86 MENDELISM CHAP. 



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. Dur- 

 ing 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 FI, 

 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 



