PARTHENOGENETIC PURE LINE OF APHIS AVEN.E FAB. 55 



will unfold themselves generation after generation within a 

 certain range of variability in constant conformity with an 

 ancestral type. This type Johannsen terms the genotype. It 

 is simply the type of the race. Theoretically, then, the germ- 

 plasm is as unchangeable as adamant, and selection with all its 

 rigors should in the end leave the race exactly as it was in regard 

 to its inheritable qualities. 



In recent years the term pure line has been applied by some 

 workers 1 to the pure strains of cross-fertilized plants or animals, 

 but incorrectly so, I believe. Johannsen did not consider it as 

 so applying; neither did Jennings, or many of the other workers 

 coming after them. Undoubtedly the theory as originally pro- 

 pounded was not construed as applying to cross-fertilized plants 

 or to sexually-reproducing animals. There is no such thing as 

 purity in Johannsen's sense where the melting-pot of amphimixis 

 alloys the germplasm of one individual with the germplasm of 

 another. In 1911 he stated that we had shown, "to excess that 

 phenotypes may seem totally ' pure ' and nevertheless be hetero- 

 geneous." He further remarks: "Thus constancy as to the phe- 

 notype of the progeny is no sure proof for genotypical purity or 

 unity." Indeed how else could it be if we are to assume the 

 frequent presence of multiple unit characters, and thus accept 

 the infinity-factor hypothesis now constantly put forward by 

 Mendelians to explain what appears to be blending in the case 

 of certain hybrid crosses. This distinction becomes doubly 

 important when we consider the confusion and possible causal 

 relation between heterozygosity expressing itself in segregation 

 of characters, even minute, and the appearance of the so-called 

 mutants. It is in relation to mutation, that the pure line ad- 

 vocates have, in the opinion of the present writer, need to be 

 alarmed at the application of the term pure line to a supposedly 

 homozygous pure strain of cross-fertilized organisms. 



THE EFFECTIVENESS OF SELECTION IN A PARTHENOGENETIC 



PURE LINE. 



In order to test the effectiveness of selection in a partheno- 

 genetic pure line several different sub-lines were bred in isolation. 



1 Pearl for example. See Pearl, 1915, " Seventeen Years Selection of a Character 

 Showing Sex-linked Mendelian Inheritance," Amer. Nat., Vol. XLIX., pp. 595-608. 



