FISSION RATE OF STYLONYCHIA PUSTULATA 487 



generations produced by both sets, the difference was 6.9 per 

 cent in part 1; 12.8 per cent in part 2; 19.3 per cent in part 3, 

 and 21.2 per cent in part 4. Table 12 summarizes the genera- 

 tions produced by each hne of each set throughout all parts of 

 the experiment. It shows that for the fast lines the number 

 of generations ranges from 178 to 187, while for the slow lines 

 the range is but from 116 to 128. Thus there is no overlapping 

 in the two sets; the slowest fast selected line has produced 50 

 more generations than the fastest slow selected line. 



To determine whether the difference in fission rate thus pro- 

 duced is heritable, parts of the two sets were removed at inter- 

 vals and subjected to culture without selection ('balanced selec- 

 tion'). In every case it was found that the difference was herit- 

 able. Also, representatives of the two sets after eighty days 

 of selection and forty days of no selection, were subjected to 

 mass culture for twelve days. Further line culture for fifty days 

 without selection showed that the inherited difference in fission 

 rate still persisted. Thus the inherited difference produced by 

 selection had lasted for one hundred and two days without 

 selection. 



Experiments with reversed selection showed that the inherited 

 difference could be reversed as readily as it is produced; the 

 originally fast set was thus caused to become the slower one, 

 and vice versa. Continuation of these sets without selection 

 showed again that the difference so produced was heritable. 



Thus in this case the selection of small individual variations 

 in fission rate has split the single clone (derived vegetatively 

 from a single parent) into two hereditarily diverse divisions 

 (diverse clones). 



2. The second series of experi7nents 



The results of the first series of experiments appeared so 

 important and in some respects unexpected that it was felt 

 necessary to control them by repetition, beginning again with 

 a single individual, and endeavoring anew to procure from it 

 by selection two hereditarily diverse sets. This was first at- 



