SELECTION WITHIN PURE LINES OF PESTALOZZIA 181 



suggestion has any validity, it follows that the two groups, while apparently 

 different, were not significantly different, even while selection was being 

 practiced. 



If two lines are isolated within a strain and cultivated separately these 

 lines will be independently subjected to the environmental influences 

 which cause variation for the character studied. The lines will tend to 

 coincide if they are subjected to these influences at the same time and to 

 the same degree. This is the case with the strain of Festal ozzia studied in 

 the author's experiment 2 and is shown in figure 7. It is apparent that 

 there is a certain rhythm, either in the organism itself or in the environ- 

 mental factors, which causes upward and downward swings of the organism 

 in respect to the character under observation. Since great care was taken 

 to have the two groups subject to identical conditions at all times the two 

 groups follow one another very closely in these swings. In experiment 1 

 the swings follow each other less closely but with some exceptions there is a 

 strong similarity of behavior in the two lines as is shown in figure 5. In 

 MAST'S work with Didynium, where two groups have been separated, 

 apparently by a mutation, these groups fluctuate in the same direction 

 at the same time, with remarkable unison. 



If two lines are isolated and cultivated separately, but the offspring are 

 produced in such a way that those of the two lines are subjected to the 

 environmental factors causing variation for a given character, at a different 

 time, or in a different degree, each of the lines will fluctuate in a different 

 way, and this will cause divergence between the two. However if the 

 lines are continued the fluctuations will compensate one another so that 

 in the end the two lines will be found to give practically the same mean 

 value for the character studied. 



To the writer it appears that JENNINGS'S results with Difflugia, ROOT'S 

 with Centropyxis, and HEGNER'S with Arcella, are explained by the above 

 assumption quite as well as are his own with Pestalozzia. If this be true 

 they tend rather to demonstrate the ineffectiveness of selection within 

 pure lines than the opposite. 



It is warranted to express the belief that the "pure-line" hypothesis is 

 still valid. The writer does not wish to express an opinion that ultimately 

 the effectiveness of selection may not be shown, but he feels that the bur- 

 den of proof lies with those who question the results of JOHANNSEN and 

 his followers. At the present time incontestable evidence of an effect of 

 selection is wanting. From, the recent investigations made in this field 

 it is obvious that the situation is vastly more complicated than was 

 formerly supposed. The truth or falsity of the "pure-line" hypothesis 



GENETICS 7: Mr 1922 



