166 INBREEDING AND OUTBREEDING 



given a special name. Atavism, or the reappearance of 

 previously existing characters, was immediately put upon 

 a Mendelian basis by hypothecating two separable factors 

 both of which were necessary for the production of the 

 end result. This was proved by the fact that later white 

 flowered plants were obtained which did not produce color 

 on crossing with either of the original white flowered va- 

 rieties and therefore lacked both factors. The light in- 

 creased. Some of the chaotic observations of the earlier 

 hybridizers began to be understood as orderly facts. 



Ten years after the rediscovery of Mendel's work a 

 symposium was held by the American Society of Natur- 

 alists on the "Genotype Hypothesis," an indication of the 

 growing importance of the ideas associated with the name 

 of Johannsen. The basis of the genotype conception is 

 that individuals which are visibly alike may be germi- 

 nally unlike merely an extension of the above Mendelian 

 concepts. Johannsen 's contribution was the idea that the 

 unit factors of Mendel are relatively constant and stable 

 in whatever combinations they occur, and that the vari- 

 ability of a constantly cross-fertilized population is 

 largely due to the segregation and recombination of these 

 unmodified factors. When such a heterogeneous popula- 

 tion is continuously self -fertilized, homozygosity is ulti- 

 mately attained, and much of the previous variability dis- 

 appears. Similar individuals making a homozygous, pure 

 breeding population, are known as a pure line, and while 

 they still vary as affected by different environmental con- 

 ditions, such variability does not respond to selection, and 

 the average condition is not changed. Although there is 

 still a question as to the degree of stability of the Mendel- 

 ian unit factor, as there is to the degree of stability of 



