CORK BREEDING 35' 
SELECTION WITHIN SELFED LINES 
Selection within self-fertilized lines, or selfed lines, as they may 
be called for brevity, is a principle rather than a method of breeding. 
It consists essentially in (1) the isolation by self-fertilization and 
selection of lines that breed true, more or less, for certain characters, 
(2) the determination of which lines are more productive, and (3) 
the utilization of such selfed lines commercially in various ways. 
Selection within selfed lines has developed into the present basis for 
corn improvement primarily as a result of advances in the science 
of genetics. Earlier efforts had shown that mass selection and ear- 
to-row selection were effective in increasing adaptation and yield 
within limits. It also had been shown that close breeding tended to 
result in decreased productiveness, whereas cross breeding frequently 
resulted in larger yields. The theory of Mendelian inheritance, 
which already has been considered, explained why the progress un- 
der mass selection and ear-to-row selection was limited. Of more 
importance, it suggested a sounder basis for improvement. 
The first suggestion for utilizing selection within selfed lines in 
practical corn breeding was made by Shull in 1908 and 1909 (68, 69), 
the following being quoted from the summary of his 1909 paper : 
The process [the pure-line method] may be considered under two heads : 
(1) Finding the best pure lines; and (2) The practical use of the pure lines in 
the production of seed corn. 
(1) In finding the best pure lines it will be necessary to make as many self- 
fertilizations as practicable and to continue these year after year until the 
homozygous state is nearly or quite attained. Then all possible crosses are to 
be made among these different pure strains and the Fi plants coming from 
each such cross are to be grown in the form of an ear-to-the-row test, each row 
being the product of a different cross. These cross-bred rows are then studied 
as to yield and the possession of other desirable qualities. One combination will 
be best suited for one purpose, another for another purpose. Thus, if the 
self-fertilized strains be designated by the letters of the alphabet, it may be 
found that the cross CXH will give 120 bushels per acre of high-protein corn, 
that FXL produces a similar yield of low-protein corn, that KXC gives the 
highest oil-content accompanied by high yield, and so on. Moreover, it seems 
not improbable that different combinations may be found to give the best 
results in different localities and on different types of soils. The exchange of 
pure-bred strains among the various experiment stations greatly increases the 
number of different possible hybrid combinations and facilitates the finding of 
the best combination for each locality and condition. 
(2) After having found the right pair of pure strains for the attainment of 
any desired result in the way of yield and quality, the method of producing 
seed corn for the general crop is a very simple though somewhat costly process. 
{69, p. 57-58.) 
The poor quality and the high cost of the seed produced on the 
weak plants of the earlier selfed lines were obstacles to the practical 
utilization of the method. In spite of this, experiments on selection 
within selfed lines were undertaken by a number of investigators. 
The use of double crosses, or crosses between crosses, suggested by 
Jones (39) did much to obviate the objections. A little later Hayes 
and Garber (27) emphasized the possibility of utilizing synthetic 
varieties. Both of these suggestions apply to the utilization of selfed 
lines and will be considered in that connection. Reference should 
be made here, however, to the effect that the change in concept of 
the cause of hybrid vigor had on the theory of breeding by selection 
within selfed lines. 
