No. 588] EXPERIMENTS IN MASS SELECTION 721 



and II) . In the third and all subsequent generations selec- 

 tion was made as rigorous as possible consistent with the 

 maintenance of a strong colony from which to make fur- 

 ther selections. Following each selection an advance in 

 the average grade of the offspring took place attended by 

 a steady movement in the direction of the selection on the 

 part of both the upper and the lower limits of variation. 

 The sixteenth plus selection produced 1,690 offspring (a 

 larger number of individuals than is contained in Pearl's 

 entire seven-year series) every one of which fell beyond 

 the original range of variation, which was from -f 1 to -}- 3 

 in the first plus selected generation and from + 3± to +5£ 

 in the sixteenth generation. What this change signifies 

 will be better appreciated when I state that +6 in our 

 grades is a wholly pigmented or "self" rat, and that the 

 extreme variation noted, + 5f , signifies a rat wholly pig- 

 mented except for a few white hairs between the front 

 legs. The ivhole race has accordingly been changed so 

 that no individual is longer produced which falls within 

 the original range of variation. Not a dozen rats in this 

 entire generation would be allowed by a fancier in the 

 category of "hooded" rats. 



In the minus selection series the results secured are 

 scarcely less striking. Only a very few individuals of the 

 1,980 sixteenth generation rats, or the 868 seventeenth gen- 

 eration rats fell within the original range of variation, 

 which in generations 1-3 went no farther than grade — 2. 

 In all other individuals of the sixteenth and seventeenth 

 generations the "hood" was reduced to an extent never 

 seen in the hooded rats of the fancier, the white areas 

 having covered the neck and in extreme cases the fore- 

 head also, leaving only the nose and a patch round the 

 eyes and ears still pigmented. 



Pearl (p. 607) commenting on the results of his selec- 

 tions states that he had no reason to think that at the close 

 of the series any individual had been produced superior 

 in productiveness to those which occurred at the outset, 

 but that he had merely secured more of them, thus raising 



