THE AMERICAN NATURALIST [Vol. XL VIII 



ous is a member of Group II and is, presumably, asso- 

 ciated or linked with all other characters belonging to 

 that group. 



Supposedly this association of the characters results 

 from an association of the factors responsible for them 

 in the germ cells. And this latter association has been 

 explained upon the assumption that factors responsible 

 for linked characters are located in the same chromosome. 

 The hypothesis has even been carried so far as to postulate 

 a linear arrangement of the factors within the chromo- 

 some — the relative position of the factors being deter- 

 mined by the degree or amount of linkage existing between 

 them. This conception and the data upon which it is 

 based have been amplified by Morgan and by Sturtevant, 

 and need not be dwelt on here. In the present case no 

 attempt has been made to ascertain the exact degree of 

 linkage between apterous and other characters in the 

 group, except black, because of the difficulty of breeding 

 the apterous flies. Apparently the linkage between apter- 

 ous and black is very close, if not complete, since no case 

 of ' 1 crossing over" was observed among the 875 F 2 off- 

 spring in this experiment (Table VI). There is a possi- 

 bility that the classification of the F 2 apterous flies as all 

 gray is not absolutely correct, because, owing to the diffi- 

 culty of distinguishing o T;1 y from heterozygous black in 

 apterous specimens, an occasional heterozygous fly might 

 have passed for pure gray. However, if there had been 

 any appreciable number of cross-overs in this direction, 

 there would also have been some in the opposite direction, 

 which fact would have been indicated by the presence of 

 winged, gray flies. And since none of these were observed, 

 it is safe to conclude that few or no cross-overs occurred, 

 and hence that apterous is very closely, if not completely, 

 linked to black. 



Experiment V .— To determine the relation between the 

 apterous mutant and the "vestigial" mutant. 



Among the mutant characters of Group II is one called 

 "vestigial wing." Flies having this character are more 



