BIRDS OF OLD 81 



and their muscles dwindled, while the bones 

 and muscles of the legs increased by constant 

 use. By the time the wings were small enough 

 to be used in so dense a medium as water the 

 muscles had become too feeble to move them, 

 and so degeneration proceeded until but one 

 bone remained, a mere vestige of the wing that 

 had been. The penguins retain their gi-eat 

 breast muscles, and so did the Great Auk, be- 

 cause their wings are used in swimming, since 

 it requires even more strength to move a small 

 wing in water than it does to move a large 

 wing in the thinner air. As for our domesti- 

 cated fowls — the turkeys, chickens, and ducks 

 — there has not been sufficient lapse of time 

 for their muscles to dwindle, and besides arti- 

 ficial selection, the breeding of fowls for food 

 has kept up the mere size of the muscles, al- 

 though these lack the strength to be found in 

 those of wild birds. 



As a swiinming bird, one that swims with its 

 legs and not with its wings, Hesperornis has 

 probably never been equalled, for the size and 

 appearance of the bones indicate great power, 

 while the bones of the foot were so joined to 



