FISSION RATE OF STYLONYCHIA PUSTULATA 475 



se\'enty- three to one hundred and seventeen and for the slow 

 set from forty-eight to ninety-six with the exception of one Hne 

 which produced one hundred and four generations. The first 

 ten hnes of both sets each produced a much larger number of 

 generations than the remainder of the lines of its own set. These 

 lines were the occupants of a single moist chamber, so that 

 there was probably some environmental difference distinguishing 

 this moist chamber from the other two. That environmental 

 differences have a marked effect on the fission rate of infusoria 

 has of course been shown by many writers. 



Figure 9-a gives the curves of variation in number of genera- 

 tions produced by each fast and each slow line during balanced 

 selection. The curve for the slow lines shows an apparent 

 bimodality. To determine whether this might be attributable 

 to the environmental difference suggested by table 7 these same 

 variation curves were plotted for the first ten lines of each set 

 alone. These curves are shown as figure 9-6; the curves are 

 mutually exclusive. Table 7 shows that only one slow line 

 among these ten produced more generations than the slowest 

 one of these first ten fast hnes. Figure 9-c gives the curves of 

 variation for fast and slow lines 11 to 30, the occupants of the 

 other two moist chambers. Only four of these twenty slow lines 

 produced nure generations during the ninety days of balanced 

 selection than the slowest fast line. 



It must be borne in mind in this connection that these ''two 

 sets of thirty lines" are parts of the same clone. That is, we have 

 got here, from a single genotype by selection two genotypes that 

 differ characteristically from each other under identical condi- 

 tions; and that retain these differences from generation tp 

 generation. 



Experiment 1, part 4. Further opposite selection, January 23 

 to March 14, 1914. 



While Experiment 1-B was in progress the original lines were 

 continued under direct selection for two reasons: 1) As a pre- 

 caution against the possible failure of the difference of fission 



