356 



Theoretically the maintenance of superior near-homozygous strains 

 and annual crossing of the best for production of Fi 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 vvould 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 FI 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, 



