564 STRUCTURE AND LIFE HISTORIES 
zygous for one or more characters, and the yield per acre 
may thus become greatly reduced. 1 If now, two of these 
simplifie^ 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- 
fertilized strains the yield per acre rose at once to 68.07 
bushels. In the F 2 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, Mendel's 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; 
1 If a high-yielding strain was separated out by selection, the yield 
would of course be increased above the average of the mixed field. 
