218 THE FACTORIAL HYPOTHESIS 



clearly that two pairs of Mendelian factors are present. 

 Pea comb was assumed to lack a factor for rose, and 

 rose was assumed to lack a factor for pea. By re- 

 combination there should result in F2 one individual 

 in sixteen that was no-rose no-pea. This is the single 

 comb. A single letter or symbol S was inserted in 

 all of the formulae so that when neither rose nor pea 

 comb was present something w^ould seem to be left 

 to represent the single comb. 



The verification of the latter point was supposed 

 to be found in the relation of the single comb to a 

 combless condition found in the Breda race of fowls, 

 which, when crossed to single, gave in F2 three 

 singles to one combless. In other words the comb- 

 less fowl was supposed to represent a race in which the 

 lowest stage of the series had been reached and the 

 last factor for comb had been lost. The series just 

 described was represented on the presence and ab- 

 sence scheme as follows: 



Rose RpS 

 Pea rPS 

 Walnut RPS 

 Single rpS 



There is, obviously, no necessity to make these 

 characters depend for their expression on losses of 

 something; for the small letters that here stand for 

 absences might just as well stand for actual factors 

 different from those represented by the large letters. 

 The formulae would then of course work out as well 

 as before. To those accustomed to the presence and 



