CORN BREEDING 25 
in obtaining productiveness, but only one tool. It is of value in 
obtaining larger absolute yields only when the sum total of the 
characters expressed in the cross make it better adapted to the en- 
vironment and therefore higher yielding than varieties already 
available. 
PRACTICE OF CORN BREEDING 
Corn breeding has been defined as a systematic effort to improve 
the crop by controlling the parentage of the seed. The control of 
the parentage is exercised in practice by selection, and the different 
systems of breeding for increased yield will be considered, according 
to the methods of selection followed, under the headings of mass 
selection, ear-to-row selection, hybridization, and selection within 
selfed lines. Corn breeding usually has as its objects an increase in 
the acre yield of marketable corn. The discussion, therefore, will 
be concerned primarily with attempts to increase the yield by selec- 
tion for general vigor, productiveness, and quality. Breeding for 
resistance to some specific disease or condition is but a special 
application. 4 
MASS SELECTION 
Mass selection consists in picking out certain individuals from the 
main crop and planting the selected seed en masse. With corn, selec- 
tion may be on the basis of the ear only, or on the basis of the plant 
and the ear. In the former case the ears most nearly approaching the 
ideal in mind are chosen without reference to the plants on which they 
grew. In the latter case selection takes place in the field, and primary 
importance is attached to the character of the plant on which the 
selected ear is produced. 
Because of the large size of the seed units — the ears — some degree 
of mass selection has been practiced by corn growers since the earliest 
times. Indeed, it is probable that long before the coming of the 
white man to America the Indians had selected their seed ears on the 
basis of some character or other. It generally is conceded that this 
selection has been of the utmost importance in improving corn and 
adapting it to the varying conditions under which it is grown. There 
also is ample evidence that varieties may still be adapted to new 
environments or may be modified in this or that character by mass 
selection. 
Because of the cross-fertilization that regularly occurs in corn, 
modification by mass selection is a shifting of the average rather than 
a true fixation of type. Even the most intensively selected varieties 
of corn are only mixtures of hybrids which, within limits, can be 
modified practically at will by selection. It is for this reason that 
varietal names mean practically nothing in corn. 
SELECTION FOR EAR TYPE 
The ideas of the earlier corn breeders differed widely as to what 
constituted the best type of ear. This is shown by such varieties as 
4 Much of the earlier literature on the results of experimental corn breeding was 
reviewed by the writer in a previous publication {62). More recent publications which 
should be mentioned and to which specific reference is not made in the text are 13, Ik, 2k, 
20, 29, J,6, 1ft, 59, 65, under " Literature cited," p. 59. 
36919°— 27 1 
