SELECTION- IN BAR-EYED RACE OF DROSOPHILA. 367 



Errors are due chiefly to the following causes: (i) The arrange- 

 ment of the facets may be irregular. Ordinarily the facets are 

 arranged in rows and one can easily count two rows at a time 

 even if the eye is large. But in the bar eye the dorsal part 

 usually contains a large number of facets without any indication 

 of rows. This area increases very rapidly with the size of the 

 eye. In heterozygous females the facets are all arranged in 

 beautiful rows, and one is thus able to distinguish readily between 

 the eye of a heterozygous female and a bar-eyed male even if 

 both eyes contain the same number of facets. Bar-eyed females 

 as a rule contain more regular facets than the males and the 

 distinction between low heterozygous females and high bar- 

 eyed females is more difficult. (2) There may be small facets 

 either among the others or at the margins. These facets are 

 most abundant in irregular eyes and may be one of the causes of 

 irregularity. All grades are found from mere prominences that 

 can only be seen with the most advantageous light to normal- 

 sized facets. As a rule, however, there are few if any doubtful 

 facets. (3) There may be colorless facets at the margins. 

 Normally the colored area of the eye extends somewhat beyond 

 the facet area, but in some rare cases a few facets may extend 

 beyond it and these may be overlooked. (4) When the eye is 

 so large that the fly must be turned to count all of the facets 

 some of the facets in the middle may be recounted or omitted. 

 All heterozygous females and some high grade males had to be 

 turned. The error in this case also is not very great, as the 

 bar eye as well as the heterozygous eye is almost divided in the 

 middle. (5)' Errors may also arise through a lack of concen- 

 tration or through the inability to make the eye retain its posi- 

 tion. These are purely personal elements. 



No mechanical device was used for marking off the counted 

 area from the uncounted area on the eye of the fly. The author 

 relied entirely on the ability of his eye to follow the rows or, 

 in the absence of rows, to mark off certain areas and hold them 

 until the eye was counted. A cross-hair in the ocular was tried, 

 but was found to be unreliable. It could be used if the eye of 

 the fly presented a flat field, but with the rounded contour of 

 the eye the hair keeps on traveling over facets as one focuses up 



