SELECTION WITHOUT ARTIFICIAL CEOSSING. 



43 



generation to generation is indicated by the space between the two 

 upper curved hues. It is seen that this type increases sHghtly until 

 the fourth generation, after which it gradually decreases. The five 

 types at the bottom of the figure decrease very rapidly from the sec- 

 ond generation on, so that by the tenth generation they have almost 

 disappeared, and the population is made up almost entirely of yellow, 

 starch-producing types which are either pure (type YYSS) or 

 heterozygote (types YYSs, YySS, and YySs) for one or both of the 

 characters with which we are dealing. Table VI and figure 3 thus 

 illustrate the effect of mass selection on crops which cross-fertilize, 

 and this effect is seen to be a gradual approach toward the type 

 selected. 



Evidently the limit of the effect of selection in a case of this Idnd 

 is reached when practically the whole crop is homozygote for the 

 character selected. No further advance can be made by selection, 

 and it must be remembered that in order to hold the crop at this 

 high degree of excellence selection must be continued, because here 

 and there plants will vary by certain characters becoming latent and 

 thus reducing the general average of superiority in the strain. 



The problem of improving cross-fertilized species or varieties 

 by selection to a particular type is complicated by the fact that 

 in plants which ordinarily cross-fertilize we are apt to lose vigor 

 when we get our plants too much alike. That is, these plants have 

 been used to cross-breeding, and when we get our population very 

 uniform, which is only another way of saying get them very close kin 

 to each other, the yield is liable to run down because of this very 

 uniformity. There is some question, therefore, whether in a cross- 

 fertilized crop we should attempt to get great uniformity. 



We can partially overcome this difhculty by each year selecting 

 the best individuals we can find and planting them in alternate 

 rows so as to get as much cross-fertilization as possible. In corn 

 the ear-row method of breeding permits this to be done. The 

 methods adopted by some of the best corn breeders in the country 

 at the present time are essentially as follows: Every year they go 

 through the seed plot and also through the cornfield and select the 

 best ears they can find to use in the seed plot the next year. As 

 many ears are selected as there will be rows in the seed plot. In 

 this way there will be the normal cross-fertilization in the seed plot 

 with the resulting vigor that comes from cross-fertilization. Careful 

 account is kept of the yield of each of the rows in the seed plot, 

 so that the breeder may learn what ears selected the year before 

 tend to produce the best yields. This enables him the next year 

 when he goes to the seed plot or the cornfield to select seed to know 

 what types to look for, and it is probably wise, from the standpoint 

 of yield, to select each year two or three types, if not more, pro- 



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