The plants, then, know enough to do two things—to grow up or down with stem or root, and to turn toward the light or away from it. This really, if you can call this knowing, is about all they do know. Now I am going to tell you about some common animals which are not much better off than the plants; which know up and down, and know the direction of the sunlight, and know mighty little else.

Happy is the community which does not know the brown-tail moth. Wherever it appears, it spreads like a pestilence, eating every green leaf off a tree, and leaving it in mid-summer as forlorn and bare as at Christmas time. A great tree that has taken a hundred years to grow, the progeny of one moth will kill in three.

The brown-tail moths, their cousins the golden-tail moths, and several other sorts of moths, lay their eggs in the late summer and early fall. The little caterpillars hatch out that same season, grow to be something like a quarter inch long, and spend the winter in a cocoon-like nest which they spin for themselves much as does a silk-worm or a spider. In the spring, having eaten nothing all winter, they leave the nest, crawl to the ends of the branches, and proceed to devour the new leaves.

But how does a little worm, no bigger round than a slender pin, finding itself in the midst of a great tree, with nothing near it but tough bark which it cannot eat, know in the first place that there are fresh green buds anywhere, and how in the second place, does it find its way to the tips of the twigs where the buds are? The answer is, that it doesn’t. The little caterpillar knows no more about buds and food and the way to them than the tree itself does. It is simply built like a tree, so that when it first leaves its nest, it always turns its head up, and when it has a choice between light and darkness, turns toward the light.

So the caterpillar simply turns up and toward the light, just as a plant would, and with no more intelligence than a plant has, and no more idea what it is about. But of course, crawling up and toward the light, sooner or later brings it to the outer ends of the branches where its food is.

You can easily prove this by putting the young caterpillars in a bottle, or a wide-mouthed jar. If you lay the jar down on the table with the closed end toward the window, every caterpillar will crawl to the closed end, and never a one will crawl back away from the light to the open end and escape. You don’t need any cover; the light holds them fast prisoners. But turn the jar round, open end toward the window, and soon there will not be a caterpillar left in it.

Suppose now, when your caterpillars are at the closed end of the jar toward the window, you take some fresh leaves, from the tree on which you found the insects (since these are presumably the sort they eat) and put them in the open end of the jar away from the window. The little caterpillars will stay where they are till they all starve to death, before one of them will turn round and crawl away from the light toward its dinner. They are even more helpless than a plant, which can at least send its roots toward water, no matter how the light comes.

Also, as I have explained, the caterpillars must crawl up. So they cannot escape from an open jar placed mouth down. Neither can they escape from an open jar placed mouth up; because when they come to the lip of the jar, in order to go farther, they must turn head down to crawl down the outside. But they cannot crawl head down so there they must stay. Moreover, if you put food in the bottom of the jar while they are at the top, they can never crawl down to get it. They cannot turn head down, and they do not have sense enough to crawl backwards.

There is, nevertheless, this difference between caterpillars and plants. If the plant grows up and turns toward the sun at all, it does so always; but the caterpillar changes its nature, and after it has reached the buds and once fed, then the impulse to move upwards and lightward, is no longer useful, and so in a large measure disappears.

Still many sorts of caterpillars keep these willy-nilly turnings, until they are full grown. Our common—our much too common—tent-caterpillar, is accustomed to leave its tent during the warm part of the day, crawl to the tips of the branches where its food is, eat until the cool of the evening begins, and then return to its tent. In no sense, however, does it go after its food, knowing what it wants. During the warm part of the day, it simply becomes like a plant stem, head up, and crawls. It has to head up, and it has to crawl. So in the end it reaches its food, but it doesn’t know anything about how it gets there.

I have often on an early summer afternoon, when the caterpillars are getting restless and just ready to start out, taken the tent, inhabitants and all, and put it on top of a post or a smooth rock. The caterpillars being disturbed, at once start to crawl away. They start in all directions. In a moment, of course, they find themselves crawling head down. That being against the rules, they turn and crawl up again. In no possible way can a single caterpillar get off the top of that rock or post, until the regular time for them to knock off eating and go back to the tent. Then they have to crawl down; and cannot crawl up if they try. So the chief difference between tent-caterpillars and plants, is that while the plant always turns its root down and its stem up, the animal turns its whole body down at certain times of day, and turns its whole body up at certain others. One can hardly say that either has any more sense, or intelligence, or knowledge, than the other.

All caterpillars, while they remain caterpillars, have to crawl toward the light. All caterpillars, also, after they have changed into butterflies and moths, when they fly, have to fly toward the light also. That is why the swarms of moths collect around the arc lights on the streets, fly into lighted rooms thru unscreened windows in the evening, circle about the reading lamp, or if the light chances to be a candle with an uncovered flame, fly into it and are burned.

People will tell you that the moth is curious, wants to see what the light is. But he isn’t; any more than the leaf is curious to look out of the window to see what is going on in the street. Both alike simply turn toward the brightest light. The moth, having turned toward the light, when he flies, flies toward it. If the leaf could fly, it also would fly into the flame and be burned.

The reason why moths only fly into the lamp is that they are about the only insects that fly at all while the lamps are lighted; most other winged insects, also, head toward the brightest light. So do vast numbers of other small animals, snails and crabs and various water bugs, earth worms, leeches, infusoria, and even minute fishes just hatched out of their eggs. But older fishes and all the larger animals with fur and feathers, have more sense. They go where they please and turn any way they like just as we do.

Many small animals, on the other hand, are like plant roots. They have to head according to the light, but they head away from it, and so move toward the darker places. In fact, it is rather the rule for the young insect, before it gets its wings, to burrow like a root and turn away from the light, but to turn toward it later after it gets its wings.

Perhaps the strangest fact of all is that some water animals which ordinarily head away from the light, turn round and head toward it, as soon as a little acid is added to the water. Alcohol, even common soda water or ordinary salt, has the same effect. But some salt water animals which normally head lightward, if put into slightly fresher water, promptly turn tail to the light. All of which shows that the creature himself hasn’t much choice in the matter, and probably doesn’t know much about it anyway, any more than if it were a plant.

These turnings of plant or animal, toward the light or away from it, up or down, the heading up-stream of many fishes, and the necessity for crowding into cracks and corners of many insects and other small creatures, all these are called “tropisms.” Tropism is merely the Greek word for turning. I tell you the name, because we human beings who have speech, if we want to think about a matter, have to have a name for it to act as a handle for our minds to take hold of.

We see, then, that the various sorts of living creatures which we have met thus far in this book, tho they are all made of much the same sort of living jelly, have really quite different sorts of minds. We ourselves, as you know, have reason, speech, intelligence, feeling, and instinct. The animals most like ourselves, dogs and cats and horses and the like, have also intelligence, feeling and instinct. Animals very different from ourselves such as fishes, insects, and the various strange sea creatures, have some intelligence, some feeling, a few strong instincts; and besides these, certain tropisms. But the simplest animals of all, and the plants, have neither intelligence nor instinct, but only feelings and tropisms.

All living things, then, plants and animals alike, have feeling. I have already explained something about instincts and tropisms; and told you, if not much about intelligence or reason, at least something about speech. Now I shall tell you something about the one thing which all living things have in common, and that is feeling.