NATURAL SELECTION 83 



birds (Cocclnella decempunctata), whose coloration varies so 

 much, might, we may hold, be formed into definite groups 

 either by isolation or by natural selection. 



(4) When once definite species have been formed, separated or (4) use- 

 from each other by inter-sterility, the importance of recognition " ^\^ n 



marks 



FIG. ii. Mosaic formed by leaves of horn-beam (carpinus). 

 From a photograph. 



marks as a means of preventing barren unions between representa- 

 tives of different groups, is difficult to over-estimate. If we bear 

 this in mind we shall cease to consider the characteristic plumage 

 of the various species of woodpecker as useless and unaccountable. 



