FISSION RATE OF STYLONYCHIA PUSTULATA 497 



Thus it is clear that the heritable difference in fission rate 

 brought about by selection during vegetative reproduction is not 

 lost when the animals conjugate, but persists through that ordeal. 



Statement of the general results of the third series of experiments. 

 This third series of experiments has entirely corroborated the 

 results of the first and second series. In a second clone, entirely 

 unrelated to that used for the first and second series of experi- 

 ments, opposite selection for thirty days produced a heritable 

 difference of average fission rate, a difference that gradually 

 increased as selection progressed, indicating again that the effect 

 of selection on this physiological character is cumulative. This 

 average difference persisted through twenty-one days of bal- 

 anced selection, twenty-nine days of mass culture followed by 

 conjugation and then fifteen days of further balanced selection. 



DISCUSSION AND CONCLUSIONS 



All the experiments thus give concordant results; through 

 selection of individual differences in fission rate it is possible to 

 divide a clone into two divisions differing hereditarily in rate 

 of multiplication. The effects of selection are cumulative; the 

 hereditary differences between the two divisions become greater 

 the longer selection continues. By reversing the direction of 

 selection the hereditary differences between the sets are reversed. 



Is this effect of selection due to the slow accumulation of 

 small variations, or to the chance isolation of mutants differing 

 markedly from the type? The whole character of the results 

 indicates strongly that the former is the case, and this indica- 

 tion is borne out by careful study of the records. There is no 

 sudden change at a definite point, indicating the appearance 

 of a mutant. The steady cumulative effect of continued selec- 

 tion can not be explained on the mutant theory without giving 

 such a meaning to the word mutant as removes any distinction 

 between it and 'slight individual variation.' It would require 

 us to assume the repeated appearance of successively faster and 

 faster mutants in each of the thirty fast selected lines, of suc- 

 cessively slower and slower mutants in each of the thirty slow- 

 selected lines; a conception which coincides with the view that 

 selection operates cumulatively on slight individual variations. 



