528 CHARLES ZELENY AND E. W. MATTOON 



by reduction from an average of 701.1 facets in the original 

 stock to 98.0 in the 'bar eye' stock. Selection, then, may have 

 an effect because of variability or because of lack of homo- 

 geneity in the race as regards these other factors without regard 

 to the 'barring' factor itself. 



Three possibilities are thus open as regards the explanation 

 of the effect of selection in this case. First, the 'barring' unit 

 factor may be variable or may have varied since its first appear- 

 ance in 1913. Second, the 'bar eye' race and by inference the 

 original normal eyed stock from which it was derived may con- 

 tain additional germinal factors affecting facet number and these 

 additional factors may be variable. Third, the 'bar eye' race 

 and by inference the origmal normal eyed stock from which it 

 was derived may not be homogeneous with regard to these 

 additional factors. Different factorial combinations may be 

 present in different individuals. Selection in this case would 

 segregate the 'high' combinations of factors on the one hand and 

 the 'low' combinations on the other, yielding finally two homo- 

 geneous races in which further selection would have no effect. 

 The 'highest' possible combination of factors as well as the 

 'lowest' possible may not exist in the original sample of the 

 general population, but by ]Mendelian recombination it would 

 finally appear. 



^Tiile the data so far obtained do not enable us to decide 

 which one of these three possibilities or which combination of 

 them is to be considered as active in this case there is some 

 evidence to support the view that the third is at least partly 

 responsible. The increase in regression of the mean toward the 

 mean of the parental population with each successive selection 

 indicates an approaching limit to the effectiveness of selection. 

 This is what we would expect in a population that is heterogene- 

 ous as regards factorial composition. If the unit factors them- 

 selves do not vary, selection must soon cease to have further 

 effect. 



