140 SELECTION IN CLADOCERA ON THE BASIS OF 



INDEPENDENT SHIFTS IN REACTION-TIME MEANS. 



Throughout this experiment every effort was made to treat alike 

 all the lines, and particularly the two strains of a line, as regards food, 

 temperature, light, method of handling stock, making of transfers, 

 etc. However, the two strains of the same line did not usually 

 reproduce on the same day, and consequently transfers were generally 

 made into culture-water of different collections from the outdoor 

 pond. Such variations as occurred in food and chemical content of 

 the culture-water, and temperature, light, etc., in the laboratory, 

 impinged upon different periods of the life of the new generations. 

 Such unavoidable differential treatment can scarcely be conceived to 

 be without influence upon the physiology of these organisms. 1 



In spite of the somewhat differential treatment of the two strains 

 of a line, there has been seen in general a fairly close correlation 

 between the reaction-time means for the two strains of most of the 

 lines throughout their entire history. In so far as there are differ- 

 ences in reactiveness between the two strains of the same line, where 

 they are temporary and hence are presumably non-genetic, they are 

 believed to be due to this unavoidable differential environmental 

 treatment. 



Pronounced independent shifts in the reaction-time curves cover- 

 ing considerable periods for the two strains of the same line are seen in 

 several cases. Three such shifts relative to each other are seen in the 

 reaction-time curves for the two strains of Line 695: (1) September 

 1912 to January 1913, in which the minus strain is consistently higher 

 than the plus strain and higher than the composite curve (figure lOo) 

 for the minus D. pulex strains for this period; (2) September 1914 to 

 May 1915, in which the minus strain is consistently, though only 

 slightly, lower than the plus strain and lower than the composite 

 curve for the period; (3) February-September 1916, during which the 

 two strains differed consistently by a widening margin due to the 

 plus curve running sharply upward. None of these three periods 

 of consistent differences in reactiveness for periods of 6 months or 

 longer are considered to have genetic significance (see pages 44-45). 



Two shifts are seen in the curves for Line 691, September 1912- 

 January 1913, during which the plus curve runs lower than the minus 

 curve and lower than the composite curve (figure lOo), and June- 

 September 1913, during which the minus curve runs abnormally high. 

 This last shift may have genetic significance, since the minus curve 

 tends to run slightly higher than the Line 691 plus and the composite 

 curves during the remainder of the experiment, but its genetic sig- 

 nificance is extremely doubtful. 



In Line 713 the plus-strain curve is particularly fluctuating, 

 there being 3 main periods of differences in reaction-time : December 



1 It is obvious that temperature and character of the food affect the reproductive index. 



