INHERITED SEED-CHARACTERS 69 



of characters dealt with by Mendel, and that of the 

 colour of the Andalusian fowl and the human eye, 

 possess in common, i.e. that in which the genealogical 

 table of the tall and dwarf peas and that of the 

 Andalusian fowl agree, namely the orderly reappear- 

 ance of the characters of the parents of the 

 hybrid, and of the hybrid itself, in the second 

 hybrid generation in definite numerical propor- 

 tions. 



There is another characteristic of the Mendelian 

 phenomenon which can only be said to be an essential 

 one in the sense that it follows from Mendel's theory* 

 to account for the composition of the second hybrid- 

 generation. This characteristic is the repetition, 

 in each successive hybrid generation, of the ratios 

 obtaining in the second. The second, and all 

 subsequent hybrid generations, should all have 

 exactly the same composition as one another, if 

 this characteristic is found to be an invariable feature 

 of Mendelian crosses. Let it be clearly understood 

 what this means. It means that, for however many 

 generations hybrid yellow peas are bred from, they 

 will produce pure yellows and hybrid yellows and 

 pure greens in the ratio of 25 : 50 : 25 per cent. What 

 is most remarkable in this generalisation, because 

 most at variance with current notions of heredity, 

 is the idea that although the parent of each successive 

 hybrid generation is as yellow as a yellow of a pure 

 strain, there will be produced an average number 

 of green, namely, 25 per cent., which will not 



* See Chapter XL 



