AN ANALYSIS OF THE EFFECT OF SELECTION. 25 
between them must be due to second-chromosome differences between 
the two P, races. This experiment may be continued further by 
mating the not-speck Dichzet males produced by the back-cross to the 
speck not-Dichet females. Such a mating should give the same result 
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Fie. 13. 
as the first back-cross and does in fact do so. In table 19 both types of 
experiment are treated together. Figure 14 is a graphical representa- 
tion of the result of one experiment of the type here described. 
It should be noted that such experiments are suited only for the 
detection of dominant modifiers present in one P; race, or, stated 
conversely, recessives present both in one P, race and in the speck race 
from which the test female came. 
TaBLE 19.—" Tests, Chromosome IT. 
Source. Means. Distributions. 
Tested 
Not-sp. sp. againsy— Not-sp. sp. Diff. Diff. x? P 
P. E. 
804. tis ssacios sp. stock | sp. stock |5.314+0.045/4.462+0.050| +0.852+0.067| 12.7 |69.4 0.000000 +| 
1002). .iaeBiners 212 sp. stock | sp. stock |5.924+ .026/5.732+ .031/+ .192+ .040) 4.8 |13.3 | .004 
BOOZ ao. 70s ove eyes sp. stock 1331 5.824 .062/5.385+ .117/4+ .489+ .132) 3.3 | 4.77) .10 
4002 2 3 esicoee. 43 1331 1331 4.983 .063/4.438+ .065)+ .545+ .091} 6.0 /17.7 | .001 
1002). cyte 1331 sp. stock |6.095+ .043/5.750+ .104/+4+ .345+ .113) 3.1 | 6.17] .05 
Cross-br. plus. .| sp. stock | sp. stock |5.412+ .063/4.686+ .097/+ .726+ .116) 6.3 /19.0 | .0003 
900s. case sp. stock | sp. stock |4.711+ .062/4.442+ .070/+ .268+ .093) 2.9 | 3.95) .27 
Cross-br. minus.| sp. stock | sp. stock |5.304+ .087|4.762+ .101/+ .542+ .133/ 4.1 | 6.67] .04 
Cross-br. minus.| sp. stock 1331 4.353 .067/4.151+ .045/+ .202= .081] 2.5 | 5.59) .24 
Table 19 gives the results of the experiments of this type that have 
been carried out. (For the raw data see Appendix.) The first two 
columns give the P; races, or in cases where the tested male came 
himself from a back-cross test, the original source of his not-speck 
and speck chromosomes. The third column gives the source of the 
test female. All tests in which the data for these three columns were 
identical have been lumped. The next three columns give the mean 
