V INTERACTION OF FACTORS 45 



From the evidence afforded by this exhaustive 

 set of experiments it is impossible to resist the 

 deduction that the appearance of colour in the sweet- 

 pea depends upon the interaction of two factors 

 which are independently transmitted according to 

 the ordinary scheme of Mendelian inheritance. 

 What these factors are is still an open question. 

 Recent evidence of a chemical nature indicates that 

 colour in a flower is due to the interaction of two 

 definitive substances : (i) a colourless " chromogen," 

 or colour basis ; and (2) a ferment which behaves as 

 an activator of the chromogen, and by inducing 

 some process of oxidation, leads to the formation of 

 a coloured substance. But whether these two bodies 

 exist as such in the gametes, or whether in some 

 other form we have as yet no means of deciding. 



Since the elucidation of the nature of colour in 

 the sweet -pea phenomena of a similar kind have 

 been witnessed in other plants, notably in stocks, 

 snapdragons, and orchids. Nor is this class of 

 phenomena confined to plants. In the course of a 

 series of experiments upon the plumage colour 

 of poultry, indications were obtained that different 

 white breeds did not always owe their whiteness to 

 the same cause. Crosses were accordingly made 

 between the white Silky fowl and a pure white _ 

 strain derived from the white Dorking. Each of 

 these had been previously shown to behave as a 

 simple recessive to colour. When the two were 

 crossed only fully coloured birds resulted. From 

 analogy with the case of the sweet -pea it was 

 anticipated that such F^ coloured birds when bred 

 together would produce an F^ generation consisting 



