THEORY OF EVOLUTION 



155 



lection could slide successive generations along 

 in the direction of selection. 



Darwin himself was extraordinarily careful, 

 however, in the statements he made in this con- 

 nection and it is rather by implication than by 

 actual reference that one can ascribe this mean- 



FiG. 80. Diagram illustrating the results of selection for 

 extra bristles in D. ampelophila. Selection at first produces 

 decided effects which soon slow down and then cease. 

 (MacDowell.) 



