ADAPTATIONS 401 



compared to a deep roar. In the closely allied Emus, on the 

 other hand, the front of the windpipe, near the middle of the 

 neck, presents a series of rings which fail to meet in the middle 

 line, leaving a long and fairly wide slit. Through this aperture 

 the lining membrane of the tube emerges to form a large sac 

 lying immediately beneath the skin, and the sac can be inflated 

 at will. As the Emu makes a very remarkable booming or 

 hollow drumming sound, such as might be made by beating on 

 a large wooden tub or barrel, it is supposed that the sac is the 

 instrument for the production of the sound, but the Cassowary 

 contrives to do as much without any such aid. Possibly by 

 acting as a resonator, it helps to increase the volume of the sound. 

 But we have no direct evidence that such is the case, and the 

 significance of this will appear presently. 



In many birds, and these in no way related forms, the wind- 

 pipe becomes enormously lengthened, so that in consequence it 

 has to be stowed away in coils, and this is done in one of three 

 ways — either these coils run between the surface of the breast 

 muscles and the skin, or they are received into a cavity in the 

 keel of the breast-bone, or they are stowed away beneath the 

 lungs (111.6, pp. 17, 18). Generally these modifications obtain 

 in the males only, sometimes in both sexes, and more rarely in 

 the female only. As an instance of superficial coiling we may 

 take the case of the Semipalmated Goose {Anseranas), in which, 

 in the males only, the windpipe is coiled beneath the skin of 

 the upper part of the breast, the coils varying in number, and 

 lying sometimes to one side and sometimes to the other, the 

 coracoid bone of that side being modified to accommodate the 

 coils. The males of the Curassows, and both sexes of a species 

 of Penelope {Penelope jacucacd), have the windpipe coiled away 

 beneath the skin, the coils extending as far back as the end of 

 the sternum. Although there are many species of Penelopes, 

 P jamcaca alone is so modified. Further, the males of the 

 Manucode Bird of Paradise have an enormous tracheal coil 

 (111. 23), covering the whole surface of the breast. Thus this 

 character has been independently acquired in several widely 

 distinct forms, but the sounds made by these birds do not appear 

 to be more remarkable than in allied species of these several 

 groups which have normal windpipes. In the Crested Guinea- 

 fowls (Guttera), and in both sexes, the windpipe forms a loop 



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