Popular Science Monthly/Volume 22/April 1883/Progress of the Backboned Family
|PROGRESS OF THE BACKBONED FAMILY.|
THERE is much uncertainty as to how the backboned or vertebrate animals began; but the best clew we have to the mystery is found in a little, half-transparent creature, about two inches long, which is still to be found living upon the English shores and the Southern Atlantic coast of the United States. This small, insignificant animal is called the "Lancelet," because it is shaped something like the head of a lance; and it is in many ways so imperfect that naturalists believe it to be a degraded form, like the acorn-barnacle—that is to say, that it has probably lost some of the parts which its ancestors once possessed. But, in any case, it is the most simple backboned animal we have, and shows us how the first feeble forms may have lived. Truly, it is only by courtesy that we can call him a backboned animal, for all he has is a cord of gristle, pointed at both ends, which stretches all along the middle of his body above his long, narrow stomach; while above this, again, is another cord containing his nerve-telegraph.
There are large fishes, too, which have this cartilaginous back-bone. The young shark has nothing but a rod of gristle or cartilage, and, though he is one of the strongest of sea-animals, he retains this gristly state of his skeleton throughout his life; however much he may strengthen it by hard matter, it never becomes true bone.
The first feeble ancestors of the shark and the sturgeon appear at a time when the crustaceans were the most powerful animals in the world, and the huge, lobster-like Pterygotus was the monarch of the
seas. The plated-scaled fish which existed at the same time were clumsy creatures, for their skeletons were probably feeble, and their armor-like shields were heavy. So, as history went on, they gradually gave way, becoming smaller and rarer, while the more active little shark-like animals gradually grew strong and powerful, and from them are descended the giant sharks of to-day.
The powerful gristly-boned fishes are much excelled in agility by the herring, the salmon, and their other bony companions, which move with much less effort in the water, and so have naturally made their way into all parts of the rivers and seas. But where have they come from? We know very little of their early history, but what little we do know leads us to think that long ago they branched off from the enameled-scaled fish, and struck out a path of their own, to make the most of the watery world.
If we wanted to pick out the strangest and strongest proof of how the shape of fish is altered to suit their wants, we need seek no further than the flat-fish. The young sole, when it comes out of the egg, is not flat like the young skate, but a very thin, spindle-shaped fish, something like a minnow. He is then about the size of a grain of rice, very transparent,
and lives at the top of the sea. He has one eye on each side, like other fish, only one eye is higher up than the other, and the single fin on its back and the one under its body reach almost from head to tail. In this way he swims for about a week, but he is so thin and deep, and his fins are so small, that swimming edgewise is an effort, and soon he falls down on one side, generally the left, to the bottom of the sea. Many times he rises again, especially at first, till he has got used to breathing at the muddy bottom, and meanwhile the eye that lies underneath is gradually working its way round to the upper side, his forehead wrinkles so as to draw the under eye up, while his whole head and mouth receive a twist which he never afterward loses. His skeleton, it must be remembered, is still very soft, and the bones of his face are easily bent; and at last this eye is screwed round, and as he lies at the bottom he can look upward with both eyes, and save the under one from getting scratched by the sand, as it must have done if it had remained below.
It is clear that if the backboned animals were ever to live upon land, after they had begun their career in the water, there must have been some among them which learned gradually to give up water-breathing, and to make use of free air; and we shall not have far to seek for creatures which will help us to guess how they managed it.
The common tadpole is to all intents a fish. He swims with a fish's tail; he gulps in water at his mouth, passing it out at the slits in his throat after it has poured over his fish's gills. Moreover, he has a fish's heart, of two chambers only, which pumps the blood into these gills to be freshened, while, like the lamprey, he has a gristly cord, enlarged at the end to form a gristly skull, a round sucking mouth, and no limbs. As he grows bigger and more active week by week, two little bumps appear, one on each side of his now bulky body, just where it joins the tail. These bumps grow larger every day, until, lo! some morning they have pierced through the skin, and two tiny hind legs are working between the body and the tail. In about another week the front legs appear, and we have a small four-legged animal with a lamprey's tail.
During this time a bag has been forming inside, at the back of the throat, which afterward divides into two, forming a pair of lungs, which he uses when out of the water, though still using his gills when below. Little by little the blood-vessels going to the gills grow smaller, and those going to the lungs grow larger; while the fish's two-chambered heart is dividing into three chambers—one to receive the blood from the body, another to receive it from the lungs, and one to drive this blood back again through the whole animal. Now that he can leap and swim with his legs, his tail is no longer of use to him and it is gradually sucked in, growing shorter and shorter, till it disappears. Thus our backboned animal has succeeded in getting out of the water on to the land.
If we glance back to the far-off time when the ancient fishes were wandering round the shores and in the streams of the coal-forests, we find that the amphibia were not then the small, scattered groups they are now, but huge and powerful creatures, which sported in the water or wandered over the land with sprawling limbs, long tails, and bones on which gills grew, while their heads were covered with hard, bony plates, and their teeth were large, with folds of hard enamel on the surface.
And now the transformation is complete, for, when we pass on to the next division of backboned animals, the "reptiles," we hear nothing more of gills, nor air taken from the water, nor fins, nor fishes' tails. We shall all allow that the tortoises are the most singular of any. Slow, ponderous creatures, with hard, bony heads, wide-open, expressionless eyes, horny beaks, and thick, clumsy legs, the tortoises seem, at first sight, to be only half alive, as they lumber along, carrying their heavy shell, and eating, when they do eat, in a dull, listless kind of way. This sluggishness would certainly be their ruin, in a bustling, greedy world, if it were not for the strong box in which they live. You would hardly guess that the shell of the tortoise is part of his skeleton. But it is so. The arched dome which covers his back is made of his backbone and ribs, and the shelly plates arranged over it are his skin hardened into horny shields, which, in the hawk's-bill turtle, form the tortoise-shell which is peeled off for our use; while the flat shell under his body is the hardened skin of his belly, and the bones which belong to it.
It is not without some struggle that the cold-blooded reptiles have held their own in the world, nor is it to be wondered at that only four types—tortoises, lizards, crocodiles, and snakes—should have managed to find room to live among the myriads of warm-blooded animals which have filled the earth. These four groups have made a good fight of it, and many of them even make use of warm-blooded animals as food. The tortoises, it is true, feed upon plants, except those that live in fresh water, and feed chiefly on fish, snakes, and frogs, while most of the lizards are insect-feeders. But the crocodile, as he lurks near the river's edge, and the snake, when he fastens his glittering eye on a mouse or bird, are both on the lookout for animals higher in the world than themselves.
We come now into quite a new life, for we are going to wander among the conquerors of the air, who have learned to rise far beyond our solid ground, and to soar, like the lark, into the clouds, or, like the eagle, to sail over the topmost crags of the mountains, there to build his solitary eyrie.
In those far by-gone times, when the huge land-lizards browsed upon the trees, the birds living among them were much more like them in many ways than they are now. Of water-birds there were some about the size of small gulls, which flew with strong wings and had fan-shaped tails, but had teeth in their horny jaws, set in sockets like those of the crocodile, while their backbones had joints like those of fishes rather than birds; and with them were other and wingless birds rather larger than our swans, but more like swimming, fish-eating ostriches.
In these and many other points the early birds came very near to the reptiles—not to the flying ones, but to those which walked on the land. And now, perhaps you will ask, Did reptiles, then, turn into birds? No, since they were both living at the same time, and those reptiles which flew did so like bats, and not in any way like the birds which were their companions. To explain the facts, we must go much further back than this. If any one were to ask us whether the Australian colonists came from the white Americans or the Americans from the Australians, we should answer, "Neither the one nor the other, and yet they are related, for both have sprung from the English race." In the same way, when we see how like the ancient birds and reptiles were to each other, so that it is very difficult to say which were bird-like reptiles and which were reptile-like birds, we can only conclude that they, too, once branched off from some older race which had that bone between the jaws, that single neck-joint, and the other characters which birds and reptiles have in common.
But where have the feathers come from—those wonderful, beautiful appendages without which the bird could not fly? They are growths of his skin, of the same nature as the scales of reptiles, or those on the bird's own feet and legs; and on some low birds, such as the penguins, they are so stiff and scale-like that it is often difficult to say where the scales end and the feathers begin. All feathers, even the most delicate, are made of horny matter, though it splits up into so many shreds as it grows that they look like the finest hair, and Dr. Gadow has reckoned that there must he fifty-four million branches and threads upon one good-sized eagle's feather.
From their skeletons and feathers which we find, we know that the strange land-birds which perched on the trees at the time that large reptiles were so numerous had not a fan-shaped tail, made of feathers growing on one broad bone, as our birds have now, but they had a long tail of many joints like lizards, only that each joint carried a pair of feathers, and like lizards, too, they had teeth in their jaws, which no living bird has. They must have been poor fliers at best, these earliest known birds, for their wings were small and the fingers of their hand were separate more like lizards' toes, two of them at least having claws upon them, while their long, hanging tail must have been very awkward compared to the fan-shaped tail they now wear.
Our backboned animals have now traveled far along the journey of life. The fish, in many and varied forms, have taken possession of the seas, lakes, and rivers; the amphibia fill the swamps and the debatable ground between earth and water; the reptiles swarm in the tropics, and even in colder countries glide rapidly along in the warm sunshine, or hide in nooks and crannies, and sleep the winter away; and the birds the merry, active, warm-hearted birds—live everywhere.
Yet still the great backboned division is not exhausted; on the contrary, the most powerful if not the most numerous group is still to
come—the mammalia, or milk-giving animals. Let us first notice two important changes which give them an advantage over other back-boned creatures. We have found the fish casting their eggs out into the water, and, as a rule, taking no more thought of them; so it was again with the frogs, so with the reptiles, whose eggs, even when carefully buried by the mother, are often devoured by thousands before the little ones have a chance of creeping out of the shell. And with the birds, in spite of the parents' care, more eggs probably are eaten by snakes, weasels, field-rats, and other creatures, than remain to be hatched.
Now, the cat and the cow, as we all know, do not lay eggs as birds do; but the mother carries the young within her body till they are born, perfectly formed, into the world. And when at last her little ones see the light, the mother has nourishment ready for them: part of the food which she herself eats is turned into milk, and secreted by special glands, so that the newly-born calf or kitten is suckled at its mother's breast till it has strength to feed itself.
Among the earliest milk-givers must have been the ancestors of the curious pouched creatures of Australia, the "marsupials," which have a large pouch of skin, into which the mothers put their little ones when they are less than two inches long, and so imperfect that their legs are mere knobs, and they can do nothing more than hang on to the nipple with their round, sucking mouths, as if they had grown to it.
There the little ones hang, day and night, and their mother, from time to time, pumps milk into their mouths, while they breathe by a peculiar arrangement of the windpipe, which reaches up to the back
of their nose. Then, as they grow, the pouch stretches, and by-and-by they begin to jump out and in, and feed on grass as well as their mother's milk.
Other singularly old-fashioned animals may be found in a Brazilian forest. The furry little opossum, the dreamy sloth, the strange ant-bear, and the armadillo, whose back is covered with long shields like the crocodile.
Having now taken leave of the curious pouch-bearers and the strange primitive sloths and armadillos, we find ourselves left to deal with an immense multitude of modern mammalia, which have spread in endless variety over the earth, and which may be divided into five great groups: the Insectivora, or insect-eaters; the Rodents, or gnawers; the climbing and fruit-eating lemurs and monkeys; the Herbivora, or large vegetable-feeding animals; and the Carnivora, or flesh-eaters.
It is clear that the Rodents and Insectivores do not hold their place in the world by strength or audacity. Both lowly groups, of simple structure and with comparatively feeble brains, they have chiefly escaped destruction from higher forms by means of their nocturnal and burrowing habits or arboreal lives, and the marvelous rapidity with which they breed, combined with their power of sleeping without food during; the winter in all cold countries. But the insect-eaters have no water-animal to match the beaver among rodents in sagacity or engineering. With his chisel-like front teeth he gnaws a deep notch in the trunk of a larch or pine or willow, and then, going round to the other side, begins work there till the trunk is severed and falls heavily on the side of the deep notch, and therefore away from himself. He always makes the deep notch in the trunk on the side near the water, so that the tree in falling comes as near as possible to the stream. Then, after stripping off the bark and gnawing the trunk into pieces about six feet long, he uses his fore-paws and his teeth to drag them into position to build his dam. He does not always clear away all the branches, but he and his companions place the logs with these lying down the stream, so that they act as supports to resist the current and prevent the dam being washed away. Thus they make a broad foundation, sometimes as much as six feet wide, and upon this they pile logs and stones and mud till they have made a barrier often ten feet high and more than a hundred feet long. The lighter branches he uses to make his oven-shaped lodge, laying them down in basket-work shape, plastering them with mud, grass, and moss, and lining the chambers with wood-fiber and dry grass.
There remain to be noticed two groups of much larger animals: first, the Herbivora, or grass-feeders; and, secondly, their great enemies, the Carnivora, or flesh-feeders. We shall see that the vegetable-feeders have filled every spot where they could possibly find a footing, and if we could only trace out their pedigree we should be surprised to find how wonderfully each one has become fitted for the special work it has to do. But three things they all require and have. The first of these is a long face and freely-moving under jaw, with large grinding teeth to work up and chew the vegetable food; the second, a capacious stomach to hold and digest green meat enough to nourish such bulky bodies; and, the third, good defensive weapons to protect themselves against each other and against their enemies.
In the three-toed group of the vegetable-eaters, the horse has the most interesting history. It was in America that the tribe began, for there we find that tiny pony not bigger than a fox, with four horn-covered toes to his front feet (and traces of a fifth), and three toes on his hind ones. Then, as ages went on, we meet with forms with only three toes on all the feet, and a splint in the place of the fourth on the front ones. In the next period they have traveled into Europe, and we find larger animals with only three toes of about equal size. One more step, and we find the middle toe large and long, and covered with a strong hoof, while the two small ones are lifted off the ground. Lastly, in the next forms, the two side-toes became mere splints; and, soon after, well-built animals with true horse's hoofs abounded, the one large hoof covering the strong and broad middle toe. For what we call a horse's knee is really his wrist, and just below it we can still find under the skin those two small splints running down the bone of the hand, while the long middle finger, or toe, with its three joints, forms what we call the foot. It is by these small splints that the horse still reveals to us that he belongs to the three toed animals.
A far different race from the Herbivora is the large army of flesh feeders, which we find throughout all past ages harassing and destroying the vegetable-feeders on all sides. And yet it would not be fair to speak of these larger flesh-feeding animals as if they had worked nothing but evil to their more peaceful neighbors, for how would Life educate her children if she put no difficulties in their way to be conquered, no sufferings to be endured? It was in the long struggle for life that the animals with the largest and strongest horns got the upper hand, that the swiftest horses or antelopes survived and left young ones; while we must remember that it is more often the sickly, worn out, and diseased animals that fall a prey to the devourers, and their life is ended far less painfully than if they dragged themselves into some hole to die.
"On revient toujours à ses premiers amours" says the French song. But who would have thought that, after rising step by step above the fish, and tracing the history of the backboned animals through their development in the air and over the land till we brought them to a state of intelligence second only to man, we should have to follow them back again to the water and find the highly-gifted milk-givers taking on the form and appearance of fishes? Nevertheless it is so, for seals and whales are as truly flesh-eating milk-givers as bears and wolves. "Do you really mean, then," exclaim nearly all people who are not naturalists, "that a whale is not a huge fish?" Certainly I do! A whale is no more a fish than crocodiles, penguins, or seals, are fishes, although they too live chiefly in the water.
A whale is a warm-blooded, air-breathing, milk-giving animal. Its fins are hands with finger-bones, having a large number of joints; its tail is a piece of cartilage, and not a fish's fin with bones and rays; it has teeth in its gums, even if it never cuts them; and it gives suck to its little one just as much as a cow does to her calf. Nay, the whalebone whales have even the traces of hind legs entirely buried under the skin, and in the Greenland whale the hip-joint and knee-joint can be distinguished with some of their muscles, though the bones are quite hidden and useless.
There was once a time when the great army of milk-givers had its difficulties and failures as well as all the other groups, only these came upon them not from other animals, but from the influence of snow and ice.
For we know that, from the time of tropical Europe, a change was creeping, during long ages, over the whole northern hemisphere. The climate grew colder and colder, the tropical plants and animals were driven back or died away, glaciers grew larger, and snow deeper and more lasting, till large sheets of ice covered Northern Europe, and in America the whole of the country as far south as New York.
True, there were probably warmer intervals in this intense cold, when the more southern animals came and went, for we find bones of the hippopotamus, hyena, and others buried between the glacial beds
in the south of England. But there is no doubt that at this time numbers of land-animals must have perished, for in England alone, out of fifty-three known species which lived in warmer times, only twelve survived the great cold, while others were driven southward, never to return.
Moreover, when the cold passed away, and the country began again to be covered with oak and pine forests where animals might feed and flourish, we find that a new enemy had made his appearance. Man—active, thinking, tool-making man—had begun to take possession of the caves, making weapons out of large flints bound into handles of wood, and lighting fires by rubbing wood together, so as to protect himself from wild beasts and inclement weather.
Many and fierce must have been his conflicts, for the wild beasts were still strong and numerous, and man had not yet the skill and weapons which he has since acquired. But, rough and savage though he may have been, he had powers which made him superior to all around him. lie had a brain which could devise and invent, a memory which enabled him to accumulate experience, and a strong power of sympathy which made him a highly social being, combining with others in the struggle for life.
At one time naturalists looked upon the animal kingdom as complete from the beginning, and, when it became certain that different kinds of animals had appeared from time to time upon the earth, the naturalists of fifty years ago could have no grander conception than that new creatures were separately made (they scarcely asked themselves how), and put into the world as they were wanted.
But a higher and better explanation was soon to be found, for there was growing up among us the greatest naturalist and thinker of our day, that patient searcher after truth, Charles Darwin, whose genius and earnest labors opened our eyes gradually to a conception so deep, so true, and so grand, that side by side with it the idea of making an animal from time to time, as a sculptor makes a model of clay, seems too weak and paltry ever to have been attributed to an Almighty Power.
- Abridged from Miss Buckley's book, entitled "Winners in Life's Race; or, The Great Backboned Family," from which also the illustrations are borrowed.