CHAP. XIV INTERMEDIATES 159 



shown by the fact that the pure parental types 

 appear in a certain proportion of their offspring. 

 In such cases as these there is but a single type of 

 intermediate, and the simple ratio in which this 

 and the two homozygous forms appear renders the 

 interpretation obvious. But the nature of the F2 

 generation may be much more complex and, where 

 we are dealing with factors which interact upon one 

 another, may even present the appearance of a series 

 of intermediate forms, grading from the condition 

 found in one of the original parents to that which 

 occurred in the other. As an illustration we may 

 consider the cross between the Brown Leghorn and 

 Silky fowls which we have already dealt with in 

 connection with the inheritance of sex. The offspring 

 of a Silky hen mated with a Brown Leghorn are in 

 both .sexes birds with but a trace of the Silky 

 pigmentation. But when such birds are bred 

 together they produce a generation consisting of 

 chicks as deeply pigmented as the original Silky 

 parent, chicks devoid of pigment like the Brown 

 Leghorn, and chicks in which the pigmentation 

 shows itself in a variety of intermediate stages. 

 Indeed from a hundred chicks bred in this way it 

 would be possible to pick out a number of indi- 

 viduals and arrange them in an apparently continuous 

 series of gradually increasing pigmentation, with the 

 completely unpigmented at one end and the most 

 deeply pigmented at the other. Nevertheless, the case 

 is one in which complete segregation of the different 

 factors takes place, and the apparently continuous 

 series of intermediates is the result of the interaction 

 of the different factors upon one another. The con- 



