IV PRESENCE & ABSENCE THEORY 37 



experiment was to prove the absence of the factor 

 for singleness in the Breda., On crossing Breda 

 with single the F^ birds exhibit a lai-ge comb of 

 the form of a double single comb in which the two 

 portions are united anteriorly, but diverge from one 

 another towards the back of the head (Fig. 6, C). 

 The Breda contains an element of duplicity which 

 is dominant to the simplicity of the ordinary single 

 comb. But it cannot contain the factor for the 

 single comb, because as soon as that is put into it 

 by crossing with a single the comb assumes a large 



Rose X Breda 



Duplex ^y Duplex 



Rose 



Rose 



1 1 



Duplex Rose Duplex Single Breda 

 Rose ' Single (Duplex 



and Simplex) 



size, and is totally distinct in appearance from its 

 almost complete absence in the pure Breda. Now 

 when the Breda is crossed with the rose duplicity 

 is dominant -to simplicity, and rose is dominant to 

 lack of comb, and the Fj generation consists of birds 

 possessing duplex rose combs (Fig. 6, A and B); 

 On breeding such birds together we obtain a genera- 

 tion consisting of Bredas, duplex roses, roses, duplex 

 singles, and singles. From our previous experiment 

 we know that the singles could not have come from 

 the Breda, since a Breda comb to which the factor for 

 single has been added no longer remains a Breda. 



