Hybrid Vigor 167 



Having considered the salient facts of heterozygosis, 

 a statement by East on the "value of heterozygosis in 

 evolution" is pertinent. 



It can hardly be doubted that heterozygosis does aid in the 

 development of the mechanisms whereby flowers are cross- 

 fertilized. Variations must have appeared that favored cross- 

 fertilization. Those plants producing a cross-fertilized progeny 

 would have had more vigor than their self-fertilized relatives. 

 The crossing mechanism could then have become homozygous and 

 fixed, while the advantage due to cross-fertilization continued. 

 But was this new mechanism an advantage ? It must have been 

 often an advantage to the species as a whole. In competition 

 with other species the general vigor of those which were cross- 

 fertilized would aid in their survival. But the mechanism may 

 not have been useful in evolving real vigor in the species, because 

 of the survival of weak strains in combination. In self-fertilized 

 species, new characters that weakened the individual would have 

 been immediately eliminated. Only strains that stood by them- 

 selves, that survived on their own merits, would have been 

 retained. On the other hand, weak genotypes in cross-fertilized 

 species were retained through the vigor that they exhibited when 

 crossed with other genotypes. The result is, therefore, that self- 

 fertilized strains that have survived competition are inherently 

 stronger than cross-fertilized strains. On this account weak geno- 

 types may often be isolated from a cross-fertilized species that as 

 a whole is strong and hardy. 



Some recent investigations have furnished striking 

 confirmation of the theory of heterozygosis. The work 

 was done originally by Collins and Kempton (i), and 

 later confirmed and extended by Jones (3). In brief it 

 is as follows. If corn sporophytes exhibit heterozygosis, 

 will the endosperm also show the same phenomenon? 

 Endosperms, as has been stated, are genetically equiva- 

 lent to sporophytes in several ways. If crossing increases 



