132 SELECTION IN CLADOCERA ON THE BASIS OF 



selections; but this effect was probably the same with all the indi- 

 viduals of the plus and minus strains and could not have interfered 

 with the choosing of the proper individuals or affected the experiment 

 as a whole, except that it added to the irregularity of the reaction- 

 time records. If, however, this disturbing influence did sometimes 

 prevent the making of the proper selections, that would only render 

 the selections less effective and would not confuse any results ob- 

 tained. 



4. Occurrence of Negatively Reacting Individuals. 



The data shown in table 2 also serve to illustrate the irregular 

 occurrence of negatively reacting individuals. There were 3 negative 

 ones of a total of 87 individuals, or 3.5 per cent, August 29; 3 out of 

 190, or 1.6 per cent, August 30; and 14 of 50, or 28 per cent, August 31. 

 While these numbers are not large, the widely differing percentages 

 indicate very well the relatively spasmodic occurrence of negatively 

 reacting individuals on August 31 (see also page 15). There is great 

 irregularity in the occurrence of negatively reacting individuals in 

 general, so that while the illustration taken from table 2 is somehwat 

 extreme, it is not unique. 1 



These illustrations, (1) to (4), bring to notice relatively tempo- 

 rary disturbing factors which are a matter of concern, but for which 

 no way of complete elimination was found; yet they do not serve to 

 discount the selection data as a whole. This is clear for two reasons : 

 (1) these factors, which in the long run presumably must have affected 

 the plus and minus strains in a similar way, could not often have 

 interfered with making the proper selections and could contribute 

 only to the fluctuations in reaction- time ; (2) the consistent and con- 

 vincing results with Line 757 could not have come about if these dis- 

 turbing factors had seriously interfered with the course of the selec- 

 tion experiment as a whole. 



5. General Influences Operative Through Longer Periods of Time. 

 The general relation between mean reaction-time and environ- 

 mental conditions is obvious in the curves plotting mean reaction- 

 times by two-month periods. This relation is seen in that reaction- 

 time curves for the plus and minus strains of the same line in a general 

 way move upward and downward together to some extent and at 

 times the nearly coincidental courses of the curves for the two strains 

 are very striking. This was not due to any large extent to tempera- 

 ture differences in the water used in the experimental tank, although 

 temperature differences in this water were sometimes a factor (see 

 pages 128-130). Temperature influences fail to explain many of the 

 broad fluctuations of the reaction-time curves, e. g., the high levels 

 obtaining for the curves for D. pulex (figure 10d) during December 



x It chances in this case that the lowest percentage of negatively reacting individuals falls 

 upon the day of highest mean reaction-time. This is a mere coincidence. 



