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.
XXV
The Five Senses and The Other Five
Traditionally, of course, we have five senses—sight, hearing, touch, taste and smell. Yet we sometimes say that we are “frightened out of our seven senses,” as if there were seven and not five. Really, the number of our different ways of feeling is neither five nor seven, as we shall now see by counting them up for ourselves.
Five at least we are sure of, the traditional five. There is no need for anyone to tell us what our eyes, ears, and noses are for; nor that we taste with our mouths as well as talk and eat, nor that we feel touches anywhere over our skins. As to this last, however, I don’t think we always realize how completely the sense of touch is confined to the skin. Headaches, for example, we feel on the outside of the head; the brain itself can be pinched, cut, burned, and generally maltreated, and we not feel it so much as we feel a pinprick on a finger tip.