RELATIVE PHOTOTROPISM. 361 



the case of flight, a total of 16 animals being affected. Of these 

 the records of 8 had to be eliminated entirely because no effort 

 had been made to distinguish their behavior in individual trials. 

 To the 80 trials thus counted out had to be added 1 1 trials dis- 

 tributed among the other 8 flies, the highest number lost by any 

 one of the 8 being 3. Thus a total of 92 insects were tested, the 

 records of 8 of them being corrected by the elimination of the 

 trials in which slipping occurred. 



It will be noted of course that the probable error in all the 

 determinations is quite high, as the character of the curves 

 would lead one to expect. This is due to the usual erratic 

 behavior of Drosophila, which the writer has always found 

 characteristic of this animal. Nevertheless, in view of the very 

 marked differences between the corresponding records of wild 

 and vestigial insects, it is felt that the results are certainly 

 significant. Thus it may be noted that even when the probable 

 errors are multiplied by five, the ranges so obtained do not 

 overlap, except in a single instance. This instance is that of the 

 range for wild flies in the horizontal unlined tube corrected for 

 use of wings, and the range for vestigials in the vertical unlined 

 tube corrected for slipping. Here the upper value for the range 

 of the wild flies just equals the lower value for the range of 

 the vestigials. 



The conclusions indicated in the table and graphs may now 

 be summarized thus: (i) The vestigials under every condition 

 are decidedly slower than the wild type; (2) the writer's work 

 fails entirely to confirm Cole's contention that the slipping of the 

 vestigials is responsible for their slower records. 



These conclusions come out even more clearly when the data 

 are analyzed in more detail. Thus not only does correction for 

 use of wings by the wild flies make no significant difference, 

 either in the horizontal or vertical records, but correction for 

 slipping in the case of the vestigials likewise produces essentially 

 no effect. Curiously enough indeed, it appears both from the 

 table and the graphs that the vestigials did much better in the 

 vertical tests when the tube was unlined, while no very marked 

 difference occurs between their records for the lined and unlined 

 tubes in the horizontal tests. In the case of the wild flies, on 



