232 BREEDING 



of the factors necessary for the production of purple. 

 It would have the formula ppbb. If mated with the 

 pure purple it would give heterozygous purples, 

 PpBb ; and if these were then self-fertilised and 

 set seed, they would produce purples, finks, and 

 white. Another possibility, and perhaps a more 

 probable one, is that the homozygous purple sud- 

 denly mutated to the heterozygous condition ; the 

 self-fertilised seed of such a plant would produce 

 whites, purples and pinks. In this case it is not 

 necessary to assume, j&rst, that a new form appeared 

 by mutation from the purple, and then was crossed 

 back with it. This question, however, of the origin 

 of Mendelian characters will be dealt with in Chapter 

 XVII. All that I am concerned with now is the idea 

 that the two factors necessary for the production of 

 purple in the power of Pisum did become separate 

 at some period, and one of them (P) lodged in a 

 pink-flowered strain, and the other (B) in a white- 

 flowered one, and that the reappearance of this 

 purple as the result of a cross is due to the re -union 

 of these two factors in the zygote resulting from a 

 cross between the two strains mentioned. 



