No. G37] FIRST GENERATION HYBRIDS 123 



bred ' varieties, are the result of recombination, the 

 breeder will be encouraged to expend the time and labor 

 necessary to eliminate them. If, on the other hand, these 

 new characters are the result of an unstable germ plasm 

 other means must be sought. 



Already the importance of deleterious recessive varia- 

 tions has found application in the breeding of maize. It 

 soon was realized that to successfully eliminate recessive 

 characters it is necessary to bring the characters into 

 expression by inbreeding. Once a strain has been freed 

 of undesirable characters, vigor may be restored by com- 

 bining the inbred lines or the full advantage of domi- 

 nance may be realized by growing first-generation hy- 

 brids of the better strains. 



This method of breeding will be relatively unsuccess- 

 ful if unfavorable mutations are of frequent occurrence. 

 It is perhaps too early to be sure that this is not the case, 

 but it is encouraging that in strains self-pollinated for 

 13 generations Jones tinds no conspicuous variations 

 after about the 7th generation. The next step is to 

 demonstrate that no unfavorable variations appear when 

 the selfed lines are crossed. This has been shown to be 

 the case in the first-generation, but a certain percentage 

 of multiple factor recessives are to be expected in sub- 

 sequent generations. 



If linkage is operative these recessive characters would 

 come to light slowly, much as they appear in successive 

 generations of an open-bred variety. As already pointed 

 out, there is as yet no way of distinguishing between 

 mutations and nuiltiplo factor characters, when the fac- 

 tors are linked. 



Nature of Degeneratiox That Follows Inbreeding. 



In discussing the nature and causes of the reduction 

 of vigor that follows inbreeding, it is necessary to choose 

 words with u'reat cave. To state the ([uestions at issue 

 ill such ;i w.iv ;is to distiiiuuish between differences of 

 fact and di tTri-ciiccs in tiic ii-*' and meaning of words 



