LINKAGE 73 
which excludes the possibility of detecting any 
linkage that is present, for obviously linkage cannot 
Fig. 29.—Photograph of P; (1 and 2) and F, (3 and 4) birdsin such across 
as that of Fig. 30. The P; male is a standard figure. 
be seen unless at least two pairs of factors are studied 
at thesame time. The steady increase in the number 
