LINKAGE 



73 



which excludes the possibihty of detecting any 

 linkage that is present, for obviously Hnkage cannot 



-r 



% 



«.#i^«*f 



s 



4 



Fkj. 29.— PhotoKnii)liof Pi (1 unci 2) and Fi (Sand 4) birds in such a cross 

 as th;it of Fin- -i"- Tlu^ 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 



