WINGS AND FEET 



the birds' wings, are used chiefly for balancing, 

 and when the fish swims fast these fins are 

 kept close to the sides. Among mammals the 

 cetaceans have developed greatest speed in 

 diving and swimming under water, and here 

 also the tail is the propulsive power, while the 

 anterior extremities are used chiefly for balanc- 

 ing. The beaver, with its posterior extremities 

 alone webbed, uses these only in swimming 

 under water. The modern screw-propeller is 

 superior to the old side-wheeler. 



In hesperornis the wing is a mere vestige, 

 but the leg bones are of great strength. It is 

 evident that hesperornis pursued its prey under 

 water by means of the feet alone, and that 

 through many generations it had gradually 

 lost the use of the wings, which must have 

 been, therefore, a hindrance rather than a 

 help in its subaqueous flight. It had long since 

 given up aerial flight. Loons and grebes, how- 

 ever, although apparently allied to hesperornis, 

 do at times, as we have seen, use their wings in 

 addition to their feet under water, yet it seems 

 to me probable from the evidence adduced that 

 as a rule they progress by the feet alone. The 

 young appear to use the wings as well as the 

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