82 MENDELISM chap.vhi 



of the fowl, these two factors must be conceived of 

 as havmg 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 Fj, 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. 



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 factors, or of reduplication 

 factors as in the case of the fowls' combs. But while 

 the data remain so scanty, speculation in these 

 matters is too hazardous to be profitable. 



