IV PRESENCE & ABSENCE THEORY 29 



than one of the two factors belonging to the alter- 

 native pair. On the other hand, Mendel supposed 

 that it always carried either one or the other of such 

 a pair. As experimental work proceeded, it soon 

 became clear that there were cases which could 

 not be expressed in terms of this conception. The 

 nature of the difficulty and the way in which it was 

 met will perhaps be best understood by considering 

 a set of experiments in which it occurred. Many 

 of the different breeds of poultry are characterised 

 by a particular form of comb, and in certain cases 

 the inheritance of these has been carefully worked 

 out. It was shown that the rose comb (Fig. 4, B) 

 with its flattened papillated upper surface and back- 

 wardly projecting pike was dominant in the ordinary 

 way to the deeply serrated high single comb (Fig. 4, C) 

 which is characteristic of the Mediterranean races. 

 Experiment also showed that the pea comb (Fig. 4, A), 

 a form with a low central and two well-developed 

 lateral ridges such as is found in Indian game, behaves 

 as a simple dominant to the single comb. The inter^ 

 esting question arose as to what would happen when 

 the rose and the pea, two forms each dominant to 

 the same third form, were mated together. It seemed 

 reasonable to suppose that things which were alter- 

 native to the same thing would be alternative to one 

 another — that either rose or pea would dominate in 

 the hybrids, and that the F^ generation would consist 

 of dominants and recessives in the ratio 3 ; i . The 

 result of the experiment was, however, very different. 

 The cross rose x pea led to the production of a comb 

 quite unlike either of them. This, the so-called 

 walnut comb (Fig. 4, D), from its resemblance to 



