Of all the wonders of the universe, the most striking, perhaps, is the spectacle of life itself – life in a tiny insect, in a lumbering elephant, in a giant sequoia tree, in man. Just what if life? There is no direct answer. We know, of course, that some things are living and others are nonliving. A man, a lion, a fish, an oak tree and a rose brush are certainly alive; rocks and icicles and such man-made objects as tables and steel pillars are just as certainly not alive. But some things are not easily classified.
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If we leave a loaf of bread and a bar of iron exposed to the open air, mold will form in time on the bread and rust on the iron. How do we tell whether the mold and the rust are living or nonliving? We cannot simply say that they are alive because they are “like men or trees,” or that they are not alive because they are “like rocks or icicles.” If we only knew in what respects living things differ from nonliving things, we could classify a mold or rust as living or nonliving. Actually, we know a good deal about such differences.
One important distinction is that practically all living things are made up largely of a complex substance called protoplasm, which is arranged in units known as cells. To study protoplasm is to study life; all the activities of living things take place in the substance. However, certain organisms called viruses do not have protoplasm and cellular structure; yet they are classified as living things. We discuss them later in the article.
The second point of difference between living and nonliving things is that living things display irritability. By this, we do not mean that men, or lions or trees are hot-tempered; we simply mean that they respond to changes in the environment.
Let us suppose that a grain of sand and a seed lie buried side by side in the ground. The grain of sand may remain there indefinitely, or it may be brought to the surface by some animal – an earthworm, perhaps. It may be exposed to heat or to moisture or to cold; but it will still remain a grain of sand. What of the seed? It, too, may lie inert for a time. But suppose its environment begins to change; suppose the earth is warmed by the sun and moistened by rainwater or melting snow. The seed will respond and it will begin to sprout. Before long, its stem will push its way up out of the ground; in the course of time, the tiny stem will become a tree. The seed, unlike the grain of sand, has displayed irritability; it has reacted to environmental changes.
Irritability is expressed in movement. Of course, movement is by no means confined to living things. The water in a river moves, yielding to external forces; gravity causes it to flow from a higher to a lower level. External forces also bring about movement in living things. When a parachute jumper leaps from an airplane, gravidity will act upon him just as surely as it will upon the water in a river. This is not what we mean by movement as an expression of irritability movements as the swelling of the germ within the seed in response to sunlight or the closing up of blossoms that go to sleep at night.
Living things respond to an internal stimulus as well as to external forces. When a child hastily withdraws his hand after touching a hot stove, it is because of an impulse from within – an impulse transmitted from the injured area along the nerves to a reflex center and from this to the muscles of the arm. There is nothing corresponding to this in nonliving things.
More about the living and nonliving world is going to be further discussed in part 2. So, until then, stay safe!