Part 29

Often, too, only the buds which are forming the limbs get divided. Then there are extra fingers or toes or claws or thumbs. Cats, for some unknown reason, are especially apt to have double paws or extra toes. Oddly enough, tho nobody knows why, this happens more commonly on the front paws than on the hind ones.

So you see, the strange power which a few creatures have of making new parts, or extra parts, like the tails of lizards or the heads of planarians, belongs also to all creatures when they are very young. Most of them lose the power as they grow old. But in some animals, like crabs and earth worms and planarians, it lasts up to old age.

XLVII

How The Animals Keep Their Tools Sharp

One can not do much of anything without tools, so of course the animals have to have them. But because they haven’t sense enough to make tools as we do, or even sense enough to use such tools as they find ready made—pointed sticks and sharp stones and shells and bits of bone and the like—as our very savage and very stupid ancestors used to do, the tools of the animals have to grow on them.

The horse’s front teeth are his mowing machine, with which he cuts down the grass. His great flat-topped grinding teeth that lie in a long straight row along each side of his jaw behind the place where the bit goes, are his millstones with which he grinds his oats and corn into meal. But the cheek-teeth of dogs and cats, which also lie along the sides of their jaws, are not millstones, but knives; and their front teeth, like their claws, are traps to seize their prey and long daggers to stab them with. For the cats and dogs do not grind corn; they kill things. They are butchers, not millers.

In fact, when you come to think of it, nearly all the larger animals are either millers or butchers. Either they grind up plants for food, as horses and cows and sheep and goats do; or else they hunt and kill other living animals and devour them, as do the cats and dogs and wolves and foxes and hyenas and such like. Naturally, these two sorts of creatures have quite different sorts of tools—claws and sharp teeth for one, hoofs and grinding teeth for the other. Tho for some reason or other, pretty much all the animals that are strong enough to do any work, are miller animals. All the horses, oxen, ponies, donkeys, buffalos, elephants, camels, goats, llamas, and I don’t know how many more, that any body can get any work out of, all have hoofs and eat plants. The Eskimo dog, so far as I know, is the only butcher animal that earns a living. The rest just lie round and growl.

So the miller animals have to grind their food. Now millstones have to be kept rough; and as fast as they wear smooth, the miller has to “pick” out the grooves to make them rough again. In like manner, the horses and cows and sheep would soon wear their teeth too smooth to be of any use, if they did not have a way of making them once more rough. In these grinding, millstone teeth, the hard enamel, instead of being on the outside of the tooth as with us, is doubled into folds and plates, and mixed in with the bone of the tooth like the streak of fat and streak of lean in bacon, or like the two colors in marble cake. So the bone wears down faster than the enamel, and leaves the enamel standing up in sharp ridges. Thus the top of the tooth is always rough and ready to do its grinding; and because these teeth are pretty long, an inch or more, they last a long time before they are worn out.

But the dogs and cats, which have teeth like ours, with the enamel all on the outside, soon wear down their teeth so that they will no longer cut. That’s why dogs and cats and the wild creatures like them, are so short lived. They are built to grow up quickly and die young, because there is no use in having them made to live longer than their teeth will hold out. At least that’s one reason Why they don’t live longer.

The elephants have a curious way of taking care of their teeth. They have to take care of them, because the elephants live to be very old—a hundred years—a hundred and fifty—some people say even two hundred. Now a hundred years is a pretty long time for a tooth to keep on grinding leaves and twigs and roots, so the elephant has to save his teeth and make them last.

To begin with, he saves his front teeth by not having any. His two great tusks are two upper front teeth grown out till they are not teeth at all, but crowbars that the elephant uses to grub up roots with. They have no enamel and no root. So they can keep growing at the inner end as fast as they wear off at the outer. Being especially hard bone, they last as long as the elephant does, and get larger and larger as long as their owner lives. After that, they get turned into piano keys and billiard balls.

As the elephant spares his front teeth by not having any, he spares his back teeth by not having many at a time. These are very large, not quite as large as a leg of ham, but quite as large, often times, as a loaf of bread.

The young elephant gets four of these big grinders, one on each side of each jaw, and grinds away on them. After he has used these for a few years and begun to wear them down, four new ones grow just behind the first four. By and by, the first set gets worn out; then the white blood corpuscles take down the roots, the crowns—what there is left of them—fall off, and that is the end of that set. By the time the elephant begins to get old, the second set of four teeth has worn out, and a third set has come in. So a really old elephant has only six teeth—the two tusks, if you call them teeth, and four grinders.

The pigs do something the same. They hold back four very large grinding teeth at the last end of the row, and don’t let them appear till after the front grinders have been pretty well used up. So too do we in a way. We don’t cut our “wisdom teeth” till we are past twenty. Moreover, the wisdom teeth, which are last in the row, and the eight grinders in front of them, two in each half of each jaw, really belong to the milk set which we began to have when we were babies. We don’t need them then. So we hold them back till we do, though that isn’t for twenty years.

The rats, mice, squirrels, beavers, and other creatures that use their front teeth as drills and chisels, have a pretty clever way of keeping them sharp. You can easily see that a squirrel who puts his teeth through the hard shells of nuts every time he gets a meal, or a beaver who cuts through trees six inches across with his, or a mouse or a rat, would use up any ordinary set of teeth in a few weeks, and have to get on as best he could for the rest of his life without any.

The grinding teeth of these creatures are like other grinders, that last till they are worn down—and that’s all. But their four front teeth, two in the upper jaw, two in the lower, are like the elephant’s tusks. They have no roots, and they keep growing out from the inner end as fast as they wear off at the outer. But in order to have them held firmly in the jaw, having no roots, these front teeth start way in at the back of the jaw close to the roots of the last grinders. They grow out along the whole length of the jaw bone, past all the other teeth, and come out at the front of the mouth. So the front cutting teeth of mouse or rat or squirrel are about as long as his legs, and start back almost to his neck. Besides this, they have the hard enamel all in one plate at the front of the tooth, instead of over the entire outside as we do. Then as the tooth wears down, the bone wears fastest and leaves the enamel as a cutting edge, always sharp.

The wild pigs do much the same thing with their four tusks. They start them clear round at the back of the jaw, curve them past the rest and bring them out at the sides of the mouth. Then they put two together to make a tusk, and each grinds on the other and keeps it sharp. But I don’t think that any animal ever does this sort of thing with more than four teeth. He can make four grow all the time, or two as the elephants do, or none. But the rest have to have ordinary roots, and when they wear out, why that’s the end of them.

But the sharks grow several rows of teeth at once, starting them inside the mouth and letting them slide over the jaw in the skin. These are not real teeth set in the bone, but only a sort of skin teeth; and the shark grows them by the dozen, new ones as fast as the old drop out. The snakes do much the same thing with their poison fangs, and keep always at least one new pair folded down behind the old ones, ready when these get pulled out. But snakes and sharks don’t chew with their teeth, they only bite with them.

Now let’s see if you can’t find out for yourselves how it is that pussy cat keeps her claws as sharp as needles. You can clip off the points, but it will not be many days before she will make new ones, just as sharp as the old. If you study the claw you will see how she does it. The dog has the same device, only his nails are not so sharp.

XLVIII

Why The Blood Is Salt

The blood is really salt. So is the sweat, as you can easily prove by putting your tongue anywhere on your skin, after you have been hot and sticky for a long time. And of course the tears are salt, as no doubt you found out long ago, sometime when everybody was especially horrid and they ran down into your mouth. In fact, pretty much everything about the body is salty, for the reason that it is all made of blood, which is itself pretty salt.

Now the blood is salt because the sea is. I much suspect that if the ocean had always been fresh water, like the ponds and lakes, then our blood, and the blood of all other animals, and all sweat and tears and the like, would have been fresh also. For the sea and the blood are salt with the same kinds of saltness. Their salt is mostly the sodium chlorid which we use for table salt, and besides these, there are calcium which makes limestone and lime and mortar and plaster, potassium which makes potash and soft soap, and various other metals including even gold. Altogether, sea water and blood are extraordinarily alike, especially when you consider that there are vast numbers of little animals in the sea that one can’t so easily tell from the white corpuscles of the blood.

The reason for all this is that the simpler creatures of the ocean do actually use the sea water for blood. Instead of having their bodies shut up tight as we land animals have ours, so that nothing gets into them unless we breathe or swallow, the inside of their bodies is open to the sea water, and the sea water flows in and out freely.

The sponges are like this. They take the water in through their smaller holes, and let it out through the large ones. They breathe the air that is in the water, and they turn their waste matter back into the water again, just as if the water were their blood. Many other creatures manage in this way, getting along without any private blood of their own, and using the great common ocean instead.

The rest of us have simply shut up our bodies and caught a little bit of the ocean inside. We call this bit of ocean, blood; and we have added various things to it. But still it is sea water, the same old sea water that is the blood of the earth and of all the lowly sea creatures that have no private blood of their own.

There are a great many other things also that we big land creatures have and do for no other reason than that some small sea creature began that way, and there has happened to be no special reason why anybody should change.

For example, all fishes, as you know, breathe by taking water in through their mouths and letting out again through a set of slits, usually five in number, at the side of what would be the neck if the fish had one. When the little fish is forming in the egg, at first it does not have any of these “gill slits” and so has to breathe through its skin. By and by, however, after the lower jaw has formed and there is a mouth, these slits punch through and become the convenient openings that we put a forked stick through when we go fishing and so bring home our fish, instead of putting them in our pockets, which is really a practice not at all to be commended.