LINKAGE 



73 



which exchides the possibihty of detecting any 

 linkage that is present, for obviously hnkage cannot 



--^ 



y 



3 



4 



Fig. 29. — Photograph of Pi (1 and 2) and Fi (3 and 4) birds in such a cross 

 as that of Fig. 30. The Pi male is a standard figure. 



be seen unless at least two pairs of factors are studied 

 at the same time. The steady increase in the number 



