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 great 
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 swimming 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 
