way that the various domesticated forms have arisen. Such, for example, is
the case in the rabbit, where most of the colour varieties are recessive to
the wild agouti form. Such also is the case in the rat, where the black and
albino varieties and the various pattern forms are also recessive to the
wild agouti type. And with the exception of a certain yellow variety to
which we shall refer later, such is also the case with the many fancy
varieties of mice.

Nevertheless there are other cases in which we must suppose evolution to
have proceeded by the interpolation of characters. In discussing reversion
on crossing, we have already seen that this may not occur until the F_2
generation, as, for example, in the instance of the fowls' combs (cf. p.
65). The reversion to the single comb occurred as the result of the removal
of the two factors {86} for rose and pea. These two domesticated varieties
must be regarded as each possessing an additional factor in comparison with
the wild single-combed bird. During the evolution of the fowl, these two
factors must be conceived of as having been interpolated in some way. And
the same holds good for the inhibitory factor on which, as we have seen,
the dominant white character of certain poultry depends. In pigeons, too,
if we regard the blue rock as the ancestor of the domesticated breeds, we
must suppose that an additional melanic factor has arisen at some stage.
For we have already seen that black is dominant to blue, and the characters
of F_1, together with the greater number of blacks than blues in F_2,
negatives the possibility that we are here dealing with an inhibitory
factor. The hornless or polled condition of cattle, again, is dominant to
the horned condition, and if, as seems reasonable, we regard the original
ancestors of domestic cattle as having been horned, we have here again the
interpolation of an inhibitory factor somewhere in the course of evolution.

On the whole, therefore, we must be prepared to admit that the evolution of
domestic varieties may come about by a process of addition of factors in
some cases and of subtraction in others. It may be that what we term
additional factors fall into distinct categories from the rest. So far,
experiment seems to show that they are either of the nature of melanic
factors, or of inhibitory {87} factors, or of reduplication factors as in
the case of the fowls' combs. But while the data remain so scanty,
speculation in these