VIII WILD AND DOMESTIC VARIETIES 79 



sive to the wild agouti type. And with the excep- 

 tion 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^ generation, as, for example, in the 

 instance of the fowls' combs (cp. p. 5 9). The rever- 

 sion to the single comb occurred as the result of the 

 removal of the two factors 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 domesti- 

 cated 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^, together with the greater 

 number of blacks than blues in F^, 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. 



