106 SELECTION IN CLADOCERA ON THE BASIS OF 



method of checking up the data (table 47). This is a test of vital 

 significance. The number of over-time individuals is extremely 

 significant of the reactiveness of a strain. For the 5 longer periods 

 of the experiment the figures for the plus strain are 247, 302, 216, 

 143, and 37 over-time individuals; for the minus strain 227, 320, 334, 

 376, and 151 individuals. These can be compared better on a per- 

 centage basis. The percentages of over-time individuals for the 

 plus strain for the 5 longer periods of the experiment are 77.2, 59.9, 

 45.8, 28.3, and 15.1 per cent; for the minus strain 87, 77.7, 65.9, 

 77.4, and 57.2 per cent. Starting near equality (though the plus 

 strain almost from the start had significantly fewer over-time indi- 

 viduals), the differences are 9.8, 17.8, 20.1, 49.1, and 42.1 per cent. 



Hence the percentage of over-time individuals in the plus strain 

 decreased throughout the experiment and particularly during the 

 last 21 months of the experiment. The final percentage was less than 

 one-fifth that for the earliest longer period. For the minus strain 

 there was also a decrease, but it was less consistent, and in the end 

 the percentage of over-time individuals was two-thirds as large as 

 at the beginning (see table 47). 



It is possibly worth while to consider the mean-reaction times 

 of the more reactive individuals of the two strains of Line 757, 

 omitting from consideration for a moment the over-time individuals. 

 While this is a point of interest, the fact remains that the relative 

 number of over-time individuals is in itself as good a measure of 

 reactiveness as might be desired. The large influence of the arbitrary 

 reaction-times of 900 seconds, assumed for the over-time indi- 

 viduals, does not ascribe too much influence in determining the mean 

 reaction-time. In point of fact, the reverse is true. This arbitrary 

 method of assuming 900 seconds as the reaction-time for the over- 

 time individuals minimizes the rightful influence upon mean re- 

 action-time of these relatively non-reactive individuals. The data 

 are given in table 47 and show that there is a progressive reduction 

 in the mean reaction-time for the more reactive individuals of both 

 strains, considered by the longer periods of the experiment. This 

 reduction for the minus strain is from 431 to 376, 334, 326, and 260 

 seconds; but for the plus strain the reduction is larger, from 410 to 

 360, 356, 284, and 209 seconds. The differences between the means 

 for the two strains, with some irregularity, became larger, the plus 

 strain having the lower reaction-time. These differences were as 

 follows: -21, -16, +22, -43, and -51 seconds. The reduction 

 in mean reaction-time for the plus strain in the final period as com- 

 pared with the first period of the experiment was 201 seconds; for 

 the minus strain, 171 seconds. The differences for the last two 

 longer periods of the experiment have statistical significance, being 

 respectively 2.97 and 4.2 times their probable errors. 



