MINUS SELECTION SERIES. 13 



throughout these generations a positive correlation between parents 

 and offspring. This amounts on the average to 0.137 as compared wit h 

 0.193 observed in the plus selection series. The absolute change in 

 amount of pigmentation is no doubt less in the minus selection than in 

 the plus selection series, but if the change were recorded as percentage 

 decrease of pigmentation in one case and percentage increase in the other, 

 the change indicated would probably be as great in one as in the other. 

 In the minus as in the plus series we observe : 



(1) The character of the offspring varies with that of the parents; 

 high-grade parents have high-grade offspring and vice versa. 



(2) The variability of the race (as indicated by the standard devia- 

 tion) undergoes some reduction and the limits of variation, both upper 

 and lower, are displaced in the direction of the selection. 



(3) The regression from a new and extreme class of parents is at first 

 large, but decreases as the selection is repeated and finally disappears 

 altogether when the average of the race becomes equal to the particular 

 grade under discussion. 



RETURN SELECTION. 



The plus and minus selection series already described make it clear 

 that one can, in a race of hooded rats, either increase or decrease the 

 average pigmentation at will, and at the same time secure more advanced 

 stages either of pigmentation or of depigmentation than those pre- 

 vious^ occurring in the race. The question now arises, are these 

 changes permanent; will these displaced means retain their new posi- 

 tion, if the race is left to itself; or will the newly obtained stages vanish 

 as soon as selection is suspended? A presumption that the changes 

 will prove permanent is afforded by the gradual decrease of regression 

 and its final reversal in the case of offspring of a particular grade, upon 

 repeated selection made in the same direction. (See page 12.) But in 

 order to test the matter more directly and thoroughly, the experiment 

 has been repeatedly made of reversing the course of selection, after it had 

 been in progress for several generations, with a view of ascertaining 

 whether the return toward the former condition of the race would be 

 made more speedily and easily than the original departure from it had 

 been. 



The first experiment of this sort was a return selection from genera- 

 tion 6 (and 6^) of the minus selection series. The parents of generation 

 6 (Table 21) averaged —1.86 in grade; the average grade of their off- 

 spring was —1.56, a regression of 0.30. The range of the offspring 

 extended from to —2.50. Some low-grade offspring were chosen for 

 a return selection series (Table 31). The mean grade of the selected 

 pairs ranged from —0.37 to — 0.S7, their mean being —0.60. These 

 parents produced 118 offspring, whose average grade was —1.28, a 

 regression of 0.68 in a direction contrary to that of the regression in the 



