CHAPTER XVIII 
SELECTION 
The oldest and most generally used means of plant improvement 
must continue to be the basic method in systematic plant breeding. 
Although selection is universally fecognized as an effective method of 
breeding, yet all too long the prevailing ideas among empirical breeders 
regarding the way in which selection effects improvement and the reasons 
why selection sometimes fails in securing the end desired have been exceed- 
ingly vague. The confusion of thought concerning this matter which 
still exists among both scientists and laymen is largely due to a lack of 
clear understanding concerning the nature of variation. The variations 
upon which selection can be used effectively owe their origin either to 
mutations or to recombinations of genetic factors. On account of the 
differences in the composition of populations in various species of plants 
the effects of selection differ greatly in different crops. In order to employ 
selection most economically the plant breeder should understand the 
nature of the population with which he is working and the genetic prin- 
ciples underlying effective selection. It is our purpose in this chapter to 
set forth the principles of selection in both allogamous and autogamous 
species. 
Selection Methods in Maize Breeding.—The maize plant is highly 
variable and many different varieties and strains have been produced by 
selection. In most of the states where corn is grown extensively the 
experiment stations have published bulletins on corn improvement and 
the subject is discussed in more or less detail in various works on plant 
breeding. We shall merely consider here certain methods of maize 
selection in order to illustrate the principles involved and to compare 
them with methods used in other crop plants. 
Inbreeding in Maize.—Self-fertilization in maize results in marked 
reduction in vigor and hence in size of plant and production of seed. This 
was first discovered by Shull, who applied the pure line method in corn 
breeding, and from his results inferred that a field of maize consists of a 
collection of genetically distinct biotypes which may be isolated by in- 
breeding. East soon corroborated Shull’s discovery and later East and 
Hayes summarized the results of inbreeding a naturally cross-fertilized 
plant substantially as follows: 
1. There is partial loss of power of development, causing reduction in 
325 
Digitized by Microsoft® 
