DIFFICULT CASES 355 



" In certain plants we can go beyond this. Two white-flowered 

 sweet peas, for instance, may when crossed give a coloured F^, 

 which by self-fertilisation will produce nine coloured, seven white. 

 This result proves that the colour depends for its appearance on 

 the coexistence of two complementary elements or factors." 



" The nine contain both complementary factors C and R ; 

 the seven are three with C, three with R, and one with neither. 

 Either factor alone is insufficient to cause colour. C and R 

 are not allelomorphic to each other, but each is allelomorphic 

 to its own absence." This conclusion has been tested and 

 confirmed by an elaborate series of experiments made by Mr, 

 Bateson, Mr. Punnett, and Miss Saunders. 



" A further complication is due to the fact that colour, once 

 formed by the meeting of the two complementary factors, is 

 modified by the operations of distinct determining elements, 

 just as is that of a rabbit. Thus, for instance, the nine 

 coloured : seven white commonly forms the series twenty- 

 seven purple : nine red : twenty-eight white." 



" In the garden stock, which has formed the subject of a long 

 series of experiments by Miss Saunders, a stiU further complica- 

 tion is met with." 



" Colour in the stock, as in the sweet pea, requires the co- 

 existence of two independent factors, each of which in the germ 

 formation is allelomorphic to its own absence. In addition, 

 the development of the hoariness or felting of hairs upon the 

 leaves is also produced by another similar pair of factors, either 

 of which alone may be present without a single hair being formed. 

 But in the stocks employed (' ten-week stocks ') both these 

 factors for hoariness may be present, but no hairs are developed 

 unless the flowers are coloured, that is to say, unless the com- 

 plementary pair which form pigment are also present." 



We have ventured on this long quotation (i) because this 

 book is meant to be a balance-sheet of facts and theories, and 

 no ex parte statement ; (2) because the increased subtlety of 



