PLUS SELECTION SERIES. 9 



PLUS SELECTION SERIES. 



This series begins with pairs ranging in average grade from +1.87 

 to +3. From these parents were obtained 150 young, which range in 

 grade from + 1 to +3, as is shown in Table 1. It will be observed that 

 the lower-grade parents have on the average lower-grade offspring than 

 the higher-grade parents. But in no case is the average grade of the 

 offspring as great as that of their parents. Thus 1.87 parents had 1.82 

 offspring (average grade) ; 2.00 parents had 1.76 offspring; 2.25 parents 

 had 1.87 offspring; and so on to 3.00 parents, which had 2.35 offspring. 

 There is a falling back in grade or "regression" of the offspring as com- 

 pared with their parents, which increases in amount as the grade of the 

 parents becomes higher. (See column "Regression" in Table 1.) The 

 parents of this first generation were chosen because of their high grade. 

 They were all probably in grade above the general average of the popu- 

 lation from which they were selected. In the case of those which 

 deviate most from the general average the regression is greatest, as we 

 should expect. 



This phenomenon of regression, which is a very general one in cases 

 of selection, was first observed by Galton in selecting sweet-peas of 

 varying size from a mixed population. Later Johannsen, who repeated 

 the experiment with beans, found that by pedigree culture he was able 

 to break the mixed population up into pure lines within which, con- 

 sidered singly, no regression occurred. We shall need later to return 

 to this subject and consider whether pure lines free from regression 

 exist or can be produced as regards the hooded pattern of rats. 



Returning to the examination of Table 1, since the high-grade parents 

 produce higher-grade offspring than do the low-grade parents, it is 

 evident that we might hope by further selection either to isolate a pure 

 line of high-grade rats which would be free from regression and therefore 

 stable, or else to advance the grade of the offspring still higher, even 

 though regression persists. As a measure of the extent to which high- 

 grade parents have high-grade offspring and vice versa, in each genera- 

 tion, we may employ the well-known correlation coefficient. This for 

 Table 1 is 0.30. 



The second generation in the plus series (Table 2) includes the off- 

 spring of parents which appear as offspring of the higher grades in 

 Table 1, together with a few individuals which appear in Table 2 both 

 as offspring and as parents of other offspring, by reason of their having 

 been mated with generation 1 individuals and so having produced 

 generation 1^ offspring, as explained on page 8. To obtain larger 

 numbers of offspring, several new pairs were added to the experiment 

 in this generation, which do not appear in Table 1 either as offspring or 

 as parents, but which were derived from the same general stock as the 

 parents of generation 1. Their inclusion here accounts for the very 



