the nature of the " walnut *' comb in fowls. 167 



Previously we had regarded the allelomorphism as existing 

 between the various factors which caused the comb to be either 

 rose, pea, walnut, or single. The critical point of difference 

 between the two views lies in the way in which single is regarded. 

 While formerly we treated it as a positive condition segregating 

 from the others, on what we may call the presence-and-absence 

 hypothesivS it is to be recognized as the original state into which 

 the factor — or pangen, if we use de Vries' term — for rose, pea, etc., 

 has been introduced. 



Since the ratios, both of characters and hereditary powers, are 

 identical in both cases, simple experiment does not discriminate 

 between the two possibilities ; though certainly if all the different 

 kinds of elaborated combs*, such as Houdan, Creve-Coeur, etc., are 

 found to give singles in F 2 from rose or pea, the fact will tell 

 distinctly in favour of the presence-and-absence hypothesis. 



In spite of the great simplification which this conception 

 effects, there are certain difficulties besetting it. In the first 

 place it is not readily conceivable that rose and pea are in reality 

 nothing but rose elements on a basis of single comb, or pea 

 elements on a basis of single comb. This difficulty is especially 

 great in the case of the Indian Game hen and the Malay hen, for 

 in each of these the comb is so low that it can scarcely be regarded 

 as formed merely by the moulding of even a low single comb into 

 the pea or walnut shape. An extreme difficulty of a similar kind 

 is presented by the fact that the walnut comb has special charac- 

 teristics not present in either rose or pea, notably the hairs or 

 feathers on its surface. We must recognize, therefore, that if 

 these forms are produced by the addition of shape- elements such 

 as rose, or pea, or both, the result is not merely that which would 

 be expected as the consequence of simple additions. 



If we take a somewhat wider range and consider the possi- 

 bility of applying such presence-and-absence hypotheses to the 

 phenomena of heredity at large further difficulties are encountered, 

 to which it may be well to make reference now. For example, 

 the phenomenon of dominance on the part of a negative character 

 is difficult to reconcile with this conception. Such a case occurs 

 even in the combs, for the Fowl x Pheasant is declared to be 

 always without any comb at all-f*. Striking cases have been 

 recorded by Biffenj in Barley crosses, where the absence of female 



* Mr C. C. Hurst has suggested to us that a particularly interesting experiment 

 on this point could be made by crossing the Breda fowl with rose or pea. The 

 Breda is described as having no comb, but a concavity in its place. In it the 

 wattles are represented as of the size proper to an ordinary single-combed fowl. 



f See for instance Tegetmeier, The Poultry Book, 1867, p. 165. 



t Biffen, R. H., Jour. Ayr. Sri.. 1905, i., p. 255. 



