A PHYSIOLOGICAL CHARACTER. 127 



cumulative at the close of the experiment. The divergence in mean 

 reaction-times is neither controlled nor noticeably influenced by the 

 relative vigor of the two strains, nor does it appear to be explicable 

 on any other grounds than as a divergence coming about through 

 selection. 



It is to be regretted that no return selection was attempted 

 with Line 757. The selection experiments were very laborious, and 

 at the time when it was decided that they had been carried far 

 enough, there did not seem sufficient justification for prolonging the 

 experiment. Though it was then realized that the two strains of 

 Line 757 were pronouncedly different in their reactiveness to light, 

 the full significance of this difference, particularly as involving a 

 changed reactiveness in both strains of Line 757 as compared with 

 the corresponding strains of the other S. exspinosus lines, was not 

 brought out until months later, when the extensive data were fully 

 worked over. Time was not then available for again taking up the 

 selection experiment. 



A further fact of importance is that the effect of selection was 

 permanent, or at any rate persisted, through a long series of genera- 

 tions. In December 1919 (32 months after selection had been dis- 

 continued) the selective effect still persisted and probably to as 

 marked a degree as at the termination of selection. The two strains, 

 meantime, had been propagated for 112 generations without any 

 regard to their past history. The persistence of the effect of selection 

 was demonstrated by testing a large number of broods of both 

 strains. In extremely few cases did a brood of the plus strain fail 

 to show more reactiveness to light than the corresponding brood of 

 the minus strain, while in most cases the broods of the plus strain 

 were markedly the more reactive. 



