40 SELECTION IN CLADOCERA ON THE BASIS OF 



The year, August 1914-July 1915, gives as an average for the 

 plus strain 368.7 seconds (478 individual records), 60 seconds greater 

 than that for the minus strain (475 individual records), a difference 

 5.62 times the statistical probable error. For the first 10 months the 

 plus strain consistently showed a higher reaction-time. For the 6 

 same-day broods the plus strain had a higher reaction-time by 2.3 

 seconds. A test series (May 1915, table 7) showed a higher reaction- 

 time for the minus strain, but the difference was only —1.1 ±6,73. 

 Another test series (June 1915) gave a higher reaction-time by 

 51.1 ±14.08 seconds for the plus strain. This difference was 3.63 

 times the probable error. 



During the remainder of the experiment with Line 695 (August 

 1915-September 1916), the minus strain fluctuated rather widely 

 in mean reaction- time (figure 2c). For the year ending July 31, 

 1916 (399 and 439 individual reaction-time records), the minus 

 strain had the higher reaction-time by 15.6 ±11.77 seconds. The 

 same-day broods gave a difference of —6.9 ±19.61 seconds. The 

 remaining short period of the experiment, a trifle more than a month, 

 shows a considerably higher reaction-time (higher by 150.8 seconds) 

 for the plus strain. 



If the experiment with Line 695 had been discontinued in 

 January 1913, one might have felt inclined to ascribe a possible effect 

 of selection to the strains of this line (see figure 2c). But it is 

 extremely improbable that such an effect really should be thus 

 ascribed for that period and that the selective effect once obtained 

 was later lost in some manner. If one were to interpret this as an 

 effect of selection acquired through a mutation or acquired in some 

 other manner, it seems quite difficult to account for its loss. The 

 selections were not relaxed and there was no period of high mortality 

 among the stock to account for the loss of a selective difference. In 

 two portions of the experiment the plus strain consistently had a 

 higher reaction-time for 8 months or more at a time. Considering 

 the curves as a whole, there are three periods in which one or the 

 other strain had a higher reaction-time for as long as 6 months at a 

 time. In two of these periods the plus strain had the higher re- 

 action-time and in one of the minus had the higher reaction-time 

 A further examination of these data will be made in order to seek 

 the explanation. 



Relation Between Reproductive Vigor and Reactiveness to Light. 



It would seem plausible to suppose that relatively shght differ- 

 ences in the general vigor of the two strains might be a factor in 

 determining the reaction-time, particularly in such periods as those 

 just referred to. 



The rate of descent of a strain may be considered one measure 

 of its general vigor. Figure 2a presents the relative rates of descent 

 of the two strains of Line 695; the base-line represents equality of 



