64 HEREDITY [ch. 



of the organism. The process is comparable with 

 a chemical reaction, where one element may replace 

 another in a compound : for example, by mixing silver 

 nitrate with sodium chloride, silver chloride and 

 sodium nitrate are produced. Or a grosser analogy 

 may be taken from bricks in a wall ; a red brick may 

 be removed and replaced by a blue or a yellow one 

 without altering the rest of the wall, and similarly in 

 pea-plants by the process described white flowers 

 may be replaced by purple, or yellow seed by green. 

 After the fact of the segregation of allelomorphic 

 characters in the production of the germ-cells of 

 a heterozygote, the most striking result of Mendelian 

 investigation is this discovery of the independence of 

 characters belonging to different pairs. 



That these results are not of merely academic 

 interest is shown by the work of Prof. Biffen on 

 wheat. Some valuable wheats are liable to the 

 attacks of a fungus giving rise to the disease called 

 'rust,' other less valuable races are immune. Biffen 

 has found that by crossing the two races together, 

 fertilising the hybrids (jP x ) among themselves, and 

 selecting the homozygous plants in the F. 2 generation, 

 wheat can be produced which combines the valuable 

 features of one race with the immunity to rust of the 

 other, and so a new and most useful variety of wheat 

 is produced. This is only one out of many examples 

 that could be given of the possibility of combining 



