GENETICS: E. C. MacDOWELL 
293 
significant in comparison with their probable errors; in some cases they 
are clearly significant. This means that in most cases there was no 
tendency for high grade parents to have higher grade offspring than low 
grade parents. In individual generations higher parents did produce 
offspring of somewhat higher grade, but then in others higher parents 
actually produced offspring of lower grade than did the lower grade 
parents. These facts indicate great independence between the grades 
of parents and offspring in all but the early generations; they lead one 
to expect to find that the selection of high variates during the early 
generations has modified the means of the race, but that later on the 
same process has failed to make further progress. What actually 
happened is shown in figure 1, which represents the means of the high 
selected race. 
24- / /' 
/ GENERATIONS 
I ' « 1 I ' ' ' < ' ' ' ' I ' I ' ' I I I I I I ' ' I 
I ' » ' ' I ' ' I ' I ' ' ' ' I ' ■ 
10 
15 
20 
2f 
FIG. 1. MEANS OF THE OFFSPRING FROM 49 GENERATIONS OF SELECTION FOR 
INCREASED NUMBER OF EXTRA BRISTLES 
Due to a change in method the means in generations following the 29th tend to be higher, 
and therefore can not be compared with the preceding ones. 
The means of the males are consistently lower than those of the females, 
but the close parallelism of the curves serves to substantiate the correct- 
ness of the means as descriptions of the different generations. As ex- 
pected, the means steadily rise for the first few generations; after this 
they fluctuate. It will be noted that the highest point in the curves 
appears in the 29th generation; this high point immediately follows the 
removal of the breeding bottles to a constant temperature room in which 
dry summer heat was automatically maintained. Moreover, due to 
a change in method, in the subsequent generations the means tend to 
be higher than those in the generation preceding the 29th. It remains, 
then, to compare the means before and after the 29th, separately. 
Where the means show regular advance, the correlation coefficients 
