WHAT IS EVOLUTION?
(From The Atlantic Monthly, March, '93.)
By PROFESSOR E.S. HOLDEN.
I was once trying to tell a boy, a friend of mine, what the scientific men mean by the long word Evolution, and to give him some idea of the plan of the world. I wanted an illustration of something that had grown—evolved, developed—from small beginnings up through more and more complicated forms, till it had reached some very complete form. I could think of no better example than the railway by which we were sitting. The trains were running over the very track where a wagon-road had lately been, and before that a country cart-track, and before that a bridle-path, and before that again a mere trail for cattle. So I took the road for an example, and tried to show my boy how it had grown from little things by slow degrees according to laws; and if you like, I will try to tell it again.
Just as one can go further and further back, and always find a bird to be the parent of the egg, and an egg to be the parent of that bird, so in the history of this road of ours; we may go back and back into the past, always finding something earlier, which is the cause of the something later. The earth, the planets, and the sun were all a fiery mist long ago. And in that mist, and in what came before it, we may look for the origin of things as they are. But we must begin somewhere. Let us begin with the landscape as we see it now,—hills, valleys, streams, mountains, grass,—but with only a single tree.
We will not try to say how the tree came there. At least, we will not try just yet. When we are through with the story you can say just as well as I can.
Suppose, then, a single oak-tree stood just on that hillside thousands and thousands of years ago. Grass was growing everywhere, and flowers, too. The seeds came with the winds. Year after year the oak-tree bore its acorns, hundreds and hundreds of them, and they fell on the grass beneath and rolled down the smooth slopes, and sprouted as best they could,—most of them uselessly so far as producing trees were concerned,—but each one did its duty and furnished its green sprout, and died if it found no nourishment.
All the hundreds of acorns rolled down the slopes, Not one rolled up; and here was a law,—the law of gravitation,—in full activity. There were scores of other laws active, too; for evolution had gone a long way when we had an earth fit to be lived on, and hills in their present shape, and a tree bearing acorns that would reproduce their kind. But ever since the fiery mist this simple law of gravitation has been acting, binding the whole universe together, making a relationship between each clod and every other clod, and forcing every stone, every acorn, and every rain-drop to move down and not up.
Just as this law operates,—continuously, silently, inexorably,—so every other law makes itself felt in its own sphere. Gravitation is simple. The law according to which an acorn makes an oak—and not a pine-tree is complex. But the laws of Nature are all alike, and if we understand the simple ones, we can at least partly comprehend the more complex. They are nothing but fixed habits on a large scale.
So the acorns fell year by year and sprouted; and one out of a thousand found good soil, and was not wasted, and made a tree. And so all around (below) the tree with which we started there grew a grove of oaks like it, in fact its children; and finally the original trees died, but not without having left successors.
First of all, the green hillside is smooth and untrodden. There is nothing but grass and flowers, borne there by the winds, which leave no track. There is no animal life even in this secluded spot save the birds, and they too leave no track. By and by there comes a hard winter, or a dearth of food, and a pair of stray squirrels emigrate from their home in the valley below; and the history of our hill and its woods begins. Mere chance decides the choice of the particular oak-tree in which the squirrels make their home. From the foot of this tree they make excursions here and there for their store of winter food,—acorns and the like,—and they leave little paths on the hillside from tree to tree.
The best-marked paths run to the places where there are the most acorns. A little later on there are more squirrels in the colony,—the young of the parent pair, and other colonists from the valley. The little tracks become plainer and plainer.
Later still come other wild animals in search of food,—squirrels will do. The wild animals do not remain in the colony (there are too few squirrels, and they are too hard to catch), but they pass through it, sometimes by day but oftenest by night.
You might think it was perfectly a matter of chance along which path a bear or a wolf passed, but it was not. He could walk anywhere on the hillside; and sometimes he would be found far out of the paths that the squirrels had begun. But usually, when he was in no haste, he took the easiest path. The easiest one was that which went between the bushes and not through them; along the hillside and not straight up it; around the big rocks and not over them. The wolves and bears and foxes have new and different wants when they come; and they break new paths to the springs where they drink, to the shade where they lie, to the hollow trees where the bees swarm and store the wild honey.
But the squirrels were the first surveyors of these tracks. The bears and wolves are the engineers, who change the early paths to suit their special convenience.
By and by the Indian hunter comes to follow the wild game. He, too, takes the easiest trail, the path of least resistance; and he follows the track to the spring that the deer have made, and he drinks there. He is an animal as they are, and he satisfies his animal wants according to the same law that governs them.
After generations of hunters, Indians, and then white men, there comes a man on horseback looking for a house to live in. He, too, follows along the easiest paths and stops at the spring; and near by he finds the place he is looking for. Soon he returns, driving before him herds of cattle and flocks of sheep, which spread over the grassy glades to feed. But everywhere they take the easiest place, the old paths, from the shady tree to the flowing spring. After awhile the hillside is plainly marked with these sheep trails. You can see them now whenever you go into the country, on every hillside.
Soon there are neighbors who build their homes in the next valley, and a good path must be made between the different houses.
A few days' work spent in moving the largest stones, in cutting down trees, and in levelling off a few steep slopes, makes a trail along which you can gallop your horse.
Things move fast now,—history begins to be made quickly as soon as man takes a hand in it. Soon the trail is not enough: it must be widened so that a wagon-load of boards for a new house can be carried in (for the settler has found a wife). After the first cart-track is made to carry the boards and shingles in, a better road will be needed to haul firewood and grain out (for the wants of the new family have increased, and things must be bought in the neighboring village with money, and money can only be had by selling the products of the farm). By and by the neighborhood is so well inhabited that it is to the advantage of the villages all around it to have good and safe and easy roads there; and the road is declared a public one, and it is regularly kept in repair and improved at the public expense. Do not forget the squirrels of long ago. They were the projectors of this road. Their successors use it now,—men and squirrels alike,—and stop at the spring to drink, and under the huge oaks to rest.
A few years more, and it becomes to the advantage of all to have a railway through the valley and over the hillside. Then a young surveyor, just graduated from college, comes with his chain-men and flag-men, and finds that the squirrels, and bears, and hunters, and all the rest have picked out the easiest way for him long centuries ago. He makes his map, and soon the chief enigneer and the president of the road drive along in a buggy with a pair of fast horses (frightening the little squirrels off their road-way and into their holes), and the route of the Bear Valley and Quercus Railway is finally selected, and here it is. See! there comes a train along the track. This is the way a railway route grew out of a squirrel path. There are thousands of little steps, but you can trace them, or imagine them, as well as I can tell you.
It is the same all over the world. Stanley cut a track through the endless African forests. But it lay between the Pygmy villages, along the paths they had made, and through the glades where they fought their battles with the storks.
Sometimes the first road is a river—the track is already cut. Try to find out where the settlements in America were in the very early days—before 1800. You will find them along the Hudson, the Juanita, the St. Lawrence, the James, the Mississippi Rivers. But when these are left, men follow the squirrel-tracks and bear-tracks, or the paths of hunters, or the roads of Roman soldiers. It is a standing puzzle to little children why all the great rivers flow past the great towns. (Why do they?) The answer to that question will tell you why the great battles are fought in the same regions; why Egypt has been the coveted prize of a dozen different conquerors (it is the gateway of the East); why our Civil War turned on the possession of the Mississippi River. It is the roadways we fight for, the ways in and out, whether they be land or water. Of course, we really fought for something better than the mere possession of a roadway, but to get what we fought for we had to have the roadway first.
The great principle at the bottom of everything in Nature is that the fittest survives: or, as I think it is better to say it, in any particular conflict or struggle that thing survives which is the fittest to survive in this particular struggle. This is Mr. Darwin's discovery,—or one of them,—and the struggle for existence is a part of the great struggle of the whole universe, and the laws of it make up the methods of Evolution—of Development.
It is clear now, is it not, how the railway route is the direct descendant of the tiny squirrel track between two oaks? The process of development we call Evolution, and you can trace it all around you. Why are your skates shaped in a certain way? Why is your gun rifled? Why have soldiers two sets of (now) useless buttons on the skirts of their coats? (I will give you three guesses for this, and the hint that you must think of cavalry soldiers.) Why are eagles' wings of just the size that they are? These and millions of like questions are to be answered by referring to the principle of development.
Sometimes it is hard to find the clew. Sometimes the development has gone so far, and the final product has become so complex and special, that it takes a good deal of thinking to find out the real reasons. But they can be found, whether they relate to a fashion, to one of the laws of our country, or to the colors on a butterfly's wing.
There is a little piece of verse intended to be comic, which, on the contrary, is really serious and philosophical, if you understand it. Learn it by heart, and apply it to all kinds and conditions of things, and see if it does not help you to explain them to yourself....
"And Man grew a thumb for that he had need of it,
And developed capacities for prey.
For the fastest men caught the most animals,
And the fastest animals got away from the most men.
Whereby all the slow animals were eaten,
And all the slow men starved to death."
