Creation by Evolution/The Evolution of the Bird

by David Meredith Seares Watson

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THE EVOLUTION OF THE BIRD

By David Meredith Seares Watson

Jodrell Professor of Zoölogy, London University

According to the story of evolution the various kinds of animals that we see to-day are not the descendants of like animals that were suddenly created in the forms they now have; some of them, at least, are the descendants of animals of far different structure and habits. We can observe that no animal is precisely like either of its parents in all respects, but in order to demonstrate certainly the larger changes covered by the theory of evolution we should have to watch carefully the natural breeding of some particular kind of animal for a long time—for thousands or even millions of years. We can get no such evidence as that, so we must turn to evidence of other kinds.

The alternative theory to that of evolution—the theory of special creation—assumes that each kind of animal was created in the form in which we now see it. If every kind of animal had this mode of origin we should expect to find that each one is a perfect machine, with all its parts arranged in the best possible way—that is, in the simplest and most effective way to perform their coöperative functions. But we find that all animals, regarded as pieces of machinery, are imperfect; each represents an attempt, more or less successful, to adapt a pre-existent structure to some new use. If the doctrine of evolution is true we should therefore be able to show that many of the present useless or anomalous structures of an animal are derived from ancestors to which they were useful, and that they have not yet been lost or fully utilized.

I purpose here to sketch the mode of origin of a single kind of animal—the bird—though perhaps I may not be able to do so very well for all my readers, because many of them may not have much knowledge of anatomy and physiology, and it is to these sciences that I shall turn for my evidence.

All zoölogists now believe that the birds, which in the flight of an eagle or an albatross show a special mode of life in its highest perfection, have arisen by a long process of change from reptiles—that is, from creatures similar in their structure and appearance to lizards and crocodiles. The actual reptilian ancestors of the birds are no longer living, but we know several animals that are closely related to these ancestors. By examining the structure of these related animals we can see what changes are necessary to convert a reptile into a bird, and we can show that by postulating such conversion many of the anomalous details of the structure of a bird may be explained.

Everyone who has watched lizards knows that soon after the sun rises they come out of the holes in the ground in which they sleep and gradually become more and more active as the day goes on. This increase in their vigor depends entirely on increase in temperature. The birds and mammals have the means of keeping their bodies warm at a uniform temperature, but the reptiles take their temperature from the air about them. After a lizard has been basking long in sunlight it may be almost uncomfortably hot to touch, and during a cold night its temperature may fall almost to the freezing point. Just as most chemical combinations go on faster at a high rather than at a low temperature, so all the parts of an animal work better when they are kept warm. The heart of a frog beats nearly twice as fast at 68° as at 50°. Thus if an animal is to be equally active under all climatic conditions it should be able to keep its body at a constant fairly high temperature.

An animal actually warms itself by moving its muscles, and it is able to keep its muscles going only by burning oxygen taken from the air, or, rather, by the natural process of breathing. The maintenance of a high bodily temperature uses up a good deal of food, and for mere economy it is desirable to reduce the quantity required by providing the animal with a coat that will allow its heat to escape very slowly. This is the original reason for the fur that covers a mammal and for the feathers that cover a bird. Probably one of the first steps in converting a reptile into a bird is to change its scales to feathers. Yet when we compare a wing or tail feather of a bird with a scale it seems at first impossible that the one should have come from the other; but the first feathers of the chick—those which it grows while it is still in the egg—consist of very short scale-like quills, whose ends fray out into fine plumes. These feathers are formed from the upper layers of the skin in exactly the same way as the scales of lizards are formed; indeed, they differ from such scales only in being longer. Between these incipient feathers and those which we know as quills we find all intermediate stages.

In order to enable the bird ancestor to utilize fully the increased activity made possible by its higher body temperature many changes of its structure were necessary. One of the most important of these has to do with the heart. A lizard can run very fast for a short distance, but it then collapses, completely exhausted, whereas a mammal or bird can hardly work so fast and so long that its muscles will no longer contract. This difference is due to the fact that the mechanism for sending a supply of oxygen to the muscles is much better in the bird or mammal than in the lizard.

The heart of a bird consists of two pumps, placed side by side. Into one of these pumps, that on the left side, blood full of oxygen comes from the lungs. This blood is then pumped forward through a great tube, which turns over to the right side of the animal and gives off blood vessels to all the muscles and all parts of the body except the lungs. All this blood, after being deprived of its oxygen, goes back to the right side of the heart and is then sent to the lungs to get a new supply of oxygen. The most peculiar part of the whole mechanism is that the great main vessel, the aorta, instead of lying in the middle line of the body, where we should naturally expect to find it, actually crosses from the left to the right side. We can explain this anomaly at once when we examine the structure of a crocodile. The heart of the crocodile is very like that of the bird, but in the crocodile there are two great vessels, one coming from the left side of the heart and the other from the right side. These cross one another in the middle line of the body and then pass upward until they join and form a single aorta. The vessel from the left side persists in birds, because it conveys only oxygenated blood, but that from the right side, although it is found in the very young chick, is blocked up in the full-grown bird, because it conveys impure blood. Thus the heart of the bird is better than that of the crocodile in that it supplies to every muscle the maximum quantity of oxygenated blood, and its peculiarities are explained by the structure of the heart of the crocodile.

By changes of the kind described, the bird ancestor was able to maintain prolonged and uniform activity. The first use to which it would put this power would naturally be in swift running in order to catch the small animals on which it lived and to escape from its enemies. It therefore began to run on its hind legs like a kangaroo. There are only two ways in which it could maintain such a bipedal gait—either by carrying its body upright or by having a long tail to balance the head and body. The bird ancestor adopted the latter plan. It had originally a spraddling walk, the feet being turned out and kept far apart. This gait made necessary a foot like that of a lizard, in which the big toe is the shortest and the fourth toe much the longest, so that the claws all lie on a straight line, at right angles to the direction of the movement of the body. The increase in the length of the toes is gained by an increase in the number of bones or joints that support them; the first has two joints, the next three, and so on, the fourth having five.

Any animal that must run fast must draw up its feet until they lie under the body, and at the same time the foot must be so shaped that the middle toe becomes the longest and the second and fourth, which lie on each side of it, become of equal length. This is the shape of the foot of a bird, though birds still retain five joints in their fourth toe, although they gain no advantage by so doing. These five phalanges are inexplicable if the bird was created as it stands, but they are easily understood if the bird was evolved from a reptile.

Any animal that uses its hind legs entirely for running or for such simple movements as scratching, that has its feet near the middle line of the body, and that runs fast, tends to simplify the structure of its foot by fusing together bones that were originally separate. In all ordinary animals the ankle joint is made up of many small bones and is, even in ourselves, a point of weakness. In a small chick these bones are represented by cartilage, but as the bird grows up they become separated into two groups, one of which fuses with the shin bone and the other becomes part of a single bone that corresponds with the bones of the arch of the foot and supports the three large toes. Thus the bird gradually comes to have a simple and very strong foot.

All animals that run very fast run on their toes, and the bird ancestor, in order to run more conveniently, permanently raised its heel from the ground. This change enabled the arch of its foot to be lengthened, so that with each stride it covered more ground.

Our bird ancestor has thus become a creature with long legs and a tail, capable of running very rapidly on its hind legs alone, the fore legs and hands being carried in the air. Such an animal, if it makes full use of its speed, must be able to use its hands for capturing prey and for carrying food. In order that it may do this its fore legs should remain rather short and the fingers should be provided with claws for holding the prey securely. But a clawed foot that is used for holding a struggling animal is an encumbrance if it retains all five fingers, and we find in many reptiles, even nascently in a crocodile, that the fourth and fifth fingers become smaller and smaller and finally disappear. A hand that is used for handling food must be capable of being turned about, and the arm that supports it must be freely movable. This stage of bird evolution is represented by many of the extinct reptiles that are called dinosaurs. These animals are not the direct ancestors of the birds, but they are close relatives, which went part of the way with them but, probably because they never developed feathers, were unable to make the last great step and begin to fly.

As the bird ancestors became able to run faster and faster they soon reached a stage when, like kangaroos, they travelled in a series of great leaps. They must then have extended the distance covered in these leaps by stretching out their freely movable fore legs so that they acted like the wings of an aeroplane and came to support more and more of the weight of the body. Their value as wings may have been greatly increased by the fact that they were already covered with feathers, which formed a fringe along the hinder edge of the arm, exactly as the scales do to the hind leg of a crocodile. The use of the fore legs as wings in this way, however, merely supports the front end of the body and head and leaves the tail trailing behind. But just as scales cover the whole of a lizard so feathers covered the whole of the bird ancestor, including its tail, and if these feathers retained the arrangement which they had as scales and followed the crocodile pattern they formed lateral keels along that organ. These keels will also act as wings, and if they become large enough are quite capable of supporting the weight of the hinder half of the animal.

Even after the bird ancestor had progressed to the point where it could rapidly move by a series of leaps, whose length was increased by gliding, while it gained speed by an increase in the power of the hind legs, the transition to true flight was a great step, which depended on the adoption of a flapping action of the wings. Such flapping, to be effective, must be regulated in a definite way, the course taken by the wing in its down stroke being different from that which it follows as it is brought up again to begin a new stroke. This perfect regulation of flight can be most easily assured by so shaping all the faces by which the bones move on one another and so arranging the ligaments by which they are tied together that no other movements are possible. In the modern birds this end is accomplished with great perfection by an arrangement that results in the fusion of all the originally separate bones of the palm of the hand and the wrist into a single element and a corresponding fusion of the two bones of the forearm.

But the adoption of a life in the air not only requires a modification of the physical structure of the wings and body but affects the relative importance and even the character of the senses as well as of the brain that makes use of the information they afford. The sense of smell becomes much less useful to a flying animal than it was to a crawling animal, which carries its head so close to the ground that it can recognise the presence of other animals by the odours which they leave behind them. We therefore find that the nose of a bird—its sense of smell—is in no way better than that of a crocodile.

In order to utilize fully the improved senses and to adjust them delicately to the conditions on which flight depends modifications must be made in the structure of the brain. The brain of a bird consists of the same parts as that of a crocodile and resembles it very closely in its fundamental arrangement, but that part (the cerebellum) which coördinates the muscular movements and adjusts them to the conditions under which the bird finds itself is much larger and is more complicated in structure. The part of the brain that is concerned with vision is larger, and the part that is concerned with smell is smaller.

More important in some respects is an enlargement of the front part of the brain to enable the bird to bring together there all the information that comes to it from its senses and to decide on its behaviour in the light of the memories of past events that are stored there. It is to the development of this part of their brains that birds owe all those competencies in building their nests and in caring for and protecting their young which have long endeared them to moralists.

In this account of the origin of the birds I have dealt only with certain selected parts of the body, and with only a few of the changes in the conditions of life. For example, I have not referred to the modifications of structure that are needed to enable a bird to perch on a twig. The features that I have mentioned were selected because they can be explained without presenting too much detail, and because they show the retention in birds of features whose presence is due to their existence as useful modifications of like features in their reptilian ancestors. The points considered may be summarized as follows:

1. The bird owes the presence of two, three, four, or five phalanges, or joints, in its toes to the fact that its reptilian ancestor, owing to its straddling gait, required toes that increased in length from the first to the fourth, whereas in the bird the second toe, which has three phalanges, and the fourth toe, which has five, are actually of the same length.

2. The bird owes the anomalous manner in which its aorta crosses from the left to the right side of the center of the body to the fact that in reptiles only this one of the pair of aortae transmits pure blood.

3. The bird owes its possession of only the first three fingers and not, as might have been expected, the fourth and fifth fingers, to the fact that the first three fingers are those which are most useful in the reptilian ancestor for clasping food between the two hands.

4. The bird owes the character of its brain to its descent from an animal having a brain like a crocodile.

This list might be extended indefinitely, but these four examples are sufficient to show that the peculiarities of the bird’s structure—the points in which it seems to be clumsily constructed—are at once explained as relics derived from its reptilian ancestor.

Fig. 1.—The earliest known bird, Archæopteryx macrura, Upper Jurassic, Solenhofen, Bavaria. (Restoration by Heilmann.)

“The earliest bird known, although obviously a bird, with wings and feathers, differs in many ways from modern birds. It has large teeth; its tail was not a fan, but a double row of feathers on either side of a long pointed axis of a bone; and most remarkable of all, it had on its wings, besides feathers, three separately movable fingers ending in claws, by whose aid it doubtless scrambled about through the branches. In all these ways it was less fully adapted to aerial life than are modern birds.”—Julian S. Huxley.

If the explanation that I have given above is true we should be justified in believing that the oldest known bird, called Archaeopteryx (Fig. 1), which is known to us by two

Fig. 2.

“Archaeopteryx was considerably smaller than a crow, with a stout little head armed with sharp teeth (as scarce as hen’s teeth was no joke in that distant period). While he fluttered through the air he trailed after him a tail longer than his body, beset with feathers on each side. Everyone knows that nowadays the feathers of a bird’s tail are arranged like the sticks of a fan and that the tail opens and shuts like a fan. But in Archaeopteryx the feathers were arranged in pairs, a feather on each side of every joint of the tail, so that on a small scale the tail was something like that of a kite; and because of this long, lizard-like tail this bird and his immediate kith and kin are placed in a group Saururae, or lizard-tailed.”—Lucas.

fossil skeletons, one in the British Museum, and the other in Berlin, is intermediate between the reptiles and the modern birds in its structure. Archaeopteryx, a queer lizard tailed bird, was found in rocks at Solenhofen, in Bavaria, which are in age nearer to the first rocks in which we find remains of ordinary birds than the rocks that were being laid down at the time when the change from reptile to bird began. We should therefore expect to find, and we do find, that it is nearer in structure to ordinary birds than to the reptiles. It has, for example, fully developed feathers, but it is in many ways a distinctly intermediate form. The hind foot, although it agrees in its proportions with the hind foot of other birds, has three separate bones in its arch, similar to the bones that are found in its reptilian ancestor. In the wing the bones of the forearm, the wrist, and the lingers are all separate, and the fingers end in big claws, so that they may have been used for capturing and handling food in exactly the way they were used by the reptilian ancestor, but in a way that no other bird does. Instead of the horny beak of a bird, Archaeopteryx has a row of little teeth that are exactly like those of a lizard. But one of the most interesting features of Archaeopteryx is its bony tail, which is longer than the rest of its body and along which there are two rows of quill feathers (Fig. 2) . In the stage that is here represented, in which the wings were not big enough and not rightly placed to support the whole weight of the body and the tail feathers had to carry the hinder part of the body, the long tail was a necessity, but an unfortunate one, because it made it impossible for Archaeopteryx to fly with the perfection exhibited by such modern birds as the eagles and seagulls. An eagle rises from the ground by a few powerful strokes of its great wings and then, as soon as it has reached a certain height, it stretches its wings outward and upward, holding them motionless, except for tiny adjustments of their tips for steering, and soars away in gradually widening circles until it finally may become almost too small to be visible. Then, seeing a small animal on the ground, it partly closes its wings, falls headlong to the ground, stops suddenly by expanding its wings and short tail, and lands directly on its prey. No aeroplane can copy this dive, sudden stop, and accurate landing, because an aeroplane becomes uncontrollable when its speed falls below very high speed; and when it stops it must run for some way along the ground.

During the last few years we have learned of the

Fig. 3.—Townsend piper, an example of a modern flying bird.

In the evolution of the birds we see many changes, such as that from the long vertebrated tail of the reptile-bird, the archæopteryx, to the consolidated, fan-like tail of the modern bird, and other marked contrasts.

conditions under which soaring flight is possible, and those aviators who have flown engineless aeroplanes (gliders) for hours at a time have been copying the eagles and giving us valuable information about the difficulties of the process, the chief of which is perhaps the recognition of the upward-directed currents of air on which it depends.

The other difficulty, the inability of the aeroplane to fly very fast or very slowly, depends on the long tail that all present-day aeroplanes have. As soon as it becomes possible to control an aeroplane that has no tail, or a very short one, man may be able to copy the manoeuvers of the eagles. From this point of view Archaeopteryx corresponds to a present-day aeroplane, the modern birds to the aeroplane of the future. It is certain that Archaeopteryx was clumsy, incapable of hovering over one spot and of alighting on a definite perch.

Archaeopteryx was therefore far inferior to the modern birds (see fig. 3) in its power of flight. It was clumsy, ill constructed, and lacked that perfection of form and motion which makes the sea gull a constant source of delight. Is it credible that a bird that was miraculously created in a moment should be so imperfect? Is not the imperfection of its machinery an evidence of evolution? Is it not more reasonable to recognize in Archaeopteryx a necessary stage in the long process by which a crawling reptile was gradually converted into the perfect flying bird of to-day?

I have here tried to bring together facts about the birds that bear witness to their evolution from more primitive ancestors. Comparative anatomy, embryology, and palaeontology unite in telling the same story. They agree in testifying that the bird is to-day a highly specialized descendant of some reptilian ancestor. Is it at all probable that there has been collusion among these witnesses and that their testimony is false? Must we not admit that the scientific research of to-day, no matter how much its results have disturbed mediaeval prejudices, has led us and is still leading us to a more reasonable conception of the order of Nature and of the true mode of creation?

SELECTED REFERENCES

• Lancaster, E. Ray. Extinct Animals. London, 1905.
• Marsh, Othniel Charles. Odontornithes: A Monograph of the Extinct Toothed Birds of North America. Washington, 1880.
• Osborn, Henry Fairfield. The Age of Mammals in Europe, Asia, and North America. New York, 1910.
• Seeley, H. J. Dragons of the Air, an Account of Extinct Flying Reptiles. London, 1901.

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“The whole architecture of a bird skeleton, indeed the whole internal anatomy, is unquestionably a modification of a primitive reptilian type.”—William King Gregory.

“If we admit that species have changed, and are changing at the present time, that is all the principle of evolution implies. The evolutionist stands for and believes in a changing world, and unless you, the reader, believe in a fixed, unchanging world, you, too, are an evolutionist. Evolution is merely the philosophy of change as opposed to the philosophy of fixity and unchangeability.”—H. H. Newman.