34 
GENETICS: W. E. CASTLE 
Metz, adopting Morgan's system of linear grouping, shows the eight genes 
in a hnear chain more than 80 units long, although the greatest distance ex- 
perimentally found between any two genes is 47.3 (yellow-rugose). This dis- 
crepancy, like the similar ones observed in D. ampelophila, shows the inade- 
quacy of the hypothesis of linear arrangement. For the maximum possible 
R 
FIG. 3. MODEL SHOWING RELATIVE POSITIONS OF GENES OF 8 SEX-LINKED CHARACTERS- 
OF DROSOPHILA VIRILIS 
cross-over percentage is 50 and this is in no case exceeded by data given by 
direct experiment. 
Reconstruction in three dimensions (figs. 3 and 4) shows even more clearly 
in this case than in that of D. ampelophila, that a linear arrangement is out 
of the question. The reconstructed figure is roughly in the form of a tetra- 
hedron. Figures 3 and 4 are views taken at right angles to each other corre- 
