large and horrible in the fairy stories—giants and ogres and dragons
and winged horses and chimeras and three-headed dogs and I don’t
know what else, all most extraordinary to imagine as well as nice
and creepy to read about. Really, however, there is no reason
whatever why a monster should be large. It must be horrid, or
unusual, or misshapen, or quite out of the ordinary. Then it is a
true and proper monster, no matter how small it is. And as a matter
of fact, some monsters, as strange as any maker of fairy stories
ever invented, are too small to be seen at all, unless one looks for
them with a microscope.
The planarian worms that I have just been telling about are
monsters. If a two-headed calf is a monster, that people who go to
the circus will pay to see, then surely a planarian is a still
greater monster, with one extra head in the small of its back,
another on the side of its tail, and four or five more hanging on at
various places anywhere over its body; and this to say nothing of as
many superfluous tails stuck on anywhere between. You know already
that these monstrous planarians are formed because the worm, instead
of healing up a cut as he should, seems possessed to grow a new head
or tail out of it. I am going to tell you now how certain other
strange monsters come to be.
You remember that earlier in this book, almost in fact, at the very
first, I told you about how the eggs of all animals are, to begin
with, single cells; and how afterwards, when they begin to grow,
they split, first into two cells, then into four, then into eight,
and so on, until finally, the single cell of the egg has become the
hundreds and thousands which build the young animal.
Some of these eggs, especially those of certain of the small sea
creatures such as star-fish and sea-urchins and the like, are
extraordinarily tough. Indeed they have to be, else few of them
would ever live to grow up at all. It is quite possible to take
these eggs when the single cell has divided into two, and shake
these two apart into two separate half eggs. You might think that
each half egg would form a half animal. Instead, much like the
planarian cut in halves, it forms a whole animal, half size.
Or we can wait till there are four cells, and shake these apart.
Then we get four complete creatures, each quarter size. Or we can in
the same way make eight. Beyond that we cannot go. Eight animals out
of what was intended for one is all that egg can manage.
On the other hand, it is possible to take a half dozen eggs, which
ought properly to have made as many separate creatures, and make
them stick together into one gigantic egg, from which will hatch out
a single gigantic animal, as large as the six or eight together
which ought to have come from six or eight eggs.
Other strange things happen when the eggs, instead of growing in
ordinary sea water, are put into water which has just a little bit
more or less of something in it. For common sea water contains some
half dozen different kinds of salt besides the one kind that people
get out and use on the table and for cooking; and more or less of
any one of these makes a salt water which is not quite the same as
the salt water of the ocean. In fact, you must have noticed that
salt water made with ordinary table salt does not taste by any means
the same as ocean water.
[Illustration: Accidents to growing fish eggs result in all sorts of
double monsters.]
A different kind of salt water (not very different either, I doubt
if anybody could tell them apart by the taste,) sometimes makes a
lot of difference with the creatures that grow in it. A little more
of one thing, for example, makes little star-fish with their
stomachs hanging out of their mouths, instead of inside their bodies
where all proper stomachs belong. A little more of another, makes
the sea minnows which live in it have only one eye, and that right
in the middle of their foreheads like the single eye of the giant
cyclops that I trust you have all read about long ago in the story
of Ulysses. If you haven’t you certainly had better right away, for
it is one of the great stories of the world, and has been told to
children, and to grown men and women as well, for at least three
thousand years, and nobody knows how much more.
Then again, by making things a little different in another way, the
baby fishes grow like other proper fishes except that they do not
have any hearts. Naturally, however, these particular monsters do
not live to grow up. People make all sorts of strange creatures
now-a-days by doing something to the young eggs when the new animal
is forming.
Curiously too, all this sort of thing may happen to almost any egg
by accident. If the two halves of the young egg get separated
entirely, then the egg brings forth twin creatures so nearly alike
that it is almost impossible to tell them apart at all. But if the
two halves get only partly separated, the result is some sort of
double monster.
So there are two-headed chickens, and two-headed snakes, and
two-headed turtles, and two-headed calves. Sometimes only the tip of
the nose is double. Sometimes it is the whole head. Sometimes there
are two heads, four front legs, two hind legs, and one tail.
Occasionally there is one body with eight legs, because the legs
doubled and the body did not.
Just about as often, the doubling begins on the other end. Then
there are two tails; or four hind legs and two front ones.
Occasionally, two complete bodies are joined at one small region
only They just missed being a pair of common twins It all depends on
how well separated the first two cells of the egg happened to get.
[Illustration: A two-headed turtle, a crab with an eye on one side and a
feeler on the other, and a child with two great toes on each foot.]
The famous “Siamese Twins” were about like any other twins, except
that they were fastened together side by side by a band of flesh
under the arm. They lived to grow up, married, and travelled about
the country exhibiting themselves for years. Finally one died;
thereupon the other died almost immediately after. But as for that,
any twins, if they are markedly alike, are likely to die at about
the same time.
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.
[Illustration: The fangs of a rattlesnake.]
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.
So the little fish, while still in the egg, has these gill slits in
the side of his head because later, when he gets hatched out and
swims round, he is going to use them to breathe with. But the little
chick in the egg, after his lower jaw has begun to grow, also has
these same gill slits in the side of his neck, although when he
hatches out, he is going to breathe with lungs, and is never going
to have the least use for gills. So, too, do little puppies and
kittens and colts and calves and all the land animals that breathe
with lungs and haven’t the slightest use for any such holes.
Sometimes the hole doesn’t even break clear through. It starts from
the inside and comes out, and from the outside and goes in, but the
two tunnels never quite meet, and the hole never gets really open.
But whether the holes open through or not, they soon close up again,
and leave no sign that they have ever been there at all.
All except the first hole on each side, the one nearest the mouth.
That never closes again, but remains open and becomes the hole into
the ear. There is a hole in from the outside, as you know, the one
you used to put beans and pencils in, only you oughtn’t. And there
is another hole from the inside, beginning high up in the throat
just where you can’t see it, and running in till it almost joins the
other hole. Between the two is just a thin skin, which is the “drum”
of the ear, and if you get a hole in it you may never be able to
hear again. Because this drum never grew. It is the place that
remained after one tunnel came in from outside, and another from
inside, and the two didn’t quite meet.
So it comes about that all of us land creatures with backbones, who
breathe with lungs, start making gill slits which we can’t possibly
use. Then, because we have them on hand, we use the one farthest in
front for the hole of the ear, and close the rest up again. And we
take all that trouble—though it doesn’t trouble us much at the
time—just because various other backboned creatures, which live in
the water and don’t have lungs, had to make gill slits to breathe
with.
There are a lot of things of this sort—parts and organs and members
which one creature, while it is in the egg, makes and doesn’t use,
just because some other creature, when it grew up, had to have them.
You know what a short tail a hen or a turkey or a pigeon has—just a
stub of a tail, only just big enough to stick its tail feathers in.
But a little hen or turkey or pigeon, while it is still in the egg,
has a tail like other animals, long enough to wag.
There is a kind of salamander, which is unlike most salamanders,
efts, newts, and the like—these are all pretty much the same thing,
and you find them almost anywhere in the brooks and the ponds and
the damp woods. All of these that you are ever likely to see breathe
with gills like a fish, and can live in the water. Only instead of
having their gills covered over with a bony plate like the fish,
these creatures often have them outside, like a sort of lace collar
that hangs down at the side of their necks. Tom, the chimney sweep,
in Charles Kingsley’s famous tale, after he turned into a water
baby, had just such tufted gills so that he could swim under water
like any newt or eft, and if you haven’t read “The Water Babies,”
it’s certainly high time you did.
What I started to say is that this particular kind of salamander
lives on the land. So he doesn’t need gills and doesn’t have them.
But the little salamander, while he is still in the egg, has gills
like any salamander, though for all the use he can ever put them to,
he might just as well have been furnished with a pair of skates; for
by the time he hatches out of the egg, the gills have been taken to
pieces by his white corpuscles and the stuff used to make some other
part of the body.
All the same, if you break open the egg, take the little creature
out, and put him in the water while he still has gills, he will swim
away, and live under water as well as any water creature. But if you
wait till he hatches out of himself and has lost his gills, then if
you put him in the water, he will drown just as you would. So the
little salamander, that is going to spend his life on dry land,
still has gills while he is in the egg and has no use for them, all
just because other salamanders that live some of the time in the
water need gills to breathe with.
Then there are the snakes, which have lungs and breathe air like any
land animal. Only a snake is so very slender that there isn’t room
in him for two lungs side by side. So he has only one proper lung,
very long and thin, that runs from his neck pretty well down to his
tail. Nevertheless, the snakes still keep the other lung, small and
quite useless, tucked away beside the front end of the one they do
use. Other reptiles have two lungs, so the snakes have to have two
lungs also, though they can’t possibly use them both, and the other
which they don’t use, merely takes up room.
We human beings are just as bad as the rest. Every little while,
somebody comes down with appendicitis and has to be taken to the
hospital to have his “vermiform appendix” taken out. The appendix
isn’t the slightest use to anybody, we are better off without it,
but in cows and dogs and rabbits and kangaroos, and various other
animals, especially those that eat grass and leaves, it is a good
deal of use for helping to digest food. So we have to have it, to be
like the rest—and then pay the doctor to cut it off.
We don’t move our ears as horses and dogs and rabbits do. But still
the muscles are there; and people say that anybody who wanted to
take the trouble could learn to wag his ears like a baboon. Anyhow,
the muscles are there, though we don’t use them and other creatures
do.