ii8 Heredity and Eugenics 



nations of the parental eharacters in the next generation. 

 And there is always the chance that new recombinations of 

 parental characters ma}- prove better fitted to survive than 

 the old combinations. In cross-bred species the stimulus 

 of h}-bridity holds a still greater advantage since even 

 homoz_\-gous strains that are weak and could never exist 

 alone, may, through combination with other strains, be 

 kept in existence as heterozygotes. For example, one finds 

 in maize literally thousands of genotj-pic strains in a single 

 commercial variety. Many could not exist alone, yet they 

 continue to exist in commercial varieties through hybridity 

 and their existence may be proven by their being partially 

 withdrawn by inbreeding. Such strains may have great 

 possibilities in certain combinations as is shown in Fig. 49. 

 In inbred or self- fertilized species such as tobacco, however, 

 strains weak in themselves perish and are lost to sight 

 because there is no probability of their being hybridized 

 and given a chance of showing their power in combination. 



This one phenomenon, alone, may account for the 

 commonness of cross-fertilized species and the rarity of self- 

 fertilized species, since it can be shown that there is no evU 

 eft'ect due to inbreeding per se. 



Passing now to the work more generally included in plant 

 breeding, we tind that commercial methods fall naturally 

 into two classes, hybridization and selection. They are 

 not really thus separable since one must use selection after 

 hybridization, but in the first category are classed all 

 cases where man produces hybrids artificialh'. The main 

 object of h}'bridization is the shuffling of unit-characters 

 in the first hybrid generation and their recombination and 

 fixation in succeeding generations. The object of selection 

 is to withdraw from mechanical mixtures or from physio- 



