126 MEXDELISM chap. 



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 F 2 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 inheri- 

 tance 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 inter- 

 mediate 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 



