SELECTION 34] 
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. 
Digitized by Microsoft® 
