SEXUAL SELECTION 



491 



whistling, or roaring noises are produced, we know tliat 

 some birds during their courtship flutter, shake, or rattle 

 their unmodified feathers together; and if the females were 

 led to select the best performers, the males which possessed 



Fia. 45.— Secondary wing-feathers of Pipra deliciosa (from Mr. Sclater, In "Proc. 

 Zool. Soc," 1860). The three upper feathers, a, b, c, from the male; the three lower 

 corresponaing feathers, d, e, f, from the female, a and d, fifth secondary wing- 

 feather of male and female, upper surface, b and e, sixth secondary, upper surface. 

 c and/, seventh secondary, lower surface. 



the strongest or thickest, or most attenuated feathers, situ- 

 ated on any part of the body, would be the most successful; 

 and thus by slow degrees the feathers might be modified 

 to almost any extent. The females, of course, would not 

 notice each slight successive alteration in shape, but oniy 



