132 THE COURTSHIP OF ANIMALS 



shape of the wing which will account for this. On the 

 other hand, a near relation of this bird, the Black-wattled 

 Guan, Aburria (^Penelope) aburri, has the four outermost 

 primaries deeply incised along their inner vanes, reducing 

 the outermost portion of the feathers to mere spines. 

 Yet, so far as is known, this wing makes no especial noise. 

 However, the males of certain little South American 

 Perching-birds known as Manakins have the shafts of 

 the secondary quills thickened to an extraordinary degree 

 so as to form solid, horny lumps, and these, when the 

 wings are brought together smartly over the back, pro- 

 duce a noise not unlike the crack of a whip, so that here 

 again structure and function are found together. In the 

 contradictory cases just cited where specialized parts are 

 found which are apparently functionless, we must suppose 

 that the habit of using them has been supplanted by some 

 new stimulant. 



The part played by musical instruments of percussion 

 would seem to be a variable one. In some cases, and 

 possibly in all, it may serve as an excitant, or stimulant, to 

 the rousing of a " sex-storm " ; in many, at any rate, 

 such sounds serve as calls to the sexes when separated. 

 This much seems to be demonstrated in the case of certain 

 of the Woodpeckers, which in this matter differ con- 

 spicuously from any other species yet referred to, in 

 that they have developed no special sound-producing 

 mechanism, but make use of hollow trees which serve 

 them as drums, the beak being used as the drumstick. 

 This is a very noteworthy fact, for one would have 

 supposed that here at any rate, where the production of 

 loud and far-reaching sounds is of vital importance, the 

 means would have been provided by some such modi- 

 fication of the wing-feathers as we have already seen to 



