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 off^r 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 ros^, to obtain 

 some single combs in F^ 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 bristlehke 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. 



