xiv ECONOMICAL 157 



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 phe- 

 nomena that we have described under the headings of 

 interaction of factors, coupling and repulsion, and the 

 recognition of these phenomena will naturally influence 

 his procedure. Or again, his experiments may show him 

 that one of the characters he wants, like the blue of the 

 Andalusian fowl, is dependent upon the heterozygous 

 nature of the individual which exhibits it, and if such is 

 the case he will be wise to refrain from any futile attempt 

 at fixing it. If it is essential it must be built up again in 

 each generation, and he will recognise that the most ec- 

 onomical way of doing this is to cross the two pure strains 

 so that all the offspring may possess the desired charac- 

 ter. The labour of analysis is often an intricate and te- 

 dious business. But once done it is done once for all. As 

 soon as the various factors are determined, upon which 

 the various characters of the individual depend, as soon 

 as the material to be made use of has been properly ana- 

 lysed, the production and fixation of the required combi- 

 nations becomes a matter of simple detail. 



An excellent example of the practical application of 

 Mendelian principles is afforded by the experiments which 

 Professor Biffen has recently carried out in Cambridge. 



