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 recognized 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 ShulPs 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 



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