SELECTION 341 



sional mass selection from the field. Seed selection of this sort is of the 

 greatest practical value to agriculture and it is applicable to most sorts of 

 field and garden crops. 



The Practical Importance of Keeping Varieties Pure. Many farmers 

 do not regard purity of varieties as a matter of great concern, but con- 

 tinue to use impure seed from year to year. Since the main object of 

 breeding work is to produce new and better varieties, and since a true 

 variety differs definitely from all other varieties, it is of great impor- 

 tance that its purity and hence its identity be maintained. The need for 

 care in this regard is of course much greater in naturally cross-fertilized 

 species than in self-fertilized forms, yet even in the latter the mixing of 

 varieties may detract greatly from the market value of the crop. It is 

 not impossible for an impure variety or a mixture of varieties to give good 

 returns for a year or two or even longer. When one considers, however, 

 the rapidity with which the number of distinct strains may be increased 

 by occasional crossing the danger of such practice will be realized. For 

 this reason all agencies supervising the collection of stock seed of com- 

 mercial varieties of corn, sorghum, cotton, etc., should exercise every 

 possible precaution against mixing varieties or collecting seed that may 

 have been crossed with other varieties. As Newman points out, 

 however, there are certain circumstances under which the planting of 

 mixed sorts may have their advantages. Thus a variety may contain 

 strains which differ from each other for example chiefly in their response 

 to different soil conditions. Were a variety of such composition sown in a 

 field in which the soil is exceedingly variable it is possible that a better 

 average would be maintained than from an absolutely pure sort which 

 demands more exact conditions. Yet even here the practice marked 

 of careful mass selection in the field would doubtless result in marked 

 improvement. In general, however, the difficulty of knowing the real 

 nature of the strains which compose a mixed variety makes it unsafe 

 to depend upon the possible virtues of maintaining the most advantageous 

 mixture. Proved sorts of general adaptability offer much greater 

 promise. 



