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Indiana University Studies 



with respect to that character. This is my own behef, but 

 the difficulty arises in knowing when the strain is homozy- 

 gous, particularly when there may be so many modifying 

 factors which influence it. 



Since the formulation of the genotype concept by Johann- 

 sen ('03), a number of papers have given corroborative evi- 

 dence. Jennings ('08) isolated a number of pure lines from 

 a culture of Paramecium and was unable by selection within 

 these lines to produce anything new. He later concluded that 

 the only method of change within the genotype is by muta- 

 tion. Calkins and Gregory ('13), however, have found dif- 

 ferences within the four lines descended from the first four 

 cells resulting from the first two divisions following conjuga- 

 tion. They hold this to be direct evidence in favor of varia- 

 tion within pure lines. Jennings ('16) does not accept it as 

 such, primarily because the two divisions following conjuga- 

 tion are different from the others. They are the ones in 

 which the eight nuclei formed by the division of the micro- 

 nucleus after conjugation are redistributed. . Jennings him- 

 self, earlier in the same year, had shown that, following con- 

 jugation, inherited variations may occur within a single 

 stock. While the work of Hanel ('08) and Lashley ('15) on 

 Hydra might be more convincing, their conclusions support 

 the pure line theory. It is further supported by the work of 

 Woltereck ('09) upon Daphnia, of East ('10) upon the potato, 

 of Love ('10) upon peas, of Shull ('11) and East ('11) upon 

 maize, of Agar ('14) upon Simocephalus, of Ewing ('14 and 

 '16) upon Aphis, of Surface and Pearl ('15) upon oats, and 

 of others. 



In biparental forms we have the work of Pearl ('15, '16, 

 and '17) upon egg production in the domestic fowl, of Mac- 

 Dowell ('15 and '17) upon bristle inheritance in Drosophila, 

 of Zeleny and Mattoon ('15) and May ('17) upon bar eye 

 in Drosophila, and a brief abstract by Sturtevant ('16) on 

 the effect of selection in a mutant race, ''Dichaete", of Droso- 

 phila, and also a brief abstract by Morgan ('16) upon selec- 

 tion in another mutant, "Notch", of Drosophila. All these 

 experiments give positive evidence of selection. They were 

 interpreted, however, in favor of the pure line hypothesis by 

 assuming the presence of two or more factors which influence 

 the character subjected to selection. The effect of sele-ction 



