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.