was placed in position. Ten 

 successive trials were given in each test, the time of movement 

 through the distance of 172 mm. being taken by a stop watch. 

 If "the passage was not accomplished within 30 seconds, a trial 

 was considered completed, and 30 seconds was recorded as the 



13 



14 16 18 



Time in Seconds. 



ao 



aa 



28 



30 



GRAPH II. The solid line shows the record of 100 wild flies, 50 males and 50 

 females, in a lined vertical tube. The broken line shows the record of 100 vestigial 

 flies, 50 males and 50 females, also in a lined vertical tube. In each case the time 

 is plotted against the number of insects. 



time made. After each trial the animal was immediately re- 

 turned to the starting point, if possible by its light or gravity 

 reaction, but frequently by shaking. The latter procedure was 

 made necessary in order that an approximately uniform period 

 .(about half a minute) might elapse between each trial. In this 

 manner each of the five flies of a given type was tested in imme- 

 diate succession, the same tube (either the one lined by paper or 

 the one unlined) being used in the same position (either vertical 

 or horizontal) for each insect. When the test of one group of 

 flies was thus completed, the vials containing the insects were 



