120 



INBEEEDING AND OUTBREEDING 



between distinct self -fertilized lines should often result 

 in high-yielding F^ generations possessing great vigor 

 and showing a high degree of uniformity. Again, crosses 

 between different near-homozygous strains, though uni- 

 form and vigorous in the Fj generations, should become 

 much more variable and much less vigorous in the F2 

 generation. These general propositions Shull tested in 

 a limited way in 1910 after his families had been self-fer- 

 tilized for ^ve generations. The variability of two such 

 strains and the crosses between them for a definite and 

 easily determined character — number of rows per ear — 

 is shown in the following table : 



Clearly the F^ generation, made with either type as the 

 mother, is as uniform as the parent strains, but the F2 

 generations are both more variable. 



To test the other corollaries, nine different self-fer- 

 tilized families of the fifth generation were compared 

 with families obtained by crossing two plants belonging to 

 each family; seven families were raised as first-genera- 

 tion hybrids between these different selfed strains; ten 

 crosses between F^ individuals were compared with ten 

 self-fertilizations in the same families ; and ten families 

 were grown in which self-fertilization had been precluded 

 for five years. The average height in decimeters, number 



