IV PRESENCE AND ABSENCE THEORY 39 



The Presence and Absence theory is to-day generally 

 accepted by students of these matters. Not only does it 

 afford a simple explanation of the remarkable fact that in 

 all cases of Mendelian inheritance we should be able to 

 express our unit-characters in terms of alternative pairs, 

 but, as we shall have occasion to refer to later, it suggests 

 a clue as to the course by which the various domesticated 

 varieties of plants and animals have arisen from their wild 

 prototypes. 



Before leaving this topic we may draw attention to 

 some experiments which offer a pretty confirmation of the 

 view that the rose comb is a single to which a modifying 

 factor for roseness has been added. It was argued that if 

 we could find a type of comb in which the factor for single- 

 ness was absent, then on crossing such a comb with a rose 

 we ought, if singleness really underlies rose, to obtain 

 some single combs in F 2 from such a cross. Such a comb 

 we had the good fortune to find in the Breda fowl, a breed 

 largely used in Holland. This fowl is usually spoken of as 

 combless, for the place of the comb is taken by a covering 

 of short bristlelike feathers (Fig. 6, D). In reality it pos- 

 sesses the vestige of a comb in the form of two minute 

 lateral knobs of comb tissue. Characteristic also of this 

 breed is the high development of the horny nostrils, a 

 feature probably correlated with the almost complete 

 absence of comb. The first step in the experiment was to 

 prove the absence of the factor for singleness in the Breda. 



