220 THE AMERICAN NATURALIST [Vol. L 



Besides this, it is found that spread is about two thirds 

 of the way between pink and ebony. Ebony, it will be 

 remembered, is an allelomorph of sooty, and therefore 

 occupies the same position. As no recombination has yet 

 occurred between C and rough in flies heterozygous for C, 

 it has not been possible to obtain these factors together 

 and so, in crosses homozygous for C, the linkage of 

 rough has not yet been discovered. The length of the 

 group between sepia and ebony is 75 in these flies, as will 

 be seen from the above data. Although these figures are 

 based on a relatively small number of flies, the difference 

 between this and the shorter value (35) found in flies not 

 containing C is marked enough to be significant, espe- 

 cially since it occurred in various crosses of this sort. If 

 the distance between sooty and rough is expanded in the 

 same way, the group would have a length of much over 

 100. If, however, this distance is of the same length as in 

 flies without C, the total length would be 95. The reasons 

 have been given which incline us to the opinion that these 

 values obtained in crosses homozygous for C may repre- 

 sent the ' 'normal" figures for this group rather than 

 those obtained in the experiments earlier cited. Further 

 investigation of this point, however, is being undertaken. 



Group IV corresponds with the pair of small chromo- 

 somes in that it contains so few factors. For this reason, 

 the author, in his account of the inheritance of bent 

 wing, in 1914, said : 



It also seems probable that when other mutations are discovered in the 

 fourth group, the genes in -which they occur will be found to be linked 

 strongly to the gene for bent wings, since the fourth chromosome is prob- 

 ably the small one, and so any genes in it must lie near together. 



One other mutant factor, "eyeless," has since been found, 

 by Miss Hoge, to lie in this group. But although Miss 

 Hoge has made numerous attempts (3) to combine eye- 



