38 



SELECTION IN CLADOCERA ON THE BASIS OF 



of this mean; its probable error; the difference between the mean for 

 the plus strain and that for the minus strain; the statistical probable 

 error of this difference ; and the quotient of this difference divided by 

 its statistical probable error. For each period considered in this 

 table the corresponding data for the plus and minus strains are given 

 in successive horizontal columns, the plus first and the minus in 

 italics immediately below it. Thus the items for making the signifi- 

 cant comparisons between the plus and minus strains are found 

 together. 



Table 8 gives a summary of the data for the "same-day" broods, 

 i. e., those broods of the mothers of the corresponding plus and minus 

 strains which reproduced on the same days and the young of which, 

 consequently, were tested under identical conditions. The arrange- 

 ment of the data is similar to that for table 7. 



Table 8. — Same-day broods. Summary of data for Line 695. 



Figure 2c shows in diagrammatic form the mean reaction-times 

 of the plus and minus strains of Line 695 by two-month periods. 

 The distances from the axis of ordinates represent two-month periods 

 of the selection data. The ordinates indicate the average reaction- 

 times in seconds for the two-month periods. The solid line represents 

 the course of the averages for the plus strain, the broken line for the 

 minus strain. The roman numerals indicate the times at which the 

 test series were conducted, and the plus and minus signs within the 

 diagram indicate the positions of the means for these test series. 



Analysis of Data for Effect of Selection. 



On the whole, there is a marked coincidence of the fluctuations 

 of the two strains, the means for the plus and minus strains in a 

 general way following the same upward and downward trends. This 



