80 SELECTION IN CLADOCERA ON THE BASIS OF 



4 occurred in succession in April 1917 and constituted the final data 

 for this line. The writer believes that considerable stress may be 

 safely placed upon the same-day-brood data and is inclined to think 

 that at the close of the experiment with Line 795 the plus strain was 

 actually and significantly the more reactive. 



It is quite possible to suppose that the considerably higher 

 reaction-time of the plus strain during the early part of this experi- 

 ment (whatever may have been the cause and meaning) was gradually 

 reduced by selection and that at the close of the experiment the plus 

 strain was actually the more reactive, due to the influence of selec- 

 tion. But in the absence of a test series at the close of the experiment, 

 and without knowledge as to what the relative reaction-time means 

 for the two strains would have been in later generations, with or 

 without further selection, the supposition is not worth consideration. 



It is not at all improbable, however, that the differences in 

 different parts of the curves for the two strains represent merely low 

 and high points in the reactiveness of the two strains due* to non- 

 genetic influences, the plus strain being relatively slightly reactive 

 and the minus strain unusually reactive during the early period, while 

 the reverse was true during the later part of the experiment. Such 

 fluctuations in the reactiveness of the two strains of the same line 

 for considerable periods are seen in several cases, notably in Line 

 695 (figure 2c) and Line 740 (figure 15). 



In this line from June-July 1915 to April-May 1916 there 

 was a general rise in the reproductive index for the plus strain and 

 a general fall in the reproductive index for the minus strain. The 

 result was a wide divergence in reproductive indices for the two 

 strains culminating in April-May 1916, when the reproductive index 

 for the plus strain was nearly twice that for the minus strain 

 (figure 12a). This may be thought a result of selection acting in a 

 cumulative way upon the vigor of the 2 strains, but there was no 

 relaxation in or change in the method of selection, and yet the minus 

 strain later reached as high a point in reproductive index as it had 

 attained at any earlier time (though in general it remained lower 

 than the plus strain). 



While the reproductive indices indicate (figure 12 a) that the 

 plus strain was in general very much the more vigorous of the two strains, 

 one fails to find (as always) any direct relation between reproductive 

 index and reaction-time. The generally much higher reproductive 

 index for the plus strain, associated as it is during most of the experi- 

 ment with a considerably higher reaction-time mean for this strain, 

 in itself serves as a general denial of an association in this line between 

 vigor and reactiveness to light. 



Line 796. 

 This was a sister line to Lines 794 and 795. Selection was begun 

 at the same time and continued for the same period as with those 



