dent satisfaction that two of Roemer's popu- 

 lations are historically traceable to individual 

 selections and that they are therefore really 

 " pure lines " (i. e., providing, of course, that 

 those originally selected individuals were 

 strictly homozygous, and that no chance cross- 

 ing has taken place since). If no genotypic 

 changes can take place within the " pure line " 

 all evolution is impossible in the large num- 

 ber of forms which naturally maintain " pure 

 lines " by obligate self-fertilization. The only 

 point to be made in regard to this feature of 

 Roemer's results, is that, if his populations 

 were really "pure lines," the numerous dis- 

 tinct biotypes he discovered by the "pure 

 line" method in those populations, were the 

 result of mutations which have taken place 

 since the original selections were made. The 

 occurrence of such mutations does not affect 

 in the least the value of the genotype theory, 

 nor the importance of the " pure line " method 

 for the study of heredity. 



Definitions: 



Genotype, the fundamental hereditary con- 

 stitution or combination of genes of an or- 

 ganism. 



Biotype, a group of individuals possessing 

 the same genotype. 



Pure line, a group of individuals traceable 

 through solely self -fertilized lines to a single 

 homozygous ancestor. 



Clone, a group of individuals uf "iilm uuy> 

 iljipiu "tmiatituljitfiifr traceable through asexual 



reproductions to a single ancestral zygote, or 

 else perpetually asexual. 



GEO. H. SHULL 





