SELECTION WITHIN PURE LINES OF PESTALOZZIA 175 



positive result. The Pestalozzia investigation has greater evidential 

 value. It was carried on for 10, 25, 10, and 6 generations, respectively, 

 in the several experiments. The characters studied were such as to admit 

 of certainty as to maturity of the observed individuals, and they were not 

 more subject to environmental influence than those considered in investi- 

 gations which have seemed to give evidence of an effect of selection. In 

 certain respects, which have already been pointed out, the different genera- 

 tions have been more fully known, and a larger number of variates have 

 been considered, than in any other investigation yet reported. Neverthe- 

 less, the different experiments consistently fail to show any effect of selec- 

 tion. The evidence secured in this study then is a direct confirmation of 

 the results of JOHANNSEN and the other early workers on the pure-line 

 problem. In attempting an explanation of the discrepancy between 

 these results and those which contradict them, it is necessary to attempt 

 to evaluate the experiments that have shown a positive selection effect. 



TABLE 10 

 Means of generations measured in experiment 4. Measurements in JJL. 



Difference between plus-selected line and minus-selected line = 0.366 + 0.337/J. 



The results of STOCKING (1915), while sufficiently striking and credible, 

 can be given little weight in a general consideration since the abnormal 

 characters dealt with so strongly suggest a pathological condition of the 

 organism. To be given weight in a discussion of selection as a factor in 

 evolution, the supposed modification produced by selection must affect a 

 normal character in a direction shown by existing forms in nature to have 

 been actually traversed by these other forms in the course of their evolu- 

 tion. 



MIDDLETON (1915) found selection for fission rate in Stylonychia effec- 

 tive to a degree which must have been surprising, even to believers in the 

 possibility of modification by selection. The great readiness with which 

 organisms could be altered in MIDDLE-TON'S experiments indicates that the 

 response of Stylonychia to selection is to some degree exceptional. ROOT 

 (1918) suggests that "the inheritance of variations in fission rate in Sty- 



GENETICS 7: Mr 1922 



