FISSION RATE OF STYLONYCHIA PUSTULATA 457 



tory culture of Stylonychia pustulata were isolated on ground 

 glass slides, one animal to each concavity, thus dividing the 

 sixty animals into two groups of thirty each; one group to be 

 selected for a fast fission rate ('plus selection'), the other for a 

 slow one ('minus selection'). For twenty-one days these thirty 

 'fast' and thirty 'slow' lines were propagated with frequent 

 selection as described above. On the twenty-second day the 

 fast and the slow lines were duplicated (by division) and on the 

 twenty-third day the first forty of the resulting sixty fast lines 

 and the corresponding slow lines were all duplicated so that now 

 we had one hundred fast lines and one hundred slow lines, plus 

 and minus selection continuing with all these. These were thus 

 propagated until the end of the third ten-day period. The 

 actual number of fissions per line and the actual number of selec- 

 tions that were made during the three ten-day periods are 

 shown for the first thirty lines of each set in juxtaposition in 

 table 1. 



When the differences per ten days of the sixty lines are aver- 

 aged it shows that on the average the 'fast' lines have pro- 

 duced 2.03 generations more than the average for the 'slow' 

 lines during the first ten-day period, 3.57 generations more in 

 the second ten-day period and 2.40 generations more in the third 

 ten-day period. Further, the fast lines have each averaged 2.67 

 generations more per ten days, during the whole thirty days, 

 than the slow lines. In other w^ords, this table shows that on 

 the average each of these thirty fast lines has produced 0.267 

 generation more per day during this thirty-day period than has 

 each slow line. Figure 2 is the curve of the difference between 

 the daily averages of the two sets. 



It is clear that the direct effect of the selections made would 

 be to produce a difference in favor of the 'fast' lines as long as 

 plus and minus selection continues, even though the differences 

 were not hereditary and were due purely to accidental causes. 

 Our next test is therefore to determine whether any hereditarj^ 

 result has been produced; to do this, selection must cease, and 

 we must determine whether the differences in rate still continue. 

 The most obvious method would be to stop selecting and pick 



