Ilyhy'id l^ii^or 171 



acter is umlcsinil^lc il is cliniiiiatt'd, Un- tin- iii<li\i(lual 

 or species that carries it is elimiiialed. This would 

 obviously apply particularly lo the <l(>niinanl characlers, 

 for undesirable recessives might well sur\ i\ l' hv escapinj^ 

 natural selection while in heterozygous combination with 

 their dominant allelomorphs. It follows that the domi- 

 nant unit characters that have survived and a])pear in 

 the plants of today are for the most part desirable ones. 



The question may be raised as to what constitutes a 

 ''desirable'' character. It may be any one of a number 

 of things, but is there not some feature which is common 

 to all desirable characters ? The common feature of all 

 desirable characters would seem to arise from their rela- 

 tion to the vigor of the organism. Each desirable char- 

 acter must add somewhat to the vigor of the plant that 

 contains it, and associated with vigor are such things as 

 size and productiveness. Is it not reasonable that those 

 plants will be most vigorous which have in combination 

 the greatest number of desirable characters ? 1'he 

 plants which have the greatest combination of such 

 characters are the hybrids. 



A diagram similar to that which was used to explain 

 heterozygosis may be considered: 



Parents Fi 



A A BBCCDD X A A BBCCdd = A A BBCCDd = 1 it Uc hybric 1 vigor 

 AABBCCDDXAAbhccdd = A A BbCcDd = si'iW more hybrid vigor 



In that explanation it was stated that the lirst case 

 showed little hybrid vigor because it had only one hetero- 

 zygous set {Dd), while the other case showed more 

 hybrid vigor because it had three such heterozygous sets. 

 Hybrid vigor, therefore, appeared in proportion to the 



