SELECTION IN PLANT BREEDING 



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by this method of selection even with a crop that is often cross-fertilized. 

 But the diagrams show other facts. The published records show that 

 the variability of the race was but little, if any, reduced by continuous 

 selection. With extreme variants comparatively as far removed from 

 each year's type, available for planting in each successive generation, 

 the gain each year should have been at the same rate, if the Darwinian 

 interpretation of the role of selection were correct. On the contrary, 

 we notice that the regular curve fitted to the crop averages for ten gen- 

 erations, is first concave showing great progress made by selection, is 

 later convex as progress becomes slower, and last becomes horizontal 



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Fig. 4. Diagrammatic Representation of the Results of the Illinois Agri- 

 cultural Experiment Station in selecting for high and for low oil content. Y, per 

 cent, oil in crop ; X, generations ; h, high oil strain ; I, low oil strain. 



as no more progress results. It is very evident that the original stock 

 was a mixed race containing sub-races of various composition inter- 

 mingled by hybridization. Selection rapidly isolated these sub-races. 

 The isolation was practically complete at the eighth generation in the 

 case of the protein strains and the ninth generation in the oil strains. 

 After this selection accomplished nothing. That the effect of selection 

 was simply the isolation of a sub-race and not a continuous response, 

 is further demonstrated by the fact that in 1903 another plot was 

 started with seed from the isolated high oil strain. After four years' 

 cessation of selection, the average composition of the crop remained the 

 same, showing that after complete isolation of a homogeneous type no 

 retrogression of the selected character occurs unless intercrossing with 

 mediocre strains takes place. Fluctuation in composition still appears, 

 but this is the non-inherited kind produced by external conditions. 



