356 GENETICS IN RELATION TO AGRICULTURE 
Theoretically the maintenance of superior near-homozygous strains 
and annual crossing of the best for production of F; seed for sale to growers 
is a practicable method of corn breeding. This plan was first suggested 
by Shull. It is certainly a desirable method not only because of the high 
degree of heterozygosity produced on crossing such strains, but because 
continuous inbreeding has a similar effect to growing the plants under 
adverse conditions. It tends to eliminate all but the strongest individuals 
and is thus an effective method of selection. However, as a more prac- 
ticable method, East suggested that combinations of the various com- 
mercial varieties be tested until the most profitable combination is 
found. There has been considerable investigation of both methods, but it 
is impossible at present to say which will be used more extensively. One 
of the most valuable features of this method of inbreeding followed by 
crossing of superior strains, as compared with ordinary ear-to-row selection, 
is the saving in time. For example, in the production of high-yielding 
strains of corn which differ in chemical composition of the grains, 
Emerson and East point out that ear-to-row selection from open pol- 
linated plants will, if carried on long enough, produce a strain of the 
desired type. It will be sufficiently homozygous to insure comparative 
constancy as regards oil, protein or starch content. At the same time 
a sufficient number of factors for other minor characters will be hetero- 
zygous to insure a fairly vigorous strain. But, on the other hand, 
by self-pollination, together with the same sort of selection, several 
practically homozygous strains of the desired type, high oil for instance, 
“could almost surely have been produced in much less time.’’ These 
strains would doubtless have been unlike for many other characters, so 
that if degree of vigor is dependent upon degree of heterozygosity, the 
crosses between them would doubtless have been abundantly vigorous. 
Or if physiological vigor is conditioned by specific factors, then crosses 
between some of the selected strains would doubtless have effected the 
most favorable combinations for maximum vigor. In either case the 
result is the same. ‘While a few years’ time may not be an important 
consideration where the character in question can be determined at 
sight, or by mere weighing or measuring, in breeding work requiring 
costly chemical analysis it is extremely important that the desired re- 
sults be obtained in as few years and, therefore, with as few analyses as 
possible.” 
Method of Comparing Yields.—The importance of accuracy and fair- 
ness in comparing the yields of #; hybrids with their parents has been 
determined by Collins. We give his conclusions verbatim: 
“So large a proportion of first-generation maize hybrids have been found to 
give increased yields and the increase is frequently of such magnitude that the 
utilization of this factor of productiveness becomes a practical question. It is, 
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