IV PRESENCE & ABSENCE THEORY 35 



come into view. Similarly a pea comb is a single plus 

 a factor which turns the single into a pea, and a walnut 

 is a single which possesses two additional modifying 

 factors. Singleness, in fact, underlies all these combs, 

 and if we write their zygotic constitution in full 

 we must denote a walnut as RRPPSS, a rose as 

 RRppSSy a pea as rrPPSS, and a single as rrppSS. 

 The crossing of rose with pea results in a reshuffling 

 of the factors concerned, and in accordance with the 

 principle of segregation some zygotes are formed in 

 which neither of the modifying factors R and P is 

 present, and the single character can then become 

 manifest. 



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 in- 

 heritance 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 domesti- 

 cated 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 confirma- 

 tion 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 singleness 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^ from such a cross. Such 

 a comb we had the good fortune to find in the 



