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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