396 HEREDITY AND ENVIRONMENT 



One of the striking results of modern work 

 in plant-breeding has been the discovery of the 

 greatly increased vigor of certain hybrids as 

 compared with either pure-bred parent. In 

 general it is not possible to tell without pre- 

 vious experience what the character of the 

 hybrid of two races or "lines" will be; some- 

 times it is more and sometimes less vigorous 

 than either parent, but not infrequently it is 

 more vigorous. East and Shull have shown 

 that hybrids between two races of corn may 

 be very much larger and more fertile than 

 either parent. In some instances the yield of 

 corn per acre has been increased from 20-30 

 bushels to 80-90 bushels, and in one case to 

 more than 250 bushels per acre (Figs. 92, 93) . 

 Unfortunately such hybrid races of corn do 

 not continue to breed true and the crossing 

 must be made anew in each generation if 

 maximum results are to be had. Nevertheless 

 this method of hybridization or "heterozygo- 

 sis," as it has been called, offers an extremely 

 important means of quickly producing very 

 vigorous and fruitful individuals, but not lines 

 or races which breed true. 



