iv PRESENCE AND ABSENCE THEORY 33 



it soon became clear that there were cases which could 

 not be expressed in terms of this conception. The na- 

 ture 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 sur- 

 face and backwardly 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 alternative 

 to the same thing would be alternative to one another — 

 that either rose or pea would dominate in the hybrids, 

 and that the F 2 generation would consist of dominants 

 and recessives in the ratio 3:1. The result of the ex- 

 periment 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), 



