20 AN ANALYSIS OF THE EFFECT OF SELECTION. 
GENERAL RESULTS OF SELECTION EXPERIMENTS. 
In every case the selected lines showed means that differed from the 
mean of unselected Dichets in the direction in which selection had 
been carried on. Owing to the apparently large environmental influ- 
ence on bristle number, it is in most cases difficult to be sure how this 
result was brought about, or, rather, at what stage in the process. 
In the case of the 1331 (speck) minus line, however, the change seems 
to have been effected fairly rapidly at first, and slowly, if at all, later 
on. In the case of the 1002 line there was probably no effect in the 
later generations. Reversed selection was uniformly successful if 
begun in the early generations, but not usually so at later stages. 
These are the results that would be expected on the view that modify- 
ing genes are involved. 
It is to be observed in the case of the plus lines that the means vary 
inversely as the standard deviations—that is, that the two curves 
are much like mirror images. In the minus lines the two quantities 
usually vary together, giving curves that are nearly parallel. These 
relations hold surprisingly closely for many of the curves, especially 
those of the plus lines. They are due to the fact that a change in the 
mean is almost always brought about by an elimination or great de- 
crease in the number of individuals at one extreme of the population 
rather than by a marked change in the position of the mode or of the 
other extreme. This is strongly in favor of the view that selection 
has been effective in eliminating ‘‘unfavorable’”’ combinations rather 
than in producing entirely new types. 
The relation between the crossbred and inbred series is too much 
obscured to repay detailed analysis. Evidently such experiments 
with this character would have to be carried out under carefully con- 
trolled environmental conditions before they could have any great 
significance. 
CROSS OF TWO INBRED PLUS LINES. 
Since the two inbred plus lines, 864 and 1002, came from slightly 
different sources (see above), and were kept separate while being plus 
selected, it seemed possible that different plus modifiers had been 
isolated in the two lines. If this were the case, crossing them should 
result in increasing the variability in F,, and the parent-offspring 
correlation when the F, individuals were bred to produce F3. The 
F, population should contain genetically unlike individuals, and 
should yield to selection in either direction. As a matter of fact, no 
such result was obtained. 
Table 17 gives the result of the experiment. The 1941 set is per- 
haps the clearest case, so we may consider it alone. The parents of 
1941 came from 1763 (Fi) of the 864 line) and 1788 (F; of the 1002 
line). As table 17 and figure 10 show, the standard deviation in F2 
