[The caterpillar changes into a moth.]

So the caterpillar does not become a butterfly simply by growing wings. It is really almost a new creature, built up out of the material of the old, as one might rip up an old coat and have it woven into a rug; and we may be very sure that no butterfly ever has the slightest recollection of the time when it crawled on a dozen legs, and chewed leaves for its food.

We, too, grow and change like frog and butterfly. When we were, say, four or five years old, we had twenty strong little white teeth, each set firmly in the jaw bone on a stout root. By and by, these teeth, one at a time, in pretty much the order in which they first appeared, began to get loose. First we could wiggle the tooth a little; then a good deal. By and by, it hung only by the thin skin of the mouth, so that one good yank, or perhaps only the push of the tongue against it, tore it out. Or possibly we bit into an apple, and left the tooth behind.

Anyhow, the tooth hadn’t any root. The hard bone had gone, and with it the soft nerve and blood vessel that used to be inside. There was only the hollow cup that once was the crown of the tooth. The rest had been eaten up, bone, nerve and all, by our blood corpuscles. We needed larger teeth. Teeth, as I told you, once formed, cannot grow any more. So the blood corpuscles chewed up the tooth, as much as they could get at, and used the old stuff to build the new one. No doubt they would consume the entire tooth, crown and all, and not leave a shred behind, if only there were some way of holding the last bit in place while they finished it off. That on the whole would be pleasanter than having to tie a string round the tooth and yank it out.

They do act that way with growing bones. The bones, as I have already explained, grow much as a tree does, by adding on new stuff to the outside. Meantime, since all bones are hollow, the hollow also has to grow. The hollow grows by the taking down of the bone on the inside, just as a tree might decay out at the heart, about as fast as it grew under the bark. This, also, is the work of the white blood corpuscles.

Then, too, the soft gristle of little children changes into the solid bone of grown men. But the gristle never turns to bone. As before, the gristle is taken out, eaten into, devoured, by these living cells of the blood and lymph; and as fast as the gristle disappears, the bone grows into its room.

Or sometimes it happens that two little boys find themselves not in entire harmony, with the result that one little boy goes home with one eye shut up; and the next day, and all the next week, that particular eye is a most remarkable study in black and blue. That is because, during the lack of harmony, something happened to hit one little boy in the eye, and smashed up some of the little blood vessels around it, so that the blood leaked out into the flesh.

Then the white blood corpuscles, which are themselves always leaking out of the lymph spaces and blood vessels, and wandering round thru the tissues, have to set to work to clean up that spilled blood. So they actually eat up the flecks of blood; until by and by, the flesh is all clean and white again, and the black eye is black and blue no more. In general, whenever we meet with an injury or a wound of any sort, these white blood corpuscles take a hand in the healing process, eating away the damaged tissue, and allowing chance for fresh growth.

The white blood corpuscles, then, live to eat. Like certain other creatures who are always hungry, they sometimes eat things that they had better have left alone. Sometimes, people’s hair turns white suddenly—corpuscles have actually eaten up the color. Some people even go so far as to say that the reason why we grow old (since, as I have already told you, growing old is mostly growing hard) is because these white blood corpuscles have gone crazy and eaten up the softer parts of us, and left us only the hard ones to grow old with. I don’t know whether this last is so or not. But at any rate, these white blood corpuscles, which let go their hold on the places where they grow, and go wandering off all over the body eating everything in sight, would be likely to make this sort of trouble if anything would; and the man who especially says it is so, probably knows more about these matters than anybody in the world.