272 THE FACTORIAL HYPOTHESIS 
clearly that two pairs of Mendelian factorsare 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 F, 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 formule so that when neither rose nor pea 
comb was present something would 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 F, 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 formulz would then of course work out as well 
as before. To those accustomed to the presence and 
