esirable 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
conception 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
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 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 {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 phenomena 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
economical way of doing this is to cross the two pure strains so that all
the offspring may possess the desired character. The labour of analysis is
often an intricate and tedious 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 individu