No. 631] FACTORS OF HEREDITY 



TABLE I 



Back Cross of Females Heterozygous for v, g axd f. (Performed by 



in Fig. 1. Other results indicate that the line would be 

 exactly straight if still smaller distances were studied. 

 Enough data have been obtained in the case of chromo- 

 some I of Drosophila to determine in this way the 

 "shape" of each part' of the linkage group, and each 

 part, by itself, is thus found to follow the rules for linear 



distances in an extraordinarily rigorous manner. That 

 is, given the factors ABCDE, etc.,— or to take an actual 

 case, y, w, A, bi, cl,— it is found that the linkages of y, w, 

 and A are proportional to their distances in a straight 

 line, so are the linkages of w, A, and bi, for A, bi, and 

 cl, etc. But, since every part of the group is thus linear, 

 it must then be true that the entire group is linear. A 

 line all of the parts of which are straight is a straight 

 line. Any differences then observed between the size of 

 the larger distances and the per cents of crossing over, 

 even if they were so irregular that they could not be 

 thought of as a function of the linear system itself, would 

 then have to be regarded as due to peculiarities in the in- 

 cidence of the crossing over, superimposed upon a sys- 

 tem of genes which was really linear in formation,— 



