A PHYSIOLOGICAL CHARACTER. 17 



line selected for its less responsiveness to light. In the beginning of 

 this selection the most responsive individual from the first brood of 

 the young mother was selected for the beginning of the plus strain 

 and the least reactive one or, perchance, one reacting negatively and 

 avoiding the light, for the beginning of the minus strain. In the 

 next and later generations selections were made from the first broods 

 (except in cases where the first brood was lost), the two most reactive 

 individuals in the plus and the two least reactive individuals (or 

 negatively reacting individuals if such occurred) in the minus strain 

 being selected to propagate and continue their respective strains. 



At certain periods during the course of these experiments the 

 rearing of the various strains was rendered difficult by poor food 

 conditions. Sometimes during such periods selections were not 

 made on the basis of reaction to light, but the individuals used 

 to continue the strains were taken at random from the mother's 

 bottle. In the tables giving the data by broods, such cases (and a 

 few others in which for some reason the selection test was not con- 

 ducted) are indicated as ''random distributions." 



Environmental Influences and Reaction-Time. 



The tables of data obtained in making the selections show that 

 there were wide differences in the general reactiveness of different 

 broods tested in making the selections (see tables 3 and 4). Some- 

 times a brood had an unusually low or an unusually high mean re- 

 action-time, when perhaps the immediately preceding and next 

 succeeding broods of the same strain had a mean near that for the 

 strain as a whole. Sometimes a considerable number of a single brood 

 reacted negatively, although on the whole only a small percentage of 

 the individuals reacted negatively. Occasionally it was noted that 

 all the broods on a certain day responded slowly to light stimulation, 

 while perhaps on the following day all responded promptly. These 

 differences are due to environmental factors. Table 2 illustrates 

 this point, as well as the spasmodic occurrence of negatively reacting 

 individuals. Reference to this table shows that on August 29, 1913, 

 the mean reaction-time of 87 individuals of Line 695 tested was 

 401 seconds, and 3.5 per cent reacted negatively; on the following 

 day the mean reaction-time was 636 seconds and 1.6 per cent were 

 negative; on the day following this the mean reaction-time was 

 398 seconds and 28 per cent were negative. Thus, on the second 

 of these three days the individuals of the same strains were slower in 

 their reactions, compared with the other two days, by approximately 

 60 per cent. And there were twice and 18 times as large percentages 

 of negatively reacting individuals on the first and third as on the 

 second of these days.^ 



^ This is an extreme case, but it illustrates differences such as were repeatedly observed, 

 though usually to a much less marked degree. 



