384 HEREDITY AND ENVIRONMENT 



of old characters, a few of which may be com- 

 mercially valuable, and sometimes actually 

 hew characters or mutations appear, possibly 

 as a result of the interaction of old characters, 

 or rather of their factors. 



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 



