GENETICS 



distinctions alone have in them a large source of error 

 which must be taken into account. 



In a museum of heredity, should such a collection 

 ever be assembled, the specimens would not be arranged 

 phenotypically as they are in an ordinary museum 

 where things that look alike are placed together as if 

 in bonds of relationship, but they would be arranged 

 historically from a genetic point of view to show their 

 true origin one from another. 



5. CASES SIMILAR TO JOHANNSEN'S PURE LINES 



Although, according to Johannsen, pure lines are 

 "the progeny of a single self-fertilized individual," it 

 is plain that in at least three other possible cases some- 

 thing quite similar to "pure lines" may be obtained. 



These are clones, partheno genetic progeny and homo- 

 zygous crosses. "In principle pure lines, partheno- 

 genetic reproduction and vegetative propagation are 

 concerned with nearly the same situation" (Morgan). 



First, in asexual reproduction where the progeny 

 are simply the result of continued fission of the original 

 individual, a pure line may be said to continue from 

 generation to generation because it is a germinally un- 

 changed sequence of individuals. Such an asexual 

 progeny is termed a clone (Webber). Shull's definition 

 of a clone is "a group of individuals of like genotypic 

 constitution, traceable through asexual reproductions 

 to a single ancestral zygote, or else perpetually 

 asexual." 



Second, in cases of parthenogenesis, the progeny 



