BIRDS 159 



and pressures applied to the trachea in the production of the 

 sound by means of muscles and air-pressure, and to these 

 special strains the modification corresponds. 



It is significant that in Anseres there are two pairs of 

 extrinsic tracheal muscles, passing from the lower part of the 

 trachea to clavicle or sternum, while in all other birds there 

 is only one pair. 1 Another important fact is that in the 

 male of CEdemia fusca, the Velvet Scoter, the modification of 

 the syrinx occurring in most ducks is absent, but there 

 is a spherical osseous enlargement above the lower end of 

 the trachea, and from this bulb a muscle passes to the 

 clavicle or merrythought. 2 



In the swans, Cygnidse, there is a remarkable contrast 

 in the condition of the trachea in different species, but in 

 no species is the peculiarity unisexual. In the species of one 

 group the trachea has a number of convolutions which are 

 actually contained in a cavity excavated in the keel of the 

 sternum. This group contains Cygnus ferns, the Whooper, and 

 C. Bewicki of North Europe, C. buccinator and C. americanus 

 of North America. Other species have a simple trachea. 

 It is known that the four species mentioned above utter 

 a loud trumpeting or whistling cry, while C. olor, the Common 

 Swan, C. nigricollis, the Black-necked Swan, and C. coscaroba 

 in which the trachea is straight, are all mute, or utter 

 only a slight grunt. It seems most probable that the 

 lengthening of the trachea is due to air-pressure, and there 

 can be no doubt, I think, that the sternum has been hollowed 

 out in course of generations by the pressure of the trachea, 

 perhaps the most wonderful illustration of the physiological 

 effect of mechanical causes. But the difficulty is to under- 

 stand how the voice, if it is produced in the syrinx, can be con- 

 nected with the compression of air in the trachea. Possibly 



1 Newton and Gadow, Diet. Birds, p. 938. 

 2 British Birds, vol. iii. p. 318. 



