126 SELECTION IN CLADOCERA ON THE BASIS OF 



differences were 77, 126, 99, 170, and 230 seconds. The difference 

 for the middle period is influenced by the exceptionally low means 

 for the Line 757 minus strain during 2 two-month periods. Other- 

 wise the divergence consistently increased. 



When the minus strain of 757 is compared with the shorter S. 

 exspinosus lines (Lines 794, 795, and 796) the divergence between the 

 minus strain of Line 757 and the minus strains of other lines which 

 may serve as checks is even more marked (table 49 and figure 18b). 

 The minus strain of Line 757 had a higher average than the combined 

 averages for the minus strains of Lines 794, 795, 796 in 14 of 15 two- 

 month periods, the differences ranging from 51 to 319 seconds and 

 all but two of them being from 4 to 13 times their probable errors. 

 The one exceptional difference was a difference of only 1.1 seconds. 



Conclusion Regarding Effect op Selection in Line 757. 



It is seen, then, in Line 757, that by all the tests apphed and in 

 spite of all the safeguards used in connection with these data, a 

 marked effect of selection is indicated. This is shown by the course 

 of the curves plotting the mean reaction-times of the two strains; by 

 a progressively increased (though somewhat irregular and once inter- 

 rupted) difference in mean reaction-time between the two strains, 

 the plus strain having the lower reaction-time; by a decrease to 94 

 seconds in the average minimum reaction-time of the plus strain as 

 compared with a decrease to 245 seconds in the minus strain; by a 

 decrease of approximately one-third in the average maximum reac- 

 tion-time in the plus strain,^ while at the same time the minus strain 

 decreased only 5 per cent; by a decrease of four-fifths in the number 

 of individuals of the plus strain which failed to respond to the light, 

 while the decrease in this number in the minus strain was only one- 

 third; by the results of the two test series applied to the strains of 

 this line; by the data for the same-day broods; by every statistical 

 test which was apphed to the data; by a comparison with similarly 

 treated strains of other lines of the same species; and, to the eyes 

 of the observer, most of all by the greatly changed behavior in the 

 experimental tank of the plus strain as compared with that of the 

 minus strain. 



There seems no escape from the conclusion that in Line 757 

 there is a marked effect of selection. This effect appeared in the 

 early period of the experiment and with some fluctuation increased 

 throughout its duration. The form of the curves would seem to 

 indicate that the effect of selection was continuous and was still 



' Tliis decrease in the maximum reaction-time means more than a decrease of one-third 

 BugKcsts. At the beginninR of selection with Line 757 some broods were loft in the experimental 

 tank for an hour or more, and those individuals which did not respond to the li<;ht within the 

 period allotted for the experiment (15 minutes) often did not move at all within the much longer 

 period. It seems probable that the maximum reaction-times for this early period misht readily 

 have been near '.ifiOO, or even 10,800 seconds in many cases, if each of the selection tests had 

 been continucfl for an hour or 3 hours. 



