80 MENDELISM . CHAP, vm 



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. 



