144 MENDELISM CHAP. 



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 concep- 

 tion 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 necessary for the determina- 

 tion 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 the 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 F 2 generation. Analysis shows 

 that the difference between the walnut and the single 

 is a difference of two factors, and it is not until this 

 has been determined that we can proceed with 

 certainty to transfer the walnut character to a single- 

 combed breed. Moreover, in his process of analysis 

 the breeder must be prepared to encounter the 

 various phenomena that we have described under 

 the headings of interaction of factors, coupling, and 

 repulsion, and the recognition of these phenomena 



