the roots of the toes, corresponds to the instep of your foot, and of course the heel comes next. You must remember the name of it—the Wise Men call it the tarsus."
"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?"
"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.
"I could go through all the inside parts of birds, and show you something like the same parts in people—stomach and bowels, to take care of the food they eat and turn it into blood to nourish them; lungs to breathe with, and keep the blood pure; heart to beat and thus pump the warm blood into all parts of the body; brain and nerves, which are what birds think and feel with, just as we do with ours; and all their bones, which together make what we call the skeleton, or framework of the body, to keep the flesh in shape and support the other organs."
"Dear me!" sighed Dodo; "there must be ever so many more things inside of birds that we can't see, than there are outside."
"Of course there are!" said the Doctor. "It won't be very hard for you to remember the outside parts, and learn the names of them all. I have told you most of them that you need to remember, to understand the stories I am going to tell you about birds. See here! What do you think of this?"
So saying, the Doctor unrolled a large sheet of drawing-paper that hung on the wall. "Here is a picture of the White-throated Sparrow, drawn so big you can see it almost across the room, with all the outside parts of which you must learn the names. You see the names are all on the picture, too; I am going to make it smaller, and put it in the book I will write for you, so you can look at it whenever you wish.
"It is almost dinner-time now, and you must be very hungry. But now I must tell you one thing more. You know there are so many, many different kinds of birds and other animals that nobody could remember them unless they were classified. To classify is to put things that are most alike closest together, then next nearest them things that are next most alike, and to keep furthest apart those things that are least alike. Now it is true that all beasts, birds, snakes, lizards, frogs, and fishes have some things alike, though each has some other things different from all the rest. If they were not all alike in some things, we could not call them all animals. One of the things in which all the animals I have named are alike is, that they all have skeletons. One of the things in which all their skeletons are alike is, that they have backbones. Backbones are the chains of bones that run along the back from the head to the tail. Backbones are called by the Wise Men vertebrae; animals that have backbones are named Vertebrates; and animals that lack backbones are named Invertebrates."
"Tell us the names of some Invertebrates, please, Doctor," said Rap.
"Well, all sorts of insects are Invertebrates, and so are lobsters and crabs, oysters and clams, worms, starfishes, jelly-fishes, corals, and even sponges. Then there are some too small to see without a microscope. But never mind about Invertebrates now. I only want you to remember that all beasts, birds, reptiles, and fishes are Vertebrates, and that there are five principal classes of them. If I should tell you as much about them as I have about the Building of a Bird, you would see that they are all built on what we call the Vertebrate plan of structure. Here is a chart of the Classes of Vertebrates—you can study it this afternoon, till you learn it by heart."
VERTEBRATES
Animals with Backbones
CLASS I. Mammals.—Warm-blooded animals which have fur or hair, bring forth their young alive, and nurse them. House People are Mammals.
CLASS II. Birds.—Warm-blooded animals which have feathers and lay eggs.
CLASS III. Reptiles.—Cold-blooded animals which have scaly skins, like lizards, snakes, and turtles.
CLASS IV. Batrachians.—Cold-blooded animals which have naked skins, like frogs, toads, and newts.
All the foregoing classes, except a few of the Batrachians, breathe air in lungs, and almost all, except snakes, have legs; none now living can fly, except bats and birds; but bats are Mammals.
CLASS V. Fishes.—Cold-blooded animals which have either scaly or naked skins, but no fur or feathers; which live in the water, breathe it with their gills, and swim in it with fins.
CHAPTER V
