v INTERACTION OF FACTORS 49 



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 wit- 

 nessed 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 white- 

 ness 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 Fj coloured birds when bred 

 together would produce an F 2 generation consisting of 

 coloured and white birds in the ratio 9:7, and when the 

 experiment was made this was actually shown to be the 

 case. With the growth of knowledge it is probable that 

 further striking parallels of this nature between the plant 

 and animal worlds will be met with. 



Before quitting the subject of these experiments atten- 

 tion may be drawn to the fact that the 9 : 7 ratio is in 

 reality a 9:3:3:1 ratio in which the last three terms 

 are indistinguishable owing to the special circumstances 

 that neither factor can produce a visible effect without 



