FISSION EATE OF STYLONYCHIA PUSTULATA 459 



at random the animals for daily transfer to the fresh slides. But 

 this method opened the possibility of the 'personal equation' 

 becoming so important a factor that I rejected it. In place of 

 it I devised what I have called the method of 'balanced selec- 

 tion.' Balanced selection is the process of compensating for the 

 effect of every selection that one is compelled to make by making 

 the reverse selection at the next opportunity; in other words, 

 one makes the same number of plus and minus selections in any 

 given line during each successive time interval adopted. Thus, 

 if on a given day one makes a plus selection, at the next oppor- 



Fig. 2 Curve of the daily differences between the average number of gener- 

 ations per line produced by each of the two sets of lines while under selection in 

 opposite directions (Exp. 1, part 1). The ordinates show the average differences 

 (in fractions of a generation) in favor of the fast selected lines; the abscissae are 

 the days. 



tunity he would make a reverse, or minus selection. In order 

 to be able to tell at a glance whether to make a plus or minus 

 selection the character of the selection to be made was also 

 recorded on the slide. 



In addition to carrying out balanced selection, in a portion of 

 the lines reversed selection was practiced; that is, the fast lines 

 were now subjected to minus selection, the slow lines to plus 

 selection. Of course, if the observed difference in rate is heredi- 

 tary and has resulted from selection, then there is no logical 

 reason why the same result should not be obtained a second 

 time, so that the difference would disappear, or eventually be 



