iS6 MENDELISM chap. 



Improvement is after alKthe keynote to the breeder's 

 operations. He is aiming at the production of a strain 

 which shall combine the greatest number of desirable 

 properties with the least number of undesirable ones. 

 This good quality he must take from one strain, that from 

 another, and that again from a third, while at the same 

 time avoiding all the poor qualities that these different 

 strains possess. It is evident that the Mendelian con- 

 ception of characters based upon definite factors which 

 are transmitted on a definite scheme must prove of the 

 greatest service to him. For once these factors have been 

 determined, their distribution is brought under control, 

 and they can be associated together or dissociated at the 

 breeder's will. The chief labour involved is that neces- 

 sary for the determination of the factors upon which the 

 various characters depend. For it often happens that 

 what appears to be a simple character turns out when 

 analysed to depend upon the simultaneous presence of 

 several distinct factors. Thus ihe Malay fowl breeds 

 true to the walnut comb, as does also the Leghorn to the 

 single comb, and when pure strains are crossed all the 

 offspring have walnut combs. At first sight it would be 

 not unnatural to regard the difference as dependent upon 

 the presence or absence of a single factor. Yet, as we 

 have already seen, two other types of comb, the pea and 

 the rose, make their appearance in the F2 generation. 

 Analysis shows that the difference between the walnut 



