One usually thinks of monsters as large. They are always, I believe, 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.

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.

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.