MODIFYING FACTORS 189 



little one from another that the variation is practically 

 continuous. 



Some gametic factors show their influence chiefly, if not 

 exclusively, m the form of changed action of other factors. 

 Thus the ordinary extension factor in rabbits produces with 

 the regular agouti factor an ordinary gray coat, but the 

 darkened extension factor produces with the same agouti 

 factor a steel gray coat. We think of the character of the 

 gray marking as a consequence of the agouti factor but find 

 in reality that it is changed by a change in the extension 

 factor, no less than by changes in the agouti factor. It is 

 assumed that there are many factors whose only discoverable 

 function is to modify the action of other factors and when 

 we find that some particular character, manifestly influenced 

 mainly by a single gene, has undergone slight change, or 

 continues to change progressively under continued selection, 

 it is safer to assume that modifying factors are concerned in 

 the matter than that the principal gene is gradually changing. 



The substance of our present knowledge as to changes in 

 genes may be summed up in the statement that such changes 

 come or go suddenly and in their entirety, and cannot, so far 

 as we know, be influenced by selection or any other control- 

 able process. Hence we may well call changes in genes 

 mutations. 



