THE PURE LINE AND SELECTION 133 



arising from a single female individual without the 

 customary maturation of the germ-cells which accom- 

 panies sexual reproduction, constitute a pure line or an 

 unmixed strain because as in clones there has been no 

 segregation nor addition of outside germplasm. 



Third, in homozygous crosses when two organisms 

 identical in their germinal determiners inbreed, their 

 progeny will form a pure line just as truly as two 

 parents that are united in a single hermaphroditic in- 

 dividual produce a pure line progeny as the result of 

 self-fertilization. 



In the case of clones and parthenogenesis it should be 

 pointed out that the "pure line" is assured only so 

 long as asexual reproduction continues. It is quite 

 possible for an organism, even heterozygotic in composi- 

 tion, to continue to breed true or to produce an ap- 

 parently pure line so long as asexual methods are em- 

 ployed. As soon as such an organism, however, changes 

 to the sexual method of reproduction, segregation of 

 characters may occur and different combinations result. 

 A pure line, therefore, implies freedom from admixture 

 of different germplasm rather than any necessary 

 equality or likeness of individuals. 



The different kinds of "pure lines" are diagram- 

 matically represented in Figure 27. 



6. SELECTION WITHIN A PURE LINE 



The basic idea of the pure line concept is that every 

 member of any pure line is genetically identical with 

 every other member of the same fraternity, therefore, 



