408 THE MECHANICAL FUNCTIONS. 



life, all their extremities are formed for swimming. Their 

 legs are exceedingly short, and placed far backwards; so that 

 these birds are compelled, when resting on their feet on the 

 shore, to raise their bodies in a perpendicular attitude, in 

 order to place the centre of gravity immediately above the 

 base of support: a posture which gives them a strange and 

 grotesque appearance. 



I have already alluded to the lengthened legs and feet of 

 the waders, the utility of which to birds frequenting marshy 

 places, and shallow waters, is very obvious. Their legs are 

 not covered with feathers, which would have been injured 

 by continual exposure to wet. But birds of a truly aquatic 

 nature have their toes webbed, that is, united by a mem- 

 brane, a mechanism which qualifies them to act as oars, and, 

 indeed, gives them a great advantage over all artificial oars, 

 that have been constructed by human ingenuity; for, as soon 

 as the expanded foot has impelled the water behind it, the 

 toes collapse, and, while it is drawn forward, it presents a 

 very small surface to the opposing water. Their plumage 

 is so constructed as to prevent the water from penetrating 

 through it; and for the purpose of preserving it in this con- 

 dition, these birds are provided with an oily fluid, which 

 they carefully spread over the whole surface of their bodies. 

 The Swan, and many other water-fowls, employ their wings 

 as sails, and are carried forwards on the water with consi- 

 derable velocity, by the impulse of the wind. 



Birds excel all other vertebrated animals in the energy of 

 their muscular powers. The promptitude, the force, and 

 the activity they display in all their movements, and the un- 

 wearied vigour with which they persevere, for hours and 

 days, in the violent exertions required for flight, far exceed 

 those of any quadruped, and implies a higher degree of irri- 

 tability, dependent, probably, on the great extent of their 

 respiratory functions, than is possessed by any other class of 

 animals. 



ST^^ 



END OF VOL. I. 



