298 DONALD F. JONES 
infection by the smut fungus has taken place during the reduction to 
homozygosity accompanying the inbreeding process. 
Strain number 1-6-1-3, which is not at all affected, is the most 
vigorous of the four inbred lines derived from variety number i. 
It has a darker green color, the plants are larger and more productive 
than any of the other three. In table 3 the data on smut infection 
of the first and second generation hybrids of this most resistant strain 
with the most susceptible strain are given, together with the figures 
for the two parents and the original non-inbred variety from which 
the inbred strains were derived. All of these were grown in five 
adjacent rows in plot III. Owing to the poor germination of the seeds 
of one of the parents and the first generation of the cross, the numbers 
are too small to put much reliance upon. Smut was not shown by 
any plants of the immune parent and of the first generation hybrid, 
but appeared on some of the plants of the original variety, the sus- 
ceptible parent, and the second generation of the cross. 
The numbers given in table 3 represent all the plants that were 
grown of the original variety and the second generation hybrid. 
Of the first generation hybrid, 439 plants in all were grown in different 
parts of the field. Of these, 2.28 percent were affected, showing that 
the dominance of immunity is not perfect. When this figure is com- 
pared with the 9.79 percent of affected plants of one of the parents, 
it shows clearly that the hybrid approaches the condition of the 
immune parent. 
As in so many other cases, those factors which enable an organism 
to attain the best development tend to dominate. The facts given 
here are considered as additional support to the hypothesis advanced 
by the writer (9, 10) that the increase in development commonly 
shown by hybrids is due to the conjunctive action of a large number of 
favorable, dominant growth factors contributed by both parents. 
Resistance is not shown by all first generation hybrids. Biffen 
(11) finds that some first generation wheat crosses are susceptible to 
rust and Norton (12) that certain tomato hybrids are susceptible to a 
wilt disease. On the other hand Vavilov (13) reports wheat hybrids 
resistant to mildew and Stuckey (14), tomatoes which are resistant 
to blossom-end rot in the first generations. All these crosses were 
between types which differed in their susceptibility. Rasmuson (15) 
with reference to the grape Phylloxera; Van Fleet (16), the chestnut 
blight; Orton, the melon wilt; Blinn (17), the cantaloupe leaf blight; 
