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.

From almost every country pond, or ditch, or swamp, a chorus of voices rises up in the springtime of the year, calling to us to come and learn how Life has taught her children to pass from the water to the air; for it is then that the frogs lay their eggs, and every tadpole which grows up into a frog carries us through the wonderful history of an animal beginning life as a fish with water-breathing gills, and ending it as a four-legged animal with air-breathing lungs.

Come with me, then, to some stagnant pool in a country lane, towards the end of March, and there we shall no doubt find a whole company of frogs, croaking to their hearts' content after their long winter sleep in the mud at the bottom of the pond. They are wide awake now, and are actively employed laying their eggs. Look carefully around the edges of the pond, especially in that part where the wind has driven the scum to the side, and you will doubtless find in some still corner a gluey mass (e,  Fig. 15), which looks like a lump of jelly with dark specks in it. Take this up carefully, for it is frog spawn; carry it home together with some weeds from the pond; put it in a glass bowl with water; and then from day to day you may study the history of a frog's life.

That jelly-like mass is a collection of frog's eggs. When they were laid, each egg was a small round dark body in a gluey covering, and they all fell to the bottom of the pond, where, by degrees, the water oozing through the envelope swelled each egg, till they clung altogether in a mass, and, rising, floated at the top. Then very soon each round dot lengthened out into a long streak, and in a few days an eyeless head appeared at one end with a soft closed mouth under it, and at the other a tail, with a soft fin round it like the tail of the lancelet; so that by the time you find the spawn, you may, most likely, be able to see the tiny creature wriggling every now and then in its watery bed. This will go on for some time, and a week or two may pass before the moving tadpole breaks through its egg skin, and coming out into the world, fastens on to a piece of weed (1, Fig. 15) by two little suckers behind its mouth. And now that it is out of the egg the interest begins. Look carefully day after day and you will see some branching tufts (2, Fig. 15) growing larger and larger on each side of its head. What are these? We have not seen them in any fish. No! but if you take a young hound-shark out of his leathery egg before his time, you will find that he has outside gills much like these, only he loses them before he comes out into the world, whereas the tadpole keeps them to breathe with a little longer. If you put the tadpole, at this stage, under the microscope, you can see the red blood flowing through these gills to take up air out of the water.

Meanwhile the tadpole's lips are gradually forming into a round mouth, much like the lamprey's, and by-and-by the inner part of this mouth is covered with two little horny jaws, forming a sharp beak (3, Fig. 15) with which he will nip off pieces of weed for food. Meanwhile, as he grows larger and larger, and eyes, nostrils, and flat ears form in the head, a covering begins to grow back over the sides of the neck, and little by little the branching tufts disappear (3, Fig. 15). How, then, can he breathe now ? Watch carefully and you will see that he gulps every moment as we saw the minnow doing (p. 23). The fact is that the outside tufts have faded away, and under the cover the tadpole has six slits in his throat, like the slits of the lamprey, which are covered in somewhat similar fashion to those of the amphioxus (see p. 11), and he breathes through them.

Here is our tadpole, then, to all intents and purposes 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, like the minnow's (p. 23), 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. All this time, however, though he has a fish's fin round his tail, he has no arm or leg fins. Wait a while and you will see that under his tender skin far more useful limbs are being prepared. As he grows bigger and more active week by week, wriggling among the weeds and feeding greedily, 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 (4, Fig. 15) are working between the body and the tail. The two front legs are longer in coming, for they are hidden under the cover which grew over the gills, but in about another week they too appear, and we have a small four-legged animal with a lamprey's tail (5, Fig. 15). These legs are something far in advance of fish fins, for they have shoulders and thighs, arm and leg bones, wrist and ankle bones, hand and foot bones; and instead of the large number of rays in a fish's fin they have four fingers on their short front legs, and five toes at the end of long hind ones; the toes being joined together by a web, which helps him wonderfully in striking the water as he swims.

The tadpole has now become fitted to jump and leap on the land or swim by his legs in the water; and, moreover, while these legs have been growing, another change has been taking place. You will notice by careful watching that at first he still gulps in water as he used to do, but he comes more often to the top, and, poising himself so that his mouth is out of the water, gives out a bubble of bad air, draws in some fresh, and goes down again. Why does he do this? Have you any recollection of another fishlike animal which comes up to take in air? Look back at our friends the mud-fishes (p. 34), and read how the Ceratodus fills his air-bladder when he is short of good air in the water. When you have re-read this, you will suspect that the tadpole, too, has something like an air-bladder, which he fills from time to time. And so he has. While his legs are growing a bag has been forming inside at the back of his throat, which afterwards divides into two, and he fills these by shutting his mouth, drawing air in at his nostrils, putting up the back of his tongue to shut it in, and then swallowing it down into the lungs; so that he is now a truly double-breathing animal, using his gills when below water and his lungs when above. Moreover, if you could watch inside his body, you would now see that little by little the blood-vessels going to the gills grow smaller and smaller, and those going to the lungs grow larger and larger; while the fish's two-chambered heart divides 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. And when at last this change is so complete that all the blood goes to the lungs to be freshened, the gills shrivel up and disappear, and our tadpole is a true air-breathing animal.

Notice, though, that he is still cold and clammy, not warm like a mouse or a bird. For his blood still moves slowly, and as he has only three chambers to his heart instead of four, as warm-blooded animals have, the good blood from the lungs and the worn-out blood from his body become mixed each time they come round, so that his breathing work is still of a low kind all his life. And 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, and the young frog is complete.

Thus our backboned animal has succeeded in getting out of the water on to the land, and in doing so lie has quite changed his habits. A peaceful vegetarian before, he is now a greedy eater of insects, slugs, and other animals. His horny beak has been pushed off; his lips have stretched back farther and farther, till they now open right back as far as his flat little ear; and he is a gaping, wide-mouthed, leaping frog—

(7, Fig. 15), with teeth in the roof of his mouth. But perhaps his tongue is the most curious of all, for instead of being fixed at the back, and free in the front, as in most other animals, the root of it is fastened to the front of his lower jaw, and the tip lies back in his mouth, so that when he wishes to catch an insect he throws his tongue quickly forward, captures his prey on the sticky point, and flings it back down his throat.

So he hops about the summer long, if he can only escape from ducks and rats and other frog-eating animals. He often takes to the water, for he can fill his lungs with air and use it very slowly, and, moreover, his soft skin is of great use to him in still breathing in the water or in the moist air; and when winter comes he takes refuge with many others at the bottom of the pond, and sinks into a state of torpor, till the spring brings croaking and egg-laying time round again.

Our little frog, then, is truly an animal with a double life, a genuine amphibian, Amphi,  all around; bios,  life.") ?> meaning by this, not merely an animal that can swim in the water and move on land, for seals and water-rats, white bears and hippopotamuses, can do this, but one that in the early part of its life would die if taken out of the water, while afterwards it lives and breathes in the air.

Have these double-lived creatures, then, such a great advantage over real water animals, or how can we account for their having adopted this strange life? If we only look upon them as they are now, we can scarcely call them particularly successful, compared to other animals. For though there are plenty of them, yet they are comparatively small and insignificant; and when we find large ones like the gigantic salamander of Japan, they are sluggish and feeble. Look at the common newts, or water-salamanders of our ponds, with their weak crawling limbs, as they wander round the edges of a pond, feeding on water-insects and tadpoles, the male with his crested back, the smooth mother, and the young eft-tadpole with its branching tufted gills (Fig. 16). They are much less active than the frog, for they never lose their tails, and they come less often out of the water, although they are true air-breathing animals. Then, when we go to other countries, there is the Proteus (Fig. 17), that curious half-transparent newt, with a round body and tiny helpless legs, which lives in eternal darkness in the still underground pools of the Carniola caverns near Adelsberg. He has become well fitted for his dismal life, for his tiny eyes are grown over with skin, and he never loses the feathery gills on each side of his neck, but lives like a tadpole all his life, although he has true lungs. Again, in America we have the Siren, with its long snake-like body, and only front legs, with which it cannot walk. It, too, keeps its gills as it wanders about the stagnant waters of South Carolina, feeding on worms and insects. Then in the Mexican lakes there are the curious Axolotls, which also wear outside gills, as a rule, all their lives, and fathers, mothers, and children remain breathing in the water together, although they have real lungs. But about twenty years ago, some of those axolotls, which were kept in the Jardin des Plantes in Paris lost their gills, came out upon the land, and astonished people by becoming true land salamanders, like some already well known and called Amblystomes, breathing only with their lungs. It was difficult for some time to make the world believe that grown-up water-breathing creatures which could lay eggs were able to turn into other creatures without gills. But at last a lady, Fraulein Marie von Chauvin, took some axolotls when they were full-grown, and kept them on land in wet moss, washing and feeding them every day, and thus succeeded in teaching them to breathe air, so that their gills shrivelled up and disappeared. Then there could no longer be any doubt that the axolotl is only the lower water-form of the amblystoma, which in the Mexican lakes, owing to the increased dryness of the surrounding country, has lost the habit of coming out on to the land, and remains in the water with its little ones all its life; but which, when brought to a moist climate where it can breathe comfortably on land, sometimes returns to its old double life.

We have, in fact, in Europe real land salamanders, which live in cool damp places, looking like lumpy soft-skinned lizards, but going down to the water to lay their eggs, that their little ones may go through their tadpole life—and one of these, the black salamander, which lives high up in the mountains of Germany, France, and Switzerland, does not even go to the water, but carries the young tadpoles in her body till they can breathe air and run alone; and yet they are still true amphibia,  for if they are taken out of their mother and put in water, they go through all their changes like common efts and newts.

Lastly, there is a strange group of legless creatures called Cxcilians, which have taken refuge underground, burrowing like worms, though they are true amphibians and their young have gills in their babyhood hidden under a slit in the neck. These cmcilians are the only amphibians which have scales something like fishes, yet they never live in the water, but in the marshy ground of tropical countries, feeding on worms and insects.

Now when we think that these sluggish newts, and salamanders, and cæcilians, with their more nimble but comparatively unprotected relations, the frogs, are all the amphibians now living, we cannot but wonder how Life came to produce such a feeble set of creatures to fight the battle of existence.

But if we glance back to that far-off time when the ancient fishes were wandering round the shores and in the streams of the coal-forests, we shall be better able to read the riddle. For in those days it was a great step for an animal to get out of the water at all, and those that did so had a much better time of it than our frogs and newts have now, when the country is full of land enemies.

And so we find that the amphibia   were not then the small scattered groups they are now, but strong lusty animals, with formidable weapons. In the hardened mud, which in those days formed the soft swampy ground of the coal-forests, but is now stiffened into the roofs and floors of our coal-mines, footprints have been left which tell us of large and formidable creeping animals, with toed feet and long flat tails, dragging themselves over the marshes of the coal-forests, and finding their way to many places which even the mud-fish with their paddles could not reach; and from time to time, in these same roofs and floors of our mines, both here and in America, we find the bones and coverings of these amphibia,  buried in Nature's catacombs for ages, and only brought to light by the rude hand of man.

These remains remind us that

for they show us 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. Some of these were fish-like, with short necks and broad flat tails, but they had true legs and toes; others, more like croc iles, and sometimes ten feet long, were able to walk fir but still dragging their bodies and long tails over the swampy ground on which their footprints are still found; some were small and more like lizards, with simple teeth, scaly armour, and" light nimble bodies; and these, probably, ran about quickly on the land, and have sometimes left their skeletons in the hollow trunks of the old coal-forest trees.

All these plated and formidable creatures were amphibia  or double-lived animals, and this was their  Golden Age, as they preyed upon the fishes in the swamps and ponds, probably not sparing even their nearest connections, the mud-fishes, who, less fortunate than themselves, had followed the road of fish-life instead of coming out upon the land. They lived so long ago that we can tell but little of their daily lives, but it is clear that they played a very different part from our small frogs and newts of to-day, and in their well-formed limbs were worthy forerunners of land and air-breathing animals.

But like the old race of fishes these large amphibians were only to have their day, for as other branches of the family tree grew up, and reptiles grew strong and mighty, and other true land animals began to flourish, these huge plated forms dwindled away, and we lose sight of them; and when we find any of their relations again it is only as our present frogs and newts, salamanders and cxcilians, which have taken up their refuge in lakes, ponds, ditches, underground waters, or damp mud. And, curiously enough, those forms of to-day which are most like the huge Labyrinthodonts,  Laburinthos,  spiral; odontas,  teeth).") ?> as they are called, of the old coal-forests, are the feeble cxcilians, with their horny scales and their numerous ribs, although they have now fallen the lowest of all amphibians, and, with their sightless eyes and ringed and legless bodies, have taken to burrowing in the ground like worms.

Not so the frogs, which, like the bony fishes, began their career in later times, and have known how to fit themselves into many nooks and corners in life. In almost all countries of the globe they hop merrily about the ponds and ditches, never wandering far from the water, into which they jump and dive whenever danger threatens. It is true they are eaten by thousands, both as tadpoles and frogs, by birds, snakes, water-rats, and fish, and even by each other, but they multiply fast enough to keep up the supply, and find plenty of insects both in and out of the ponds. Nor have they kept entirely to a watery life, for their near relations, the toads, which have toothless mouths and toes less webbed, have ventured much farther on to the land, protected partly, no doubt, by the disagreeable acrid juice which they can throw out from a gland behind the eye whenever they are attacked.

It is curious to notice the quiet leisurely waddle of the sluggish toad, as he spreads out his short fat legs and puffs out his warty skin, and to compare him with the nervous, anxious, little frog, starting at every danger. And still more curious is it to see him getting out of his skin, as he does several times a year. For his skin does not peel off in pieces as it does in the watery frogs, but splits along his back; then he wriggles about till it lies in folds on his sides and hips, and, putting one of his hind feet between the front ones, draws the skin off the leg like a stocking off a foot. With the other leg he does the same, and then, drawing out his front legs, pulls the whole skin forward, and stripping it over his head, swallows it; thus deliberately putting his old coat inside him, and appearing in one that is glossy, fresh, and new. The toad has many enemies in spite of his acrid taste, and he shows his wisdom by hiding in walls and under stones in the daytime, and coining out in the dusk of evening to hunt the beetles and grubs so often out of reach of the water-loving frog.

But the toad is not the only land relation of the frog; there are others of the group that venture even farther from water; for in most parts of the world (though not in England), tree-frogs, with sucking disks at the ends of their toes and fingers, climb the trees and hunt for insects among the leaves and branches; while in Borneo Mr. Wallace found one (Fig. 19) with webbed feet, which it spread out, and so flew down from the trees. There are plenty of the ordinary tree-climbing frogs to be seen in the south of France, their small green bodies peeping out from under the dull gray olive-leaves; and to be heard, too, in an endless chorus all night long when the spring arrives.

But how can these tree-dwellers bring up their little ones in water? Some of them come down and lay their eggs in the ponds, and even sleep down in the mud in winter. Others lay their eggs in little puddles of water in the hollows of the trees, and there the young ones live their tadpole life; while in one curious tree-frog of Mexico, called the Nototrema,  the mother has a pouch in her back, and the father places the eggs in it for the little tadpoles to live in a moist home till they leap out as perfect frogs.

Nor is this the only case in which fathers and mothers take care of their young. In one species of frogs living near Paris, the father winds the long string of gluey eggs round his thighs, and buries himself in the ground till the young tadpoles are ready to come out, and then he leaps into the water. And in one of the tongueless toads, the Surinam toad, the mother's soft skin swells up, forming ridges and hollows, and when her eggs are laid the father clasps them in his feet, and, leaping on her back, puts an egg into each hollow. Then the mother goes into the water, and remains there while each tadpole completes its changes in its own hole, jumping out at last a finished toad.

Yet, in spite of curious habits such as these, the frogs and their companions on the whole lead a very monotonous life. They are, it is true, more intelligent than fish, and have learned to know more of the world, but in the long ages that have passed since their ancestors roamed in the coal-forest marshes, other and higher animals have taken possession of the land, and left room only for a few scattered groups of amphibia.  Still, however, they remain hovering between two lives, and filling such spots as neither the fishes nor the land animals can occupy; and when we hear them croaking in the quiet night, or see them leaping on the marshy ground, they remind us that we have still living in our day, a link between the fish whose world is a world of waters, and the air-breathing animals which have become masters of the land.