249 THE AMEEICAN NATURALIST [VOL. XLV 



crosses show, are attributable to this lack of purity, 

 rather than to any advantage gained by crossing per se. 



The columns of the table representing the Fj and F 2 

 show very plainly the superiority of the former over the 

 latter in regard to both height and yield per acre. The 

 fall in average height from Fj to F 2 from 25 decimeters 

 to 23.4 decimeters and the corresponding fall in yield 

 per acre from 68.07 bushels in the Fj to 44.62 bushels per 

 acre in the F 2 show in a most striking way the economic 

 advantage of using first-generation hybrids for produc- 

 ing the corn crop. A comparison of the Fj hybrids with 

 the ' ' cross-breds ' ' shows the average yield of the former 

 to be 6.55 bushels per acre greater than that in the fami- 

 lies in which self-fertilization had been avoided. 



The relation of these results to the experiences of eco- 

 nomic breeders of corn may now be considered. Perhaps 

 in no other class of plants has the evidence been so strong 

 for the possibility of gradual improvement through con- 

 tinued selection as in corn, and this method has been 

 generally followed. The selections of particular physical 

 and chemical qualities which have been carried on at 

 various experiment stations have produced noteworthy 

 results. Most important instances of this kind are in- 

 volved in the breeding experiences of Hopkins, Smith and 

 other breeders at the Illinois State Experiment Station, 

 which have been already mentioned. Here selections for 

 high oil content, low oil content, high protein and low 

 protein, high ears and low ears, and the angle which the 

 ears make with the axis of the plant, as well as selection 

 for increased yields, have all led to the production of 

 strains which possessed the desired qualities to a much 

 higher degree than that in which they existed in the foun- 

 dation stock when the selection began. All of these re- 

 sults may be readily explained on the ground that some 

 hybrid combinations of genotypes have greater capacity 

 for the production of the desired qualities than other com- 

 binations, and that the selection has gradually brought 

 about the segregation of those genotype-combinations 



