FOUR years ago, when I brought a snake into the living room and put it down to observe its way of locomotion, my natural feelings toward it were tempered somewhat by the view of the evolutionist. That view is that the reptile is the ancestor of the bird. Therefore, a scale is a scale, whether upon the body of a bird or a reptile; and anyone who shrinks from contact with a snake had better be careful not to touch the legs of a chicken.
In this regard, every little bird upon the lawn is standing up to its shanks in its own reptilian ancestry; and every little bird in its nest, opening its mouth so astonishingly wide to receive the worm that you feed it, is confronting you with the reptilian gape. Bird and snake have each the reptilian quadrate bone whose function is to throw the jaw open at the back as well as at the front; and thus it is that the snake can engulf its prey and the pelican find room to swallow a fish; and the mother robin, opening her jaws like parallel pliers, instead of like the human jaw, is enabled to take home many pieces of angleworm with no danger of dropping them. In these and many other ways the bird is ‘a glorified reptile,’ and the snake is an ancient cousin of the canary.
From the reptile to the bird would seem to be the farthest call in nature. They are extremes of the most unlike. But the common sentiment in favor of the bird is a view that hardly goes as deep as the feathers. If one were to make a list of all the reptilian qualifications of birds, as Launce did of the virtues of his prospective wife, the catalogue would stand about as follows: — Item. Birds and reptiles both have scales.
Item. They both lay eggs.
Item. The little chick in its shell has a flintlike end on its soft bill wherewith to peck its way out into the world; the little snake has the same; and in both cases this little tool is discarded after it has made that single hole.
Item. Among its characteristic structures the bird has a gizzard. So has the alligator.
Item. The little bird forming in the shell has a tail like a lizard and other reptilian features.
Item. While the bird has a fourchambered heart, the great systemic artery, originating in the left side of the heart, makes a bend over to the right, a feature that is not found in other four-chambered hearts. Any comparative anatomist, familiar with the pipe lines of life, can see at once how the bird’s heart has been altered from that of the reptile in order to give it pure oxygenated blood, advantageous in flying.
Item. The proud American eagle, which, although it prefers to live on fish, does not catch its own (for which reason Benjamin Franklin so strongly disapproved of it as an emblem), is not considered by European naturalists to be a true eagle. It is rather an erne. A distinguishing difference is that the true eagles are feathered to the toes, whereas the American or bald eagle has its legs bare like a chicken. Between these extremes, other birds wear their leg feathers at different heights, and the extra feathering can be bred in, as in pigeons. By this encroachment of the feathers down the leg we may see the scales in metamorphosis; and as the scale or young feather frays out and goes through its stages we may see the process by which the scaled creatures of the past became the feat hered creatures of to-day.
Item. We might set aside all such considerations and simply take a look at the Archæopteryx, a lizard-like bird which was found, with all its delicate tracery complete, in the lithographic limestone of Bavaria. The tail, with twenty vertebræ and a pair of feathers growing from each, trailed behind and balanced it like the tail of a kite. While it was feathered, it had the mouth and teeth of a lizard; and it resembled a present-day bird, the hoaetzin, in having claws on the wings for grasping a branch or handling prey. And the skeleton had a feature which no animal but a bird has — the merrythought, or wishbone. Anyone who loves birds and sets his heel upon the snake might here ask himself a question. If he were to see an Archæopteryx in the orchard, would he go into ecstasies over its feathered beauty, or would he rather run from it as from that most horrible of imagined things, a winged reptile?
Item. The snake, like the bird, is the descendant of a lizard-like form. The snake is a former quadruped; it is not a decadent or primitive type, but one that is highly specialized. Like the whale, which has small hip bones and legs buried deep beneath its fat in memory of the days when it was a land animal, some snakes, the boas and pythons, have the hip girdle still remaining; and in one kind the legs show themselves outwardly in the form of spurs. The snake is a quadruped that has come completely down off its legs. It is little more than a specialized spinal column; and the wonder is that it can do everything necessary, and do it so deftly, after having thrown away nearly all the conveniences of other animals. It was this that made Solomon marvel. ‘The way of a serpent upon a rock’ was to him one of the few special wonders of the world.
As I was about to say, this problem of locomotion interested me; and, having studied snakes out of doors with little result, I caught one and brought it into the house. I put it down on various surfaces — first upon a large rug, then upon the polished oak floor when the rug was taken up, and finally upon a glass door of my bookcase which I took off its hinges and rested horizontally between two chairs so that I could lie beneath and look up. And then, as I did not seem to get at the secret of snake machinery, I put it in a cage and kept it for repeated observation. I did not know how a snake worked. And I had become satisfied, through the reading of many books, that no one else knew.
Although I made many experiments in the course of four years, and devised some apparatus to bring forth facts, nothing that I did in this way proved so interesting as did the voluntary doings of the snake itself. I soon discovered that the most scientific procedure was to wait and see what the snake would do next; and that the one great experiment consisted in having the snake under constant observation. And a snake in the hand is worth any number of snakes in theory.
Of all the things that animals have taught me about themselves, the fact that a snake can walk a wire was the most surprising. Not by training or any sort of suggestion on my part, but naturally; and with that flowing grace and consummate ease which distinguish anything a snake may do. The wire in the case was the shortest distance between two points; and the snake, an animated line of beauty, was simply engaged in going from here to there.
When you consider that a snake virtually skates through life, being thereunto provided by nature with a belly as smooth as ice, it would hardly seem fitted for such tricky footing. But there is no mental difficulty to be dealt with as a snake approaches the subject. When it encounters a wire it simply flows out on it with its head weaving from side to side; thus it distributes itself along the length with every few inches of its body falling alternately to this side of the wire and to that. These downward curves of the body are but slight — merely enough to put more of the weight below the wire than there is on top of it. Thus draped along, with five or six curves on each side, the snake is really hanging from the wire, though one would say that it was poised on top of it. The weight on each side must be equal. It now goes along, not by shortening and lengthening itself or changing the curves, but by flowing through the curves of its body as thus accurately distributed.
Anyone who has watched a snake winding its way through the grass or on the ground must have noticed how the head, darting from side to side, marks out the path. On a wire the regularity of this motion is brought to the attention; and we now see that the head, instead of merely finding a path through obstructions, is striking out curves upon some necessary principle of locomotion. Gracefully it veers from side to side and marks out a measured pathway through the air; and through that very course each part of the body comes flowing along. It is done as easily as an old-fashioned writing master, with his lined copy book before him, would conduct a swelling and flowing scroll along the printed track.
Before the snake walked the wire, which I was kind enough to provide, it had demonstrated its ability to do so by getting up on the edge of ordinary fly screen, of which its circular pen was made, and going round and round on that. I left the top of the pen or cage open for easy access; and, since the screen was fourteen inches high and the snake only eighteen inches long, it was not quite able to get over the top. But it had not been long in confinement when it discovered that it could get out by taking advantage of a squat custard cup in which I kept its water, and in which it liked to lie coiled for hours at a time. When the cup happened to be set close to the screen, the snake would prop its length on top of the flat handle, and, standing up stiffly, would get the couple of inches of added stature necessary to hook its chin over the edge. After several escapes, one of which involved a long search and the final discovery of the snake in a bread pan, I was more careful to set the cup at a point where the snake could not use it as a stepladder.
For people who like mechanical puzzles, I would suggest a study of the method by which a snake is enabled to flow over the edge of a wire screen.
First it gets its chin hooked over. On one side, now, is its head; on the other side hangs the whole length and weight of the body. Then, without any wiggling or climbing or working of evident mechanism, it seems to defy the law of gravity by flowing over the top. It is as easy as if the head were made of lead and bore the other part down. The working of the broad scales or scutes upon the thin edge could hardly do it, because no two scutes are in contact at one time. They cannot get the necessary alternate motion, one catching and holding while the other is brought forward for further progress. The working back and forth of one slippery scale would not result in advancing the snake against the pull of its weight. And besides, there was no movement of the scales evident. Problem: How was it done?
One may experiment all he wishes, but in the end he has got to reason. I finally came to my own solution of the matter. Instead of the live body of a snake, let us consider an iron hook — a very smooth one, bent upon the end of a smooth and slippery rod. When hung over the edge of the wire screen, the rod would naturally, because of its weight, be supported at the deepest point in the hook. Now, if the rod or bar had the live faculty of shifting the point of curvature progressively down its length, it would automatically slip on the edge of the screen and finally transfer its whole weight over, and fall on the other side. As the snake changes its point of curvature, mere gravity causes the edge of the wire screen to slip to the deepest part of the curve, and locomotion is achieved with no mechanical contact other than that of a slippery surface. Observe as closely as I might, I could see no other explanation; and I submit this as a possible contribution to recent studies in snake locomotion.
We shall learn more about the snake as it performs upon my finger — a support somewhat thicker than the edge of the wire screen. I take up the cage and pour the snake out on a rather broad window sill. The snake immediately turns toward the light and stops with its nose pressed against the pane. As its head is raised an inch or more on account of the wooden frame, there is a V-shaped vacancy under its throat; into this I slip my finger. I lift slowly, whereupon the snake hooks its head over the finger and holds on while it hangs at full length. It wishes to get down and go on its way; but it does not let go and drop, as one might expect. It continues onward over the finger. Here the alternate working of the belly scales or scutes might come into play; it seems logical that they would; but I have never been able to feel such working even with a three-foot snake.
The snake expects to continue on over and be on its way; but when it has gone over nearly all its length I raise my hand so that the sill is out of reach, whereupon the snake gives its slender tail a turn or two around my finger and firmly puts on the brakes. A snake can stop at any point, hanging like a hook; and it flows over at even speed through any part of its progress, which shows that the running-hook action, which I have described, here comes into play. If, while it is hanging head downward almost full length, I tempt it by letting it down nearer the sill, I can feel the slender coils loosen while the snake drops a little — it may be a small fraction of an inch. In t his use of its tail it has a brake band like that of an automobile, and it has admirable control. If, now, it finds that it has let itself out as far as possible and still cannot get to the sill, it will turn about, doubling on its length, and come up and rest upon my hand. In any case, it will not drop. It must be remembered, though, that I am speaking of snakes that were not frightened. A snake that is panic-stricken will throw itself instantly from a height.
When I speak of a snake’s tail, I refer to a definite part which is as distinct from the body as is the tail of a quadruped. On the under side of the body are scales or scutes extending the full width of the belly, and they are capable of working back and forth to some extent, their ends being attached to the ends of movable ribs. These scutes terminate at a point which marks the end of the body, which is also the end of the intestinal canal, as in a quadruped. From that point on, the bottom of the snake has small scales like the top, so that, when you view a snake from below, the tail is plainly in evidence. It is prehensile like the tail of a New World monkey, and the snake uses it in the same way. This feature marks the snake as a creature of arboreal skill. We regard the snake as a creature built close to the ground and forget its ability to make its way about in trees and bushes, and to live, as many do, upon the eggs of birds. In thick patches of mesquitc and like bushy growths a snake may take to flight right through the maze of branches; and it is a common sight, on sunny banks, to sec a bush draped full of snakes taking their Falstaffian ease.
A snake’s disinclination to let go and fall came to my attention every time my snake succeeded in getting up on the edge of the tall cage. In that case it would look about for the next means of escape, and it was this endeavor that brought me the demonstration of wire walking. The cage was set in front of a window, near which was a sewing machine with a box on it; and between the two was a longhandled broom leaning against the wall. The snake, mounted on the thin edge of the screen, and going round with its skillfully balanced curves, would stop and reach out almost to the handle of the broom and over toward the box on the machine. It evidently wanted to make the crossing, and was in need of a path. I moved the machine farther away and provided a path consisting of a piece of wire from the cellar. The wire, three sixteenths of an inch in diameter, was somewhat rough and rusty. Whether the snake could have gone across so well on a path as shiny as a knitting needle might be open to doubt; but from what I have seen I am prepared to give a snake a great deal of credit.
The snake is not the only reptile that is legless. There are lizards of serpent shape, notable among them being the so-called glass snake of the United States and the slowworm of Europe. A notable difference between a snake and a lizard is that the lizard has eyelids. The snake’s eyes are covered with insensate epidermis of glasslike transparency, like the crystal of a watch. It thus makes no difference to a snake if it happens to hit its eyes on the side of a narrow hole; it has a protection much better than eyelids, which must do their work by opportune blinking.
When a snake sheds its skin, this covering of the eyes turns milky white and shuts off vision to the extent that the snake is temporarily blind. But the blind snake has a skill that serves it; and if you throw a fishworm into the cage the blind snake’s ability will soon become manifest. When it is not blind, a snake will watch the worm awhile to make sure that it is alive, and will then snatch it with a move as quick as lightning. But when the snake is blind it examines the worm by putting its chin on top of it and feeling about. Then it seizes the worm by the middle or any part of its length, and it works it along till the end is reached.
Sometimes a worm, because of its ability to stretch out and become thinner in any part, will get itself partly through one of the small openings in the screen, — a fishworm can easily go through a hole half its size, — and then, swelling up on the other side, it has a great advantage in a tug of war. In this case the snake will brace itself and pull until it has won the victory. It is as determined in the work as is a robin bracing itself against a worm that does not want to come out of its hole. This persistent battle with a stubborn worm is the most birdlike thing I have seen a snake do.
According to the most accepted authorities, a snake gets ahead by either of two methods. If it is in a hurry, it takes hold of the ground with the forward part of its body by raising the edges of several of the broad scales or scutes upon the belly, and while these prevent it from slipping backward the rest of the body is brought forward in a series of lateral curves. Next, the scales at the rear of the belly engage the ground while the body is being straightened out; and by these two operations it makes a complete step forward. If the snake is not in a hurry it gets ahead by the scutes alone, working them back and forth all along the belly and pushing against the ground with the raised edges so that there is a constant movement forward.
All this looks as reasonable as it sounds. The information would seem to come straight from the tree of knowledge. But after spending much time in following snakes that were undoubtedly in a hurry I have failed to see any of this pulling together and reaching forward by means of opening and shutting the curves. The body of the snake flows forward along the same curves, like a train on a winding track or a stream within its banks.
With the encyclopædia bearing witness to the contrary, a man of modesty will naturally distrust his own eyesight; and he will wish for an instrument of precision with which to come closer to facts. I was about to devise such an instrument when it occurred to me that I had one all ready for use. It was only necessary to follow a snake through the grass, and especially through June grass, to get a safe answer to the question. In a meadow closely grown with the tall, straight, delicate stems of June grass, a snake, changing its curves, opening and shutting like a collapsible hatrack, would necessarily push the grass aside and bend it over; and the tops would make the slightest movement visible. But a snake will flow right through a thick maze of June grass and hardly disturb a stem.
A host of books will tell you just how a snake operates, some going into the authoritative particulars, and others saying quite easily that it is done by ‘a sort of wriggling movement.’ A man who willingly takes words for knowledge would soon be enlightened upon the subject, for on paper there seems to be no difficulty whatever. But, as Galileo said, the world we are living in is not a world of paper.
Thinking it over, one would suppose that this method of getting ahead by opening and shutting the curves would be impractical, if not impossible. In drawing itself together to make another reach forward, the body of the snake would be sliding sidewise against obstructions all along the curves; it would be pushing against stones on open ground and moving them aside or crowding them together; and in grass it would be pushing over and flattening a swath equal to its whole width of travel. This would take much force, especially in making speed. It is doubtful whether the shallow scutes would catch and hold powerfully enough to move against such obstacles. A snake thus operating would seem to be a most inept and ill-adapted piece of machinery.
Next we take up the theory that a snake can move forward in a straight line by the use of the scutes alone; and we wish to know what a snake actually does when it cannot move in curves. In a state of nat ure we should expect a snake to move in this way through long passages or holes no wider than itself. I therefore built two tunnels twelve feet in length and of a width exactly to fit the girth of two garter snakes, one of them thirty-five inches in length and the other eighteen. This tunnel was made of smooth pine and was roofed over with wire screen so that the snake’s motions could be observed. I must confess that it is not easy to take hold of an unwilling snake and insert its whole length into a groove into which it does not wish to go; but with the assistance of Mr. August W. Pollworth, of Milwaukee, a friend who caught snakes with me when we were boys together and who later became not only tolerant of snakes but actually fond of them, the trick was done, and I put the snakes through many courses along this straight and narrow way.
What was my surprise to find that in such a place a snake does not go straight ahead, and that it does not have to. It takes hold with the forward part of its body while the rest of its length is pulled ahead into a large number of kinks or curves all touching the sides of the tunnel. In other words, it makes all the curves it can in the spare space at the sides of its tapering form. It then takes hold in the rear and straightens out all these curves, thus making a definite advance with each step. The whole process is beautifully reduced to slow motion in such a place. The alternate pulling together and reaching forward would be well described in the darky phrase ‘inching along.’
Thus the snake in strait quarters uses the very method that has been described as useful in making speed, and which it evidently does not use when it is out in the open and in a hurry. It also becomes evident that, as a snake tapers from its middle girth toward both ends, it never, in a state of nature, gets into a place so narrow that it cannot use its kinks or curves as a means of taking steps. There is always room for it, inevitably; and I have never been able to get a snake in a tunnel to advance in any other way.
Certainly a snake can raise the rear edges of its scutes; and it can move them forward and back to some extent. When you place a snake on a large sheet of glass and look at it from below, the movement as the snake struggles to get ahead on the smooth surface might be described as a shrugging of the belly. The movement cannot be seen from above, because the movable ribs, to the ends of which the scutes are attached, slide back and forth beneath the skin like pairs of crutches underneath a cloak. We have therefore been told that a snake ‘walks on the ends of its ribs’ — a rather poor description. But, as it certainly has this mechanism, it logically ought to be able to move ahead on its scutes, either in a straight line or in curves, as by a multiplicity of little steps. If it does not make haste in the open by alternately pulling together and straightening its curves, it must be wholly dependent upon the mechanism on its belly, though this would seem to be a difficult way of making speed.
Assuming that it does, a question now arises, and a most pregnant one. A snake using the belly mechanism ought to be able to move ahead on a straight line. Why, then, does it always travel in wide curves? It is as if a man were always to follow a roundabout course instead of going straight to the object; or as if a railway track were always built crooked where it might be straight. Certainly, if a snake were to straighten out, its scutes would carry it much farther with the same number of steps; it would t ravel at least twice as far with the same amount of effort. Why, then, does it always travel in curves? If a snake can ‘walk on the ends of its ribs,’ why does it not do so and go straight to its destination? To follow a curved route does not fit in with the economy of Nature, faulty as her works often are.
Ruskin was greatly puzzled to know how a snake traveling by the movement of its scutes, which he assumed to be its method, could make such speed with steps so very short. He said that the locomotion of a snake ‘seems to involve a motion of the scales too rapid to be conceived.’ As a mechanical problem he had to give it up; and he describes it inspirationally as ’one soundless, causeless, march of sequent rings, and spectral processions of spotted dust, with dissolution in its fangs, dislocation in its coils.’ This is a beautiful group of words, but Ruskin can hardly be considered to be speaking as a man of this world. Owen said that a snake can ‘outclimb the monkey, outswim the fish, outleap the zebra, outwrestle the athlete, and crush the tiger.’ True enough. But how does it perform the ordinary, everyday function of moving from place to place?
The reader who has hopefully followed me thus far must now be prepared to have me say what I might have said sooner. I could not find out how a snake gets ahead. Rather I came to have a thorough and well-founded ignorance upon the subject. Besides having various snakes in my study for nearly four years, I at last took into my hands seven varieties of snakes in a traveling snake pit, and the whole result of my experiment and speculation was that I had just begun to find out how much I did not know. One snake, a garter snake eighteen inches long, which finally came to be known as Pete, I had for a year and a half; and then I had to release him in the fall because the dry season brought such a scarcity of angleworms that I could not lay in his store of winter food. This snake kept active all winter instead of hibernating, and shed his skin three times each summer.
Quite recently — less than two years ago — the University of Michigan completed some experiments intended to solve the problem of snake locomotion. This means, of course, that it had not been solved — a piece of reliable news that was quite gratifying to me. When a man thinks he does not know a thing it is very pleasing to him to find out that he is right. But now the problem has been solved, and there is a large body of information upon the subject that has not yet been published.
While I might be able to give an idea of it through studying an abstract of a lecture given by Dr. Walter Mosauer, of the University of California, I shall not attempt to put it formally in words, for reasons which the reader will appreciate. The snake, like other things in life, is complex in its simplicity. Let us suppose that I were to undertake to explain, in strict engineering detail, how a slippery plum seed pops forth from the pressed-together fingers. A diagram in demonstration of the forces would require arrows to show the direction of pressure of the finger and thumb upon the opposed sloping surfaces, and yet other arrows to show how these forces were translated into forward movement. It would be an engineering problem requiring one to knit his brows, whereas we all know that a plum seed just snaps forward naturally when pressed.
It is somewhat the same with a graph illustrating the forces acting upon a snake’s progress. With its body curves and the sides of its smooth, flattish belly, it is always pressing sidewise and slantwise on little stones or hummocks or stems of grass, and constantly slipping past. That is what the wide curves of the body are for — to give the angle of incidence upon the passing object. It is popping forth from between these things on either side of it somewhat like the plum seed; but, instead of a sudden impulse, the snake’s head is striking out new curves ahead and the body is following along with a constant outward pressure and a constant slip forward. Simple as it might look, it took astute mechanical insight and careful experiment to solve the problem — a thing I should probably not have been successful with in my experiments at home. When I learned that the University of Michigan had taken hold of the problem and worked it out in engineering fashion, I had a feeling of personal triumph. I did not know how a snake worked, and I continued to insist that I did not know in spite of all the books and all the words. And that is a sort of intellectual feat in itself.
In the Age of Reptiles, Nature produced some astonishing examples of animal design. There was the fiftyton beast of the field whose stature was such that it ‘could easily have looked over the top of a four-story building.’ On the same scene were the carnivorous cousins running on two legs for the advantage of speed, and seizing their prey with sawlike teeth a foot in length. There were flying reptiles which, from running so fast on two legs, had learned to glide and soar and to fly on a spread of batlike wings that were twenty-eight feet across. All of them, little and big and of great variety, were cold-blooded creatures that grew more active in the heat of the sun; and they laid eggs of a size proportionate. It would seem as if the sun-warmed earth, with all its celestial appurtenances, had been made with the special needs of reptiles in view.
Of all these reptilian creatures, some of which had already risen from the mud to a mastery of the air, we still have with us the turtles, the lizards, the crocodiles, and the snakes, belly-going creatures that walk so humbly that they seem to swim on land. But there are exceptions to these earth-bound appearances. The snake can do everything but fly. The alligator, when excited and really in a hurry, will stand up and surprise one with its abilities as a two-legged runner. It remembers when it was a dinosaur, a creature that went on two legs like a bird. But, of all the descendants of that experimental age, the bird of the air seems the most successful. When one views its specialized performances, the foundations of which were put down in that egg-laying age, it would seem that the Ruling Reptiles had not lived in vain.