CAUSE OF HYBRID VIGOR 171 



of the parental varieties was medium in height, because 

 it possessed one of these factors ; e.g., that for thick stems, 

 but lacked the other. Such a plant had the formula Till. 

 The other variety was of medium height, because it lacked 

 this T factor, but possessed the factor for long internodes, 

 and was given the formula tt LL. Both of these factors 

 showed dominance over the allelomorphic condition; 

 hence, the first generation of the cross was taller than 

 either parent because both factors were present. Whether 

 or not later investigations have justified this precise in- 

 terpretation makes no material difference in the discussion 

 here. Taken as it stands, it is a beautiful instance of the 

 way in which complementary action of dominant factors 

 may increase a character in a first generation hybrid over 

 its expression in either parent. 



The investigators attempted to generalize from this 

 experiment and to apply a dominance interpretation to 

 the many other cases in which an increase in growth is 

 occasioned. As the matter stood at that time, however, 

 it was impossible to see why recombination of all the 

 dominant factors concerned in the increased growth of 

 the first generation could not readily be obtained, and 

 hence some individuals be produced having maximum 

 size and vigor, yet unaffected by inbreeding because 

 of their homozygous condition. In other words, in gen- 

 erations after the first it ought to be possible to obtain 

 some strains having all the dominant factors and others 

 with all these dominant factors lacking. Any such race 

 could be rendered homozygous ; thereafter, self-fertiliza- 

 tion would not result in a less vigorous progeny. And 

 while such results may have been obtained in the peas, 

 investigators have not been able to duplicate them in the 



