36 APPLICATION OF PEINCIPLES OF HEREDITY TO BREEDING. 



In these homozygote forms, which constitute the major part of a 

 tield of any crop which habitually self-fertilizes, there is little, if any, 

 more variation than in plants which are propagated vegetatively. 



CROSS-FERTILIZED SPECIES. 



The eifect of individual selection on cross-fertilized species, such 

 as corn, is very different from what it is in self -fertilized species. 

 Here the plants chosen are more or less cross-fertilized with other 

 plants and the seeds obtained from a single plant are not all alike 

 in content of hereditary characters. Hence we may get distinct 

 differences in the individuals grown from this seed. Either mass 

 selection or individual selection in a crop of this character may make 

 decided changes in it for the reason that, in the seed of every plant, 

 combinations of hereditary characters will occur that are unlike 

 those in the original plants selected. Some of these may be superior 

 to the original plants. For instance, the plant with which we start 

 may be heterozygote with reference to a particular character which 

 we will call "AJ'' That is, it inherited from one of its parents the 

 presence of this character and from the other its absence. Its 

 formula with reference to this character would therefore be Aa. 

 Such a plant will produce progeny one-fourth of which has the 

 formula AA, one-half Aa, and one-fourth aa. Now, the combina- 

 tion AA may be superior to Aa and aa, so that in the seed of our 

 selection we may get something better than the plant selected. On 

 the other hand, we may also get something not so good. Selection 

 alone, therefore, enables us to make positive improvements in crops 

 which regularly cross-fertilize. 



The work done on the corn plant at the Illinois Agricultural Ex- 

 periment Station is perhaps the best illustration of the effect of 

 selection on crops that cross-fertilize. Bulletin 128 of that station 

 gives the results of ten years' selection of corn for high and low oil 

 content and for high and low protein content. Some of these results 

 are given in Table IV. 



Table IV. — Effect of selection in a cross-fertilized species. The figures of column 2 

 give differences in percentage content of oil between two strains of corn of similar 

 origin, one selected for high and one for low oil content. Column 3 gives similar differ- 

 ences between two other strains selected, one for high and the other for low protein content. 



Years. 



Oil 

 differences. 



Protein 

 differences. 





Per cent. 



Per cent. 



1896 



0. 00 



0. 00 



1897. 



.67 



.55 



1898 



1.16 



.50 



1899 



1.82 



1.60 



1900 



2. 55 



2.98 



1901 



2. 6() 



4.08 



1902 



3. 39 



4.12 



1903 



3. 53 



4.52 



1904 



4.08 



5. 70 



1905 



4.71 



6.15 



1906 



4.71 



5.62 



1 0.^) 



