“Then hasn’t a bird got any legs, Uncle Roy, only just feet?” asked Dodo.

“Oh! yes; legs too, with a knee-joint and a hip-joint, like ours.  But all these parts are up closer to the body, and hidden by the feathers, so that you cannot see them.”

As the Doctor said this there was a great commotion.  Quick, who had been watching the mouse hole all the while, gave a sharp bark and pounced on something.  There was a feeble squeak, and it was all over with a mouse which had ventured too far from its hole.

“Poor little mousey!” said the Doctor, as he took the limp body from the terrier’s mouth.  “It is quite dead.  I am sorry, but it might have nibbled some of my birds.  Besides, this is exactly what I wanted to teach you something about.  Who can tell me the difference between a mouse and a Sparrow?”

“I can!” said Dodo; “it’s all difference; a mouse 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?”