76 SELECTION IN CLADOCERA ON THE BASIS OF 



series shows effectively that there was up to July 1916 no certain 

 basis for assuming an effect of selection within this line. 



During the last longer period (August 1916-May 1917) of 

 selection with Line 794 the means were 309.9 and 373.8 seconds. The 

 difference ( 63. 9 15. 3 seconds) was 4.18 times its probable error. 

 The 2 same-day broods gave means differing by 134 49. 66 

 seconds. As for the preceding year, the means are suggestive of an 

 effect of selection, but the irregular course of the curves (figure lie), 

 together with the result of the test series conducted in July 1916 

 (table 33) and the much higher reaction-time for the plus strain 

 during 6 months of the experiment, make this interpretation ques- 

 tionable. It is noteworthy, however, that in spite of considerable 

 irregularities in the curves, the curve for the minus strain is appreci- 

 ably lower than that for the plus strain in only one of the 9 two- 

 month periods after the first 12 months of the experiment. After 

 this early period all the data except the test-series data indicate that 

 the minus strain was on the whole the less reactive throughout the 

 experiment. 



Since the mothers from which the two strains of Line 794 

 originated were sisters from the same brood, it is hard to understand to 

 what the difference in reactiveness (the minus strain being markedly 

 the more reactive) during the period of the experiment (June- 

 November 1915) can have been due. It is possible to think of it 

 as a mutation in the minus strain, the effect of which was later 

 eliminated by selection or by a second mutation in the same strain 

 in the direction of less reactiveness to light; but these are mere sur- 

 mises for which there is no real evidence and the differences them- 

 selves are not pronounced and certain enough to be considered as of 

 great significance. It is probable, however, that the rather wide 

 differences in reactiveness in the minus strain of this line at different 

 periods of the experiment are due to non-genetic factors, such as 

 produced similar effects in Lines 695 and 740 (see figures 2c and 15) 

 and elsewhere. Nevertheless, the data leave room for the suggestion 

 that selection was possibly responsible for the elimination of the 

 greater reactiveness on the part of the minus strain, however it 

 may have occurred, and a development of a relatively greater re- 

 activeness on the part of the plus strain. 



Figure HA indicates graphically the general reproductive levels 

 for the two strains during the different two-month periods. There 

 is no obvious relation between reproductive vigor and mean reaction- 

 time unless the generally lower reproductive vigor of the minus 

 strain be assumed to account for the generally higher reaction-time 

 of the minus as compared with the plus strain; but with Line 795 

 (see figure 12s) the plus strain in general showed an even greater 

 superiority in vigor, yet it was much less reactive than the minus 



