ouse hasn't any feathers,
or any wings, and it has four feet, and a long tail and whiskers and
teeth--"

"That will do, little girl, for differences; do you see anything alike
between a Sparrow and a mouse, Rap?"

"I think the fur is something like feathers, Doctor," answered Rap; "and
you told us how a beak was like a mouth without any teeth or lips; then
a mouse has four feet and legs; but a bird has only two feet, and two
wings instead of four legs and feet like a mouse."

"That is just what I want you all to think about," said the Doctor. "Now
listen. If a Sparrow has a pair of feet that correspond to a mouse's
hind feet, what do you think a Sparrow's wings correspond to in a
mouse?"

"I should think they would be something like a mouse's fore feet,"
answered Rap, after thinking a moment.

"That is exactly right. Birds and beasts are alike in many respects.
They have heads, necks, and bodies; they have tails; and they have
limbs. Beasts have two pairs of limbs. We call them fore legs and hind
legs. People have two pairs also. We call them arms and legs. So you see
our arms correspond to the fore legs of beasts, though we never use them
for moving about, except when we go on our hands and knees, or climb
trees, or swim in the water. And as for birds--why, their fore limbs
are turned into wings, to fly with, so that they walk or hop on their
hind limbs only, just as we do. Animals that go on all fours are called
_quadrupeds_. Animals that go on their two hind limbs only, like Bird
People and House People, are called _bipeds_. A Sparrow's wings are just
as much like a mouse's fore legs, as a Sparrow's feathers are like a
mouse's fur."

"How funny!" said Dodo. "But how are a bird's wings like fore legs, when
they haven't got any paws or toes--or fingers--or claws--only just long
feathers?"

"They have fingers, and some birds' wings have claws; only you cannot
see them, because they are all wrapped up in the skin and covered over
with the feathers. Some day--not to-day, because you have had a long
lesson already--I will show you a bird's wing with only its bones. Then
you will see that it has finger-bones at the end, then hand-bones next,
then bones that run from the wrist to the elbow, and then one bone that
runs from the elbow to the shoulder--almost the same bones that people
have in their fingers, hands, wrists, and arms. So you see wings are the
same to a bird that fore legs are to a mouse or arms are to us