into a melodious love song. In the case of the modified feathers, by which the drumming, whistling, or roaring noises are produced, we know that some birds during their courtship flutter, shake, or rattle their unmodified feathers together; and
Fig. 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 corresponding 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 f, seventh secondary, lower surface.]]
if the females were led to select the best performers, the males which possessed the strongest or thickest, or most attenuated feathers, situated 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 only
