The Woodpeckers

Any one who has lived in a granite country knows the deep round holes that stone masons make when they split rock. Did you ever wonder why they are as large at the bottom as at the top? If you remember the shape of a mason’s drill, you will recollect that it looks a little like a stick of home-made molasses candy bitten off when it was just soft enough to stretch a little. The mason’s drill is a round iron rod with a thin, flat end, sharpened on the edge and a little pointed in the centre. In the flattening of the sides and the width across the tip its end resembles that of a typical woodpecker’s bill. The woodpeckers that drill for grubs, especially the largest, the logcock and the ivory-billed woodpecker, have the tip remarkably flattened. The likeness to the drill does not go farther because the woodpecker’s bill is a combination tool; but it is drill-pointed rather than pick-pointed.

What is the advantage of this compressed tip? Can the bird pick as well as he could with a sharp point? The bird and the mason reap the same benefit from this form of tool. A sharp-pointed drill would bind in the hole and could neither be driven ahead nor removed without difficulty, but the sharp-edged tool cuts a hole as wide as the instrument. There is, of course, some difference between working in stone and in wood, but the principle is the same. The mason strikes his drill with his hammer and cuts a crease in the stone; then lifts and turns the drill, cutting a crease in another direction; and so by continually changing the direction of the cuts until they radiate from a centre like the spokes of a wheel, he finally reduces a little circle of stone to a powder fine enough to be blown out of the hole. In drilling for a grub the woodpecker must do much the same thing. He wishes to keep his hole small at the top so as to save work, yet it must be large enough at the bottom to admit the borer when nipped between his mandibles; therefore he needs an instrument that, like a drill or a chisel, will cut a straight-sided hole. Indeed, we might call it a chisel just as well if it were not a double-wedge instead of a single wedge and if it did not move when it is struck instead of being held stationary beneath the blows.

When he is digging his house the woodpecker uses his bill as a pick-axe. When he is digging for grubs he uses it as a drill. Now some species drill very little and some a great deal, according to the number of grubs they feed on; but all dig holes to nest in,—that is, all use their bills as picks but only a few employ them as drills. The flickers, for example, seldom drill for grubs, their food being picked up on the surface or dug from the earth; yet they excavate the deepest, roomiest holes made by any woodpeckers of their size; they use their bills effectively as pick-axes, but seldom, very seldom, as drills. And what do we find? No drill-point—not a truncate, compressed bill fit for drilling, but a sharper, pointed, rounded, curving bill. Notice the ordinary pick-axe and see how much nearer the flicker’s bill than the logcock’s or the ivory-billed woodpecker’s it is. Why is a flicker’s bill better for being curved also? Why do the drilling woodpeckers have a perfectly straight bill? We should find by studying the birds and their food that there is a direct relation between the shape of the bill and the amount of drilling a woodpecker does; that the grub-eating or drilling woodpeckers have a straight bill, for working in small deep holes, while the flickers have a curved bill for prying out chips. And we should note that the flicker’s bill is most like the ordinary bill of perching birds, while the drilling bill, as typified by the logcock’s and the hairy woodpecker’s bills, is a more specialized tool, limited to fewer uses, but more effective within its limits.

There is another detail of the woodpecker’s bills which casts light upon their habits. The species that drill most have their nostrils closely covered by little tufts of stiff feathers, scarcely more than bristles, which turn forward over the nostril. The density and the length of these tufts agree very well with the kind of work the woodpecker does; for in the hairy and the downy, which are continually drilling and raising a dust in rotten wood, they are very thick and noticeable, while in the red-head and the sapsucker they show as scarcely more than a few loose bristles, and in the flicker they barely cover the nostril. This seems a plain provision to keep the dust out of the bird’s lungs; and we might cite as additional evidence the fact that the only other birds of similar tree-pecking habits, the nuthatches and the chickadees, have their nostrils protected in the same way. But we must always be cautious before drawing inferences of this sort to see what may be said on the other side. When we recollect that the crows and ravens and many kinds of finches, among other birds, none of which dig in the bark of trees or raise a dust, have their nostrils as completely covered, we see that we have perhaps discovered a use for these nasal tufts but not the cause of their being there. We must be careful not to mistake cause and accompaniment in our endeavor to explain differences in structure.

Let us see what we have learned and how to interpret it:—

That the woodpecker’s bill is a combination of drill and pick-axe.

That the shape varies with the use to which it is most commonly put.

That the use varies with the food principally eaten; or, what is a step farther back, that the different kinds of food must be sought in different places and by different methods, and therefore require different tools.

Therefore the shape of the woodpecker’s bill has a direct relation to the kind of food he eats. Please notice that we do not assert that it causes him to eat a certain kind of food nor that a certain diet may not have affected the shape of the bill, causing it to be what we now see. Both may be at least partially true, but to prove either or both would need profound study, and all that we have observed is that the shape of the woodpecker’s bill is adapted to his food and that it varies with the kind of food he eats, or, to be more exact, with his ways of procuring it.

XII

THE WOODPECKER’S TOOLS: HIS FOOT

We have studied the woodpecker’s bill and have found that it is a very serviceable tool. We shall find that his feet are equally well adapted to their work.

Here is the foot of a woodpecker. Observe how it differs from a chicken’s foot, or a sparrow’s foot. What is it that especially fits it for climbing? Perhaps you will notice that the tarsus is short, and you may be able to explain why it would be a disadvantage for a climbing bird to have long legs, as well as why it is a help for him to have long toes. Toes long and legs short is the rule with the woodpeckers.

I never see a woodpecker’s foot without thinking of an iceman’s nippers with their short handles and long, sharp-toothed jaws. They are designed for similar uses,—to lift heavy weights by laying hold of smooth, flat surfaces. The iceman sets his nippers into the ice and lifts the block; but the bird sets his claws into the tree and lifts his own body.

Suppose the nippers had one short jaw and one long one, would they then take as firm hold as they do with jaws of equal length? In perching birds the hind toe is much the shortest, but they sit balanced upon a limb and have merely to hold themselves in position. The woodpecker climbing a tree-trunk is out of balance; he would fall off unless he had a firm grip; and he could not get this firm hold if his hind toes were not long enough to give his foot a nearly equal spread back and forward. Other birds grasp a limb with the whole under surface of their toes, but the woodpecker when on a smooth, upright tree-trunk nips it only with his toenails. Try with your own hand to hold a stick as large and heavy as you can grasp, and you will see that when you clasp your hand around it as a perching bird takes hold of a perch, it makes little difference that the thumb is shorter than the fingers, but when you try to nip it with your finger tips alone, you must bend your fingers until they are not much longer than your thumb,—that is, a pair of nippers must be equal jawed.

This simple illustration shows why the woodpecker’s foot reaches as far backward as forward. But a sensible objection may be raised, namely, that as there are two hind toes of unequal length, it is by no means certain which is the more necessary.

Scientists tell us that a woodpecker’s foot, though it looks so unlike a chicken’s, is really very much the same. When we ask how one of the front toes disappeared and how the extra hind toe came to be where it is, they tell us that there has been no addition and no loss, but the extra hind toe is only a front toe turned backward. They call it a reversed fourth toe. A bird’s toes are numbered in order starting with the hind toe and going around the inside of the foot to the outer or fourth toe. The hind toe is the thumb, and the others are numbered in the same order as the fingers of our hands. So we see that the woodpecker’s real hind toe is rather small, like that of most birds. It looks very much as if it had been found too small and as if another had turned back to help it do its work. Do you say that a bird cannot turn his toes about in this way? Most cannot, to be sure, but all of the owls can do it. An owl will sit either with two toes forward and two backward, or with three forward and one the other way. The owls have a reversible outer toe, and perhaps the woodpeckers did also before it became permanently reversed.

That this is exactly what had happened is curiously confirmed. There are a few woodpeckers in this country which have but three toes. They are the only North American land birds with less than four toes (though many sea and shore birds have but three). Compare this picture with a four-toed woodpecker’s foot. One toe is gone completely, when or how no one can tell. But in some way the first toe, the thumb, the one we always begin to count from, has disappeared. The one left is the reversed fourth toe, as we know by the number of joints in it. Undoubtedly this woodpecker needed a hind toe, but he must have needed a longer, stronger one than his natural first toe. A toe of the right length was supplied by turning one of the front toes back, and the short hind toe in some way disappeared.

This may seem a roundabout way to show that a woodpecker’s foot is a pair of nippers. First we studied nippers till we found out that they were not good nippers unless they were nearly equal-limbed. Next we studied the woodpecker’s foot to learn about that extra hind toe. Then it occurred to us that four toes were not necessary, since some of our best climbers have but three. What was the essential point? Might it not be a foot equally divided without reference to the number of toes? But that is the principle of a pair of nippers. Then came the question, is there any similarity in their use? Yes, the nippers are used to lift heavy weights, and the woodpecker’s foot is used to lift his heavy body in just the same way, by taking hold of a flat, smooth surface. We conclude that a wide-spread, equally divided, nipping foot would be the best device possible for the woodpecker’s way of living, and we find by examination that every woodpecker shows this type of foot.

There is additional evidence that this is the right explanation. Our only other North American birds that climb on the bark of trees professionally, as we may say, are the brown creepers and the nuthatches. In both these the tarsus is short, as we found it in the woodpeckers, and the hind toe and its claw are fully equal to the middle toe and claw, making an equally divided foot. On the other hand, the foot with two toes forward and two toes backward is confined neither to woodpeckers nor to climbing birds. The parrots, which climb after a fashion, have it; but so do the cuckoos, which do not climb, some of which, like our road-runner, or ground cuckoo of the West, are strictly terrestrial. The “yoking” of the toes may occur by the reversion of the fourth toe, as ordinarily, or of the second toe, as in the trogons; the arrangement appears to be definitely related to the distribution of the tendons that control the toes. But though accounting for the structure may give a clue to its descent, it does not justify its efficiency. The yoke-toed foot is not exclusively a climbing foot. All our families of climbers have at least one representative with but one toe behind, and this clearly proves that the yoke-toed structure is by no means necessary even though it may be an honorable inheritance among climbers. The natural conclusion is that the important point in climbing is not the number nor the arrangement of the toes, but the length of at least one hind toe so as to give an equally divided foot.

There is an interesting point to notice about the woodpeckers. This reversed fourth toe is curiously variable in length. In the flickers, with its claw, it is a little shorter than the middle (third) toe with its claw; in the red-heads and their friends it a little exceeds the middle toe and claw; in the downy and the hairy it is much the longest toe, and in the ivory-billed woodpecker it is abnormally developed. We at once judge that it is some indication of the bird’s manner of life, and we look for it to be largest in the species that live continually upon the trunks of trees, obtaining most of their food by drilling. We expect to see the finest development of drilling bill accompany this enormously developed toe, and we find them both in the ivory-billed woodpecker. In imagination we clearly see the use of it. The great bird, keen in his quest of grubs, sidling hastily round the tree, in an unsteady balance and unsupported by his tail, throws one long hind toe downward to steady himself, hooks the other into the bark above him, and hangs between the two as firmly supported as in his ordinary position. No doubt he does do this, but does it prove the supposition that the heaviest and most arboreal woodpeckers have the greatest development of the fourth toe? Not at all. There is our rare acquaintance the logcock, or pileated woodpecker, a bird nearly as large as the ivory-billed, one of the most persistent of our tree-climbers and more than any other woodpecker I ever observed given to scratching rapidly round and round a tree-trunk, clinging at ease in almost any position except head-downward, and drilling incessantly and at all seasons for grubs; he is a typical woodpecker of the largest size, but his hind toe and claw are, if anything, a trifle shorter than his middle toe with its claw. He throws it out and uses it as we have described, but it has not that disproportion to the other toes which we expected to find as the result of a strictly arboreal life.

What have we proved? We have not shown that the long toe is not more useful than the shorter one,—that is a matter of observation; but we have failed entirely to show that it is so, and this can be done only in one of two ways: either by proving that the logcock’s habits are not what all previous observers have believed them to be,—which would be assuming a great burden of proof; or by demonstrating that his ancestry explains why his feet do not illustrate our theory,—and this, though it is undoubtedly the true solution, could be settled only by a very learned man.

But we have encountered one truth which must always be held in mind in science—that a theory is not proved while a single fact remains rebellious and unsubdued. We might have examined every other woodpecker in the continent but just one; we might have seen that every other one agreed with our theory, as it does; we might have supposed that the explanation was good past doubting; but that one exception—if it was a logcock—would still over-turn the whole theory; and the very facts that we relied upon to strengthen us—its resemblance in size, habits, shape, and color to the ivory-billed woodpecker—have been the strongest possible means of totally demolishing our fine theory. We have learned, if nothing more, that all the facts must be examined and accounted for before an explanation is accepted as indisputable.

XIII

THE WOODPECKER’S TOOLS: HIS TAIL

If we study the woodpecker’s anatomy and observe his broad, strong, highly-arched hip-bones and the heavy, triangular “ploughshare” bone in which the tail feathers are planted, as well as the stiffness and strength of the tail itself, we must conclude that it is not by accident that he uses his tail as a prop. The whole structure shows that the bird was intended “to lean on his tail.” What we wish to discover is how good a tail it is to lean on.

Our first impression is that the woodpecker’s tail might be improved. Why are not the tips of the feathers stiffer? Why is it so rounded? Most of the work seems to fall on the middle feathers, and in some species, as the downy and the hairy woodpeckers, these end in decurved tips so soft and unresisting that they seem quite unfit to give any support. Would it not be better if the woodpecker’s tail had been cut square across and made of feathers equally rigid and ending in short stiff spines? For we see that the woodpecker’s tail is not only weak in its inner feathers, but weaker still in its outer ones, and it is stiff, in most species, only in the upper three fourths of its length.

When we propose a change in nature it is wise to inquire whether our improvement has not been tried before and to learn how it worked. How many kinds of birds have we that use their tails for a support? What are their habits and what sort of tails have they?

Besides the woodpeckers we have but two kinds of land birds that prop themselves with their tails,—the swifts and the creepers. The creeper has a tail very much like the woodpecker’s as it is; while the chimney swift’s is precisely like the woodpecker’s as we thought it ought to be. But we observe that while the creeper’s habits are almost precisely like the woodpecker’s,—so much so that when we first make his acquaintance, some of us will be sure we have discovered a new kind of woodpecker,—the chimney swift has but one habit in common with the woodpecker, that of clinging to an upright surface and propping himself by his tail. If the bird with the tail most like the woodpecker’s has the woodpecker’s habits, is it not a fair inference that this form of tail is better fitted to this way of living than the other would be?

Next, what variations in shapes do we observe among the woodpeckers themselves? The logcock and the ivory-billed woodpecker have the longest tails—because they are the largest birds. When we compare the length of the tails with the length of the birds we are surprised at the results. On measuring sixteen species, representing seven genera, I find that the tail is from three tenths to thirty-five hundredths of the entire length; that it is, in proportion, as long in the flicker as in the ivory-bill, as long in the downy as in the logcock, and longer (in the specimens measured) in the almost wholly terrestrial flicker than in the wholly arboreal logcock. Without much more study all that we can safely infer is that the woodpecker’s tail is not far from one third the length of his whole body measured from the tip of the bill to the tip of the tail. Probably this is the proportion most convenient for his work.

All woodpeckers’ tails agree in one particular: they are rounded at the end. At first sight we would say that some are but slightly rounded and others very deeply graduated; but as nearly as I can determine this is at least partly an optical illusion, explained by the great difference in the shape of the feathers making up the tail, which in some, as the flicker, are very broad and abruptly pointed, and in others taper gradually to the end and are very narrow for their length. The larger birds naturally appear to have longer tails, and the effect of narrow feathers is to make the tails appear longer and more sharply graduated than they really are. This diagram shows the shape of the curve in six species, and indicates that, while the curvature is less than we might expect, it bears some relation to the bird’s way of living; for we see that the strictly arboreal woodpeckers have more pointed tails than the terrestrial species, and that the amount of gradation bears a direct relation to the amount of time spent upon the tree-trunks.

There is a third difference, the shape of the individual feather, to which we shall refer again; but now we wish to examine the uses and meaning of the curved end.

I will show you how to prove this point so that you may be satisfied about it even if you should never see a woodpecker. We will make a little experiment, so simple that even a child can understand it.

First, how many shapes can any bird’s tail have? It may be one of three general patterns, and it can be nothing else unless we combine those patterns. It may be square across the end, it may have the middle feathers longest, or it may have the outer feathers longest. To one of these patterns every form of birds’ tails may be referred; you can invent no other shape.

Let us assume that you know nothing whatever of a woodpecker’s tail except that it has ten feathers, is used as a prop, and is held at an angle of thirty or forty degrees with the tree-trunk. Now, take three strips of paper of the same width and length, and of any size not inconveniently small. Fold them all down the centre. Cut one square across; cut one with a rounded end and the third with a forked end, making them of any shape you please so long as the three papers are of the same length. To give our models a fair test they must be of the same width and length. Next, pin a sheet of paper of any size you please into the form of a cylinder and stand it on end to represent a tree-trunk. Then fit the patterns to the tree-trunk and see which is the form that would give the most support.

But first, in how many ways is it possible for a bird to use his tail as a prop? He may of course hold it open or closed; and the open tail may be held in a single plane, “spread flat,” as we say; or curved up at the edges, like a crow blackbird’s; or curved down at the edges. And the closed tail may be held in a single plane; or, by dropping each pair of feathers a little, in several planes. Thus we see there are five positions in which each shape may be held against the cylinder of paper. Try each one against it, holding it first in the open positions and then after folding the paper like a bird’s tail with the outer feathers underneath, in the closed positions. The size of the model tree-trunk and the shape you cut your curves will make the results vary a little, but you will be surprised to observe, if your models are not too small, how many times you will get the same answers. Note the number and position of the pairs that touch:

Which shape brings the most feathers into use in all positions? Which positions bring most feathers into use? We see at once that the rounded end has a decided advantage, that the middle pair of feathers is used in all possible positions, that the pair next outside is the next important, and that the spread tail curving downward at the edges and the closed tail in different planes are the two shapes which give the best support. There is therefore a reason for the rounded end which we said was the rule among the woodpeckers.

Our little experiment is what we call a deduction. It shows us what we ought to expect under certain imaginary conditions. But it does not show us what actually exists, so there often comes a time when our deductions are faulty because Nature has done some unexpected thing, as when we found the single exception of the logcock’s foot upsetting a fine theory of ours. A deduction must always be compared with facts, and is worth little or nothing if a single fact of the series we are studying is not explained by it. This time all the facts do agree; for I had, before we made our experiment, examined the tails of every species of woodpecker ever found in North America, and there was no exception to the rounded end. I had already drawn my conclusion that this form was better adapted to life on a tree-trunk than the square or the forked tail would be, reasoning by a different process called induction. An induction examines many, and, if possible, all the facts before drawing any conclusion; a deduction examines the facts after the conclusion is reached. There is no hard-and-fast line between the two kinds of reasoning, but we may say that a deduction is reasoning out a guess and an induction is guessing out a reason. Deductions are easier and quicker; inductions are surer, and in preparing them we often make other discoveries.

The rounded tail is no doubt the best; but we have yet to decide whether the sharper curve is more advantageous than the lesser curve, as we thought probable from our observations. And there is still another deduction from our experiment which we did not make. If in the rounded tail the middle pairs of feathers do most of the work, and if use increases the size and efficiency of a part, which is almost an axiom in science, we should expect to find the middle tail feathers not only strongest in all woodpeckers but also strongest in increasing ratio in the species that use them most. To determine this we must study the use of the tail and the structure and shape of the individual tail feathers.

We should remark, perhaps, that the woodpecker’s tail is always composed of twelve feathers—ten pointed rectrices and two tiny abortive feathers so short and so hidden that no attention is paid to them. The ten principal feathers are arranged in corresponding pairs numbered from the outside to the centre as first, second, third, fourth, and fifth pairs.

In the flickers all ten feathers have wide vanes and are similar in everything but the shape; all are more or less pointed. The flicker’s tail looks and feels very much like that of any other bird except that the shafts are stiffer and the vanes contract to an acuminate tip. But as we take up the other species we notice a change, not only in the shape of the feathers but much more in their texture and in the difference between the various pairs. While in the flicker four pairs out of five are pointed and all are rigid, in the downy and the hairy three pairs out of five seem to be too soft to give any support, the sharp points have disappeared, and the tail has lost much of its stiffness. The two middle pairs of feathers are the only ones capable of doing much work and they are wavering and infirm at the tips where we should expect them to be strongest. In the logcock it is about the same,—two pairs are apparently unfit for work, one pair is infirm, and the two middle pairs are compelled to give all the support, except the little contributed by the third pair. In the ivory-billed woodpecker the two outer pairs are of no assistance and the three central ones do the work, and here again we find the base of the rectrices rigid and inflexible and the last fourth of their length weak and yielding. But what a difference in the individual feather! It is well able to do all the work; for, except for that weak tip which we cannot now explain, it is one of the toughest and strongest feathers to be found. The shaft is broad and flat, as elastic as a watch-spring; it looks like a band of burnished steel as it runs down between the vanes. And the vanes themselves are of a very curious pattern. They curl under at the edges so that we do not see their whole width, and the barbs crowd so thickly upon each other that they over-lie until they present an edge three or four broad. Indeed, the under side of one of these tail feathers reminds one of nothing so much as of the under side of a star-fish’s arm with its two long lines of ambulacral suckers on each side of a central groove, so thickly do the spiny vanes of these strong rectrices over ride and crowd together. These spines lay hold of the bark of the tree, rank after rank, hundreds of bristling points that cannot be dislodged except by a forward motion of the bird or by lifting the tail. Compared with this, the spiny points on the flicker’s tail were a poor invention. This device, which takes hold like a wool card, or a wire hair-brush, cannot slip from place. We begin to see, too, the use of that weak and flexible tip; it is to press down upon the tree-trunk a flat surface sufficiently large to hold hundreds of these little spiny points against the bark. The ivory-bill braces against this with the stiff upper part of the shaft and has a support that will not slip. The upper part of the shaft acts like a spring also, and adds tremendous force to the blow of the bill. Watch a hairy woodpecker when hard at work and see how his legs and tail form a triangular base by bracing against each other, and how his blow is delivered, not with the head alone, but with the whole body, swinging from the hips, the apex of the triangle on which he rests. He swings like a man wielding a sledge hammer, and to the strength of his neck adds the weight of his body, the spring of his tail, and the momentum of a blow delivered from a greater height. When the little hairy woodpecker does so much with his weak body, we can imagine what great birds like the logcock and the ivory-billed woodpecker, with their tremendous beaks, their huge claws, their springy tails, and their great physical strength can do. They are magnificent birds, the terror of all the grubs that hide in tree-trunks.

One point we have left unexplained: What is the advantage, if there is any, in the sharper curve to the tails of the arboreal woodpeckers? It is a simple question. The curve is caused by the unequal length of the tail feathers; each tail feather is a prop, and by their inequality they become props of different lengths. Now ask any carpenter which will best support a tottering wall—props all of the same length set at the same angle, or props of different lengths set at different angles? His answer will help you to solve the problem. But if a little is good, why are not all the pairs used as props? Partly, perhaps, because the woodpecker is always crowded for houseroom, and while he must have tail enough, he cannot afford to have any which he does not use. Did you ever think what an inconvenience any tail at all must be in a woodpecker’s hole?

XIV

THE WOODPECKER’S TOOLS: HIS TONGUE

We have seen how the woodpecker spears his grubs: now we will study his spear.

There are many interesting points about a woodpecker’s tongue, and they are not hard to understand. If a woodpecker would kindly let us take hold of his tongue and pull it out to its full extent we should be afraid we were “spoiling his machinery,” for the tongue can be drawn out almost incredibly—between two and three inches in a hairy woodpecker and more in a flicker. A strange-looking object it is, much resembling an angle-worm in form, color, and feeling; for it is round, soft, and sticky, except at the flat, horny, bayonet-pointed tip, and as it lies in the mouth it is wrinkled like the wrist of a loose glove; but it grows smaller and smoother the more we pull it out. Evidently we are only drawing it into its skin. But where does so much tongue come from? Does it stretch like a piece of elastic cord? Or is a part hidden somewhere? And if so, where is it kept?

These questions are answered by studying the bones of the tongue, for without bones it could not be guided as swiftly and surely as it is. Indeed, all tongues have bones in them, as you will discover by cutting carefully the slices near the root of an ox-tongue; but no other creature has such long and elaborate tongue-bones as some of the woodpeckers. They are the slenderest and most delicate little bony rods, joined end to end, but not really hinged nor needing to be, because they are so elastic. Here are the bones of a flicker’s tongue. The little knob at the end, marked a, bore the horny point of the tongue and directed it; the straight shaft marked b was inside the round part of the tongue as it lay within the bird’s mouth; but what was done with these two long branches, fully three quarters of the entire length of the bones? They are too sharply curved to pass down the bird’s throat, and, not being jointed, they cannot be doubled back in his mouth. They were tucked away very neatly and curiously. As the hyoid or tongue-bone lies in the mouth its branches diverge just in front of the gullet, and, traveling along the inner sides of the fork of the lower jaw, pass up over the top of the skull, looking in their sheath of muscles like two tiny whipcords. But still the bones are too long by perhaps half an inch for the place they occupy, and the ends must be neatly disposed of. Usually both pass to the right nasal opening and along the hollow of the upper mandible. Very rarely they may curl down around the eyeball in a spiral spring. So when the flicker thrusts out his tongue he feels the pull in the end of his nose, for the tip of the tongue being run out, the long slender bones are drawn out of their hiding-places, down over the skull until they lie flat along the roof of his mouth. As soon as he wishes to shut his bill, back fly the little bones guided by their hollow sheaths of elastic muscle into their hiding-place in the top of the bill. The muscular covering is a part of the same soft envelope that we saw lying in wrinkles at the root of the tongue. It covers the whole length of the little bones just as the woven outside covers an elastic cord.

Not all woodpeckers have tongues precisely like this. The sapsucker’s is the shortest of any, and reaches barely beyond the hinge of the jaws. In the Lewis’s woodpecker and others of his genus the branches of the hyoid extend part-way up the back of the skull but in the kinds that live principally upon borers they are very long and resemble the flicker’s in arrangement. The only other North American birds that have a tongue built upon this plan are the hummingbirds, in which also it is extensile. The flicker, in proportion to his size, has the longest tongue of any bird known.

XV

HOW EACH WOODPECKER IS FITTED FOR HIS OWN KIND OF LIFE

We have studied the woodpeckers at some length: first, what all of them do; next, what some that are peculiar in their ways do; lastly, how each is fitted for a particular kind of life. At first we were inclined to think they were all alike; but now we begin to see that there are very real differences between them,—in tails, feet, bills, and tongues, and at the same time in their food and habits.

The flicker’s tail is less sharply curved than that of any other woodpecker,—a sign that he is probably not exclusively a tree-dweller; his bill is curved and rounded, a pick-axe rather than a drill,—an indication that he does not dig for grubs; his feet do not tell us much; but his long extensile tongue shows that, whatever he feeds upon, he seeks it in holes. We find a tongue like this in no other bird, but among mammals the aard-vark, the ant-bear, and the pangolins are all similarly equipped, and all live on ants which they extract from their mounds and burrows in hundreds by means of these round, sticky, and extensile tongues. This is precisely the way the flicker gets his living. He lives principally upon the ground or near it, pecks very little except when digging his nest, and feeds largely upon ants, thrusting his head into the ant-hills and drawing out the ants glued to his tongue rather than speared by it. As he has been known to eat three thousand ants for a meal, we see how much easier this is than spearing them one by one.

The red-head is another type. The bill is still nearly of the pick-axe model, the feet not especially different from the flicker’s, the tail rather better adapted to life on a tree-trunk, and the tongue entirely unlike the flicker’s,—not very extensile and heavily clothed near the tip with long, thick, recurved bristles. We infer that though he may climb well, he is not a drilling woodpecker to any great extent, and that his tongue is adapted neither to extracting borers nor to eating ants from their burrows. His habits bear out the inference. He is arboreal, but his food is either vegetable or picked up from the surface, rasped up rather than speared.

The sapsucker presents still another variation. The points to the tail feathers are more acuminate and the tail itself more resembles that of the tree-dwelling woodpeckers in shape; the feet are fitted for clinging to the trunk; the bill, now perfectly straight and no longer smoothly rounded but buttressed by strong angles that spring from the base and run down toward the tip, is the bill of a woodpecker that lives by drilling; but the tongue is wholly unadapted to catching grubs. What kind of food can an arboreal woodpecker with a drilling bill find upon a tree-trunk when his tongue can be extended only a fifth of an inch, and is furnished with a brush of bristles at the end? We have answered that question before: he eats the inner bark of trees and laps up the sap, for which this brushy tip is excellently fitted. It has been observed that the tongue much resembles the tongues of insect-eating birds, which cannot be extended beyond the end of the bill. It is true that the sapsucker catches great numbers of insects, taking them on the wing like a flycatcher. But he also eats nearly as many ants as the flicker, though their tongues are totally unlike. We have made the mistake perhaps of thinking that ants live only underground and can be obtained only by tongues like those of the flicker and the ant-bear, which hunt them there. But ants are abundant on the surface of the ground, and they excavate long tunnels in rotten wood. The black bear is a famous ant-hunter, yet his tongue is like a dog’s and he gets his ants by lapping them up after he has torn open the rotten logs in which they live. This is the way that the sapsucker obtains his ants, and the brush of stiff hairs is a help to him in such work. We see, then, that it is not so much the food as the manner of feeding that explains the form of the tongue.

The downy and the hairy are a step farther along in their development. The fourth toe is longer than the others, a condition that we do not find in any of the woodpeckers not strictly arboreal; the tail is of the improved pattern, holding by a brush of bristles rather than by one stiff point at the end of each feather; the bill is heavier, broader at the base, more heavily ridged, and in every way a stronger tool; and the tongue is highly extensible and of the spear pattern, sharp-pointed and barbed with recurved hooks. Everything about these birds indicates that they are fitted to live on tree-trunks and to dig for borers. This, indeed, is what they do.

But the great logcock and the ivory-billed woodpecker, though of the same type as the other larvæ-eating woodpeckers, are more highly developed along the same line. We notice the great strength of the feet; the claws, as large and as sharp as a cat’s; the enormous weight and strength of the bill, compared with that of the other woodpeckers, which enables them to cut into the hardest wood and even into frozen green timber; and the great development of the tail, which now becomes a strong spring to support and aid the bird in his work.

As we try to group these particulars under general heads, we see that we have observed three things:—

That the structure of a bird is adapted to its kind of life.

That the structure varies by small degrees with the kind of life.

That the kind of life is conditioned largely upon the kind of food and upon the method of procuring it, more particularly the latter.

These are not so much different truths as three aspects of one truth. When we study the first we see why birds are grouped together into orders and families: we study their resemblances. When we observe the second we see why they are divided into species, for we note their differences. But when we consider the third and reflect that birds have the power to choose new kinds of food or new places and means of getting it, we see how it is that there can come to be new kinds of birds, new subspecies and species, springing up from time to time. Wonderful and improbable as it seems, there is more reason to believe than there is to doubt that new kinds of animals and plants are constantly in process of making; that the laws of change are constantly at work, adapting creatures to their surroundings or crushing them out of existence because they will not learn new ways. And it is probable that these differences which we mark in the woodpeckers have been the result of efforts to adapt themselves to a peculiar kind of life where food was abundant; and also that by acquired habits and by acquired tastes for different kinds of foods they will be subject to still further variations in the future.

XVI

THE ARGUMENT FROM DESIGN

But if the birds are making themselves into new species, where is the place for God in the universe? Did not God make all kinds of creatures in the beginning? How can they go on being made without God?

These are questions every one ought to ask, but—did God leave his world after He had made it and go a long way off? Did He wind it up like a watch to go till it should run down? Is the world a machine, or is it alive?

Long ago the wise and good man Socrates argued that if you did not know there was a God at all, you could at least infer it because everything was so wonderfully made. “There is our body,” said he: “every part of it so perfect and so reasonable. Consider how the eyes not only please us with agreeable sensations but are protected in every way. The eyebrows stand like a thicket to keep the perspiration from them, the lids are a curtain to shut out too great light, the lashes screen them from dust,—everything is planned for some wise and reasonable end. And where the evidence of design is so convincing must we not believe that there was a Designer?” Words like these he spoke, and we know because everything is so perfectly contrived that there must have been a contriver, who knew all from the beginning. We are compelled to believe that there is a God.

Shall we believe it less because we find in the creatures about us intelligence and the power to care for their own lives? Has God gone on a visit because these living creatures are looking out for themselves? Were they made less perfectly in the beginning because when new conditions surround them they are able to change to meet the strange requirements? This is not less evidence of a Designer, but more. It was long said that the existence of a watch was proof of a watchmaker who had planned and put together all the parts so that they worked harmoniously. But if the watch had the power to grow small to fit a small pocket, or large to fit a large one, to become luminous by night, and to correct its own time by the sun instead of being regulated by outside interference, what should we have said—that it was proof there was no watchmaker? or that it showed a far more skillful one, since he could make a living, self-regulating, adaptive watch?

And so of the world and the creatures in it. Every evidence we get that they can care for themselves, that they can adapt themselves to new conditions, that they are intelligent and reasonable, capable of improvement in habits or in structure, is so much surer proof that a wise God made them what they are. Evolution—for that is the name by which we call these changes—does not take God out of the universe but makes the need of Him stronger. The argument from design is immensely strengthened when we consider that we have not only an obedient machine acting according to a few fundamental rules, but one that is intelligent also and capable of self-modification.

APPENDIX

Explanation of Terms.