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SCIENCE PROGRESS 



dividing specimens in the high line and rapidly-dividing speci- 

 mens in the low line were selected generation after generation. 

 This type of selection was also found to be effective, and the 

 original high line became the slower one, and the original low 

 line became the faster. The difference in fission rate was 

 here likewise found to be heritable, since it persisted when 

 the specimens were cultivated without selection. In order 

 to be certain that heritable diversities in fission rate were not 

 restricted to the descendants of one parent, but were character- 

 istic of the species in general, another " wild " specimen was 

 studied, and similar selections were practised on its descen- 



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dants. The results in every way resembled those already 

 described. By selection, high and low lines, with respect 

 to fission rate, were obtained in which the difference gradually 

 increased as selection progressed, and this difference was shown 

 to be heritable, since it persisted through fifty days of non- 

 selection, followed by conjugation, and then through fifteen 

 days more of non-selection. The final conclusions are that in 

 Stylonychia pustulata heritably diverse lines may be derived 

 by selection from a single specimen dividing by fission ; that 

 these new lines (genotypes) have resulted from the accumu- 

 lation of small heritable variations ; and that the selection of 

 such variations is effective for evolution in this species. 



