564 STRUCTURE AND LIFE HISTORIES 



zygous for one or more characters, and the yield per acre 

 may thus become greatly reduced.^ If now, two of these 

 simplified strains, homozygous for many characters, and 

 having a low yield per acre, are crossed, there results an 

 Fi hybrid progeny that is heterozygous for all of these 

 characters. This heterozygosity is correlated with a 

 greatly increased vigor; the plants are much larger, and 

 the yield per acre is enormously increased (Fig. 409). 

 Thus in one experiment of this kind the average yield of 

 the heterozygous horticultural variety was 61.25 bushels 

 per acre. After self-fertilization for several generations 

 the yield became reduced to 29.04 bushels per acre; but 

 in the Fi generation of a cross between two of these self- 

 fertiUzed strains the yield per acre rose at once to 68.07 

 bushels. In the F2 generation the yield again fell to 44.62 

 bushels. From this, and numerous other experiments, it 

 is found that the biggest corn crop is to be obtained by 

 finding the strains that will produce the largest yield 

 when crossed, and thus using for seed the grains of the 

 first-generation hybrids each year. 



486. Unsolved Problems. — ^Like all truly great con- 

 tributions to science, MendeFs discoveries have raised 

 more questions than they have answered. Therein lies, 

 in part, their great value. So, also, the most important 

 effect of Darwin's work was that it set men to asking 

 questions. The history of botany, as of all natural science 

 since 1859, is chiefly the attempts of men to answer the 

 questions raised by Darwin, or stimulated in their own 

 minds by his books. So with Mendel and de Vries; 



^ If a high-yielding strain was separated out by selection, the yield 

 would of course be increased above the average of the mixed field. 



