168 INBKEEDING AND OUTBKEEDING 



But all strains in maize are so greatly reduced by in- 

 breeding that none can be compared in productiveness to 

 the normal cross-pollinated plants. Something besides 

 ordinary segregation must be involved in this well-nigh 

 universal effect of inbreeding. 



It was apparent that when germinal heterogeneity 

 was at the maximum the greatest vigor was shown. When 

 this heterogeneity was reduced by inbreeding, vigor was 

 lost. Hence, the fundamental fact that hybrid vigor 

 varies directly with heterozygosity was clearly estab- 

 lished. To account for the greater vigor and increased 

 development of hybrids, it was only necessary to postu- 

 late that a developmental stimulation was evolved when 

 different germ plasms were united. This hypothesis 

 (East and Hayes, 58 Shull 196 ) satisfied all the essential 

 facts, and for the first time the effects of inbreeding and 

 cross breeding were clearly understood in their true rela- 

 tion to each other. Inbreeding was not a process of con- 

 tinuous degeneration; it was a process of Mendelian 

 segregation, and its effect was directly related to the 

 number and type of characters existing originally in a 

 heterozygous condition. If unfavorable characters were 

 covered up by favorable characters, inbreeding brought 

 them out whenever a simplification of the germ plasm al- 

 lowed them to appear. Inbreeding was in effect the isola- 

 tion of homozygous hereditary complexes from an hetero- 

 zygous hereditary complex. If the best of these combina- 

 tions failed to attain the development of the original 

 stock, it was thought to be because they were deprived of 

 a stimulus which only accompanied heterozygosity and 

 which seemingly was impossible to fix. 



This hypothesis, by associating all the facts of inbreed- 



