170 



THEORY OF EVOLUTION 



as Dexter has shown, that a number of mutant 

 factors that act as modifiers have been picked 



Fig. 84. Two flies showing- headed wings. 



up in the coiu'se of the selection, and Avhen it 

 is recalled that durino- those five vears over 

 125 new characters have appeared elsewhere 

 it does not seem improbable that factors also 

 have appeared that modify the wings of this 

 stock. 



(3) The third method is one that has been 

 developed principally by East for plants; also 

 bv ^lacDowell for rabbits and flies. The 



