22 AN ANALYSIS OF THE EFFECT OF SELECTION. 
two lines both came in large part from the sepia, spineless, kidney, 
sooty, rough, and peach, spineless, kidney, sooty, rough stocks, and 
therefore selection presumably had similar material to work with in 
both cases. That the result was the same is, then, only a somewhat 
unexpected coincidence. It may be pointed out that the identity of 
the two lines is borne out by their very similar behavior after the 
seventh and tenth generations, respectively. (See figs. 3 and 4, above.) 
CROSS OF PLUS AND MINUS LINE. 
When two races that differ in quantitative characters are crossed, 
the usual result is an increased variability in F, and an increased 
F,-F; parent-offspring correlation. This result was obtained in the 
present case, as is shown by table 18 and figures 11 and 12, which 
give the data for a cross of the 1002 plus and 1331 minus lines. 
TaBie 18.—Cross of Inbred Plus and Inbred Minus Lines. 
Generation. n M a r 
Bie essere sce 53 | 5.679+0.049 | 0.542+0.035 
Faexs wawce 369 | 4.694 .037 | 1.052 .026 | +0.193+0.034 
Ps cccalsuates® 1,133 | 5.524 .016 .787+= .O11 | + .258= .019 
Lt eee eee 1,078 | 5.492 .013 -610+ .009 | + .330+ .019 
1,555 
1Calculated after elimination of aberrant culture 3077. 
Such a result is capable of explanation in either of two ways. It 
may be due to the segregation of modifying factors, or it may be due 
to contamination of unlike allelomorphs in the F, individuals. 
The contamination hypothesis presents some unusual features in 
the present case; for the F, Dichets were not heterozygous for one 
plus Dichet gene and one minus one; homozygous Dichets always die. 
Half of them had one plus selected Dichzt gene and one minus selected 
normal allelomorph of Dichet (7. e., not-Dichext), the other half had 
one minus selected Dichet and one plus selected not-Dichet. Both 
not-Dichets, when homozygous, give for the most part 8-bristled 
flies, which are more “plus” than any Dichet race. Nevertheless, 
on the contamination view, each must contaminate its mate in the F, 
fly, in the direction in which it has been selected. Even the minus 
selected not-Dichet, that makes for 8 bristles, must contaminate the 
plus selected Dichext, that makes for 6 bristles, in such a way that 
the resulting Dichet gene makes for only 4 or 5 bristles. That. is, 
“‘plusness” or ‘“minusness” and “Dichetness” must be separable, 
and a degree of “‘minusness” that affects the result produced by a 
not-Dichet gene only very slightly must nevertheless be capable of 
