358 



ROBERT S. MCEWEN. 



set aside in a dim light, and the test of the other group begun. 

 In this way each of the two groups were given during the day 

 four tests, so spaced that the time elapsing between the successive 

 tests of any given insect was approximately two hours. Each 

 of the four tests was of course varied as regarded either the type 

 of tube used or its position, the four possibilities involved being: 



t3 12 



Witt Flies. 



VestfaUl Fii 



13. 



2.3, 



3.6, 



14 16 % 18 



Time in Seconds. 



GRAPH III. The solid line shows the record, corrected for flight, of 93 wild 

 flies, 43 males and 50 females, in an unlined horizontal tube. The broken line 

 shows the record of 100 vestigial flies, 50 males and 50 females, also in an unlined 

 horizontal tube. Only three instances of slipping were noted in the case of the 

 vestigials, and as correction for these would have made no significant difference it 

 is not shown. 



the unlined tube horizontal, the unlined tube vertical, the lined 

 tube horizontal, and the lined tube vertical. Furthermore, for 

 the purpose of overcoming any biased effect which might result 

 from always testing the same type of fly in the same condition 

 with respect to time of day and previous tests, the order of the 

 tests was rearranged each day. Under this plan during the 

 course of the experiments each of the two types of insects was 

 tested under almost (though not quite) every possible combina- 

 tion with regard to the above factors. The variations in the 



