BIRDS OF OLD | 71 
of the feathered race, the exclusive prerogative 
of the bird being not flight but feathers; no 
bird is without them, no other creature wears 
them, so that birds may be exactly defined in 
two words, feathered animals. Reptiles, and 
even mammals, may go quite naked or cover 
themselves with a defensive armor of bony 
plates or horny scales; but under the blaze of 
the tropical sun or in the chill waters of arctic 
seas birds wear feathers only, although in the 
penguins the feathers have become so changed 
that their identity is almost lost. 
So far as flight goes, there is one entire order 
of mammals, whose members, the bats, are 
quite as much at home in the air as the birds 
themselves, and in bygone days the empire of 
the air belonged to the pterodactyls ; even frogs 
and fishes have tried to fly, and some of the 
latter have nearly succeeded in the attempt. 
As for wings, it may be said that they are 
made on very different patterns in such animals 
as the pterodactyl, bat, and bird, and that 
while the end to be achieved is the same, it is 
reached by very different methods. The wing 
membrane of a bat is spread between his out- 
