No. 637] FIRST GENERATION HYBRIDS 127 



which the Fj was measurably larger than the mid- 

 parental value are length and diameter of ear and length 

 of internode. With respect to length of ear (Tables XIII- 

 XV), there are 13 F2 progenies that may be compared 

 with the Fj. In 10 the mean was higher than the mean of 

 Fj. Four F3 progenies were grow from F2 individuals 

 above the mean of F^ and in 3 of these the mean exceeded 

 the mean of Fj. 



With respect to diameter of ear (Tables XVIII and 

 XIX), 8 F2 progenies may be compared with the Fj. In 

 2 of the 8 instances it would appear that the mean was 

 above that of F^. Seven of the 18 F3 progenies were 

 grown from F2 individuals above the mean of F^ and in 

 every case the Fg mean exceeded the mean of F,. 



Length of internode is the character showing the most 

 decided increase in F^ over the mid-parental value. In 

 the two crosses reported (Tables XXXIII and XXXIV) 

 this increase was 33 and 27 per cent. None of the F^ 

 progenies gro^\^l the same season as the F^ equalled the 

 Fj. The mean of the Fj, however, was exceeded by the 

 mean of 5 of the F3 progenies grown the following season, 

 although the parents were not selected for internode 

 length. The results with these characters give little or 

 no evidence of non-heritable vigor in the Fj, neither is 

 there any proof that it is diflicult to select progenies 

 with the vigor of F^. 



Expectation or Obtaining Fo Peogenies with the 

 Vigor or Fj 



In the absence of experimental data, to secure which 

 will require very extensive experiments extending over 

 many years, it may be instructive to consider what re- 

 sults may be expected if suppression of semi-lethal char- 

 acters is the true explanation of heterosis. 



The difficulty of obtaining individuals homozygous for 

 all or even a limited number of characters has been fre- 

 quently pointed out, but the bearing of this on the diffi- 

 culty of retaining the vigor of the first generation seems 

 not to have been appreciated. 



