THE burn is in spate. It swirls round a sharp curve, then sweeps on its course, with occasional eddies. A burn is not a river, nor the miniature of a river. It is to a river as childhood is to maturity. Its changes of mood are sudden. It is volatile. It plays as children play. And this afternoon it is having a fit of temper. It has for us much of the interest we feel in childhood.

It is very brown. The sides fall in to make it browner. The worst is past. Against the far bank the level is sinking. I n the gleams of sunshine it is like a child smiling itself back into good humour. A dipper is flying up and down, lighting, for a little, on the alder branch, where it bobs impatiently. Its perch on the boulder is out of sight ; the larder at the bottom is closed. It is fain to drop down where the bank softens into a sandy or gravelly stretch for such larvae as are left by the shrinking water. Though birds like to feed in their own way. Sweeter is that brought up from the dive. Silver runs-broken and made musical by the gravel-and the clear shallow pools are the natural haunt of the dipper. The northern streams are of this character. Though not a Scots bird, perhaps it gets more of what it needs here, and is most at home. A Scots stream without a dipper were almost a play without Hamlet.

It has its nesting area, where it is found year after year. It may shift up and down the burn a little as winter approaches. It has certain narrow limits of migration. Though often the same pair may keep to their summer quarters all year round ; the food is perhaps more certain in the upper reaches. But hard weather makes its own conditions.

A sedge-warbler is either scolding or prattling, or both. The harsh notes sound like scolding, and may well be meant for the stream which has been creeping up the grass stems, floating out the long pendent twigs of the white willow, and coming within measurable distance of its nest. Between the harsh notes is a long string of prattle, much of it pleasant prattle, with some notes very like they had been stolen from our sweetest singers.

Each summer the bird is there, always at the same eddy. It comes in May; it sings all through the lingering twilights of June, and intermittently throughout July. The young flit about with the old, six or seven in all; then they vanish. They go south, somewhere in Africa. Next year only a pair are in the willows. Does it mean that this is the annual waste of life? Out of every seven only two are left? Or do the young go elsewhere? Over the whole burn one season's warblers are not greater than another. Whence the mortality? It is mainly in the young. Buffeted by winds on the passage, or heedless of risks, do they lose touch with the sage guides of the flight ?

This is our Scots warbler. Each patch of marsh lodges its summer pair. On the watercourses which network the land, it is familiar; save only where some wilful bend has left a dry place. I have not yet been where it was absent. So rich in warblers is the south that it may well afford us, who have so few, this one all to ourselves. It has been named the Scots nightingale -not because it sings so well, but because it is our only night-singing bird. The peasant, in the country cottage near where water is, hears it, on his pillow, as a not ungrateful lullaby. From its perch on the willow it catches the step of the late wanderer by the streamside. It chatters to him as he passes, scolding, with its harshest notes, that he is out at such untimely hours. Who has not gone down in the delightful dim coolness just to hear, and could not point out the very turn in the path where the first greeting note will sound; and, when the scolding stopped, has not sent a little pebble at the bush that he might begin again?

In Scotland, if we boast not the finest singer, we have the most charming setting. What environment may be to a song, and how the sweetest outpourings may suffer in a poor and unhelpful scene, the prince of observers tells us :

The magic light of the lanes, the deeper shadows under the hedges, the mystery, the stillness, disturbed only by the passing moth, are all in the song. It is a poor imagination that hears but the sound and finds no space for the rest.

If it be so in the sweet south, how much may the magic of our northern night-drawn out and mystic, and still, beyond the dream of those of other lands-add to the northern night song? Almost enough to make a sedge-warbler's chatter by a Sutherland stream arresting as a nightingale's lay in a Surrey lane.

A delicate lad is fishing. The nervous hand drops the bait just where the burn surges out of the curve. I watch the line in its swift career in the straight run, and round the eddy, where the tall grasses grow, and the white willow bends. More than once the line surges down. Each time it swirls in the eddy, the sedge-warbler scolds, because its nest is there.

At length is a check. The hook may have caught on something in the bottom. If a bite, it is a slow bite. The angler watches the tremor curiously; so do I. When the strain is put on, it comes away like a twig ; and like a black twig it appears on the surface-only it is a living twig. It is an eel which has lived in the eddy where the white willow leaves dip just under the nest, and in a kind of summer comradeship with the sedge-warbler.

It is the penalty for dabbling in brown water; at any time unsportsmanlike. Eels are alive in a spate. The rush and swirl stir up the muddy bottom, and there is so much to eat. The angler gazes helplessly at his catch; it is a yard long, hopelessly hooked and twisted in a dozen coils round the gut cast. The only resource is to make a present of so much as it has swallowed and give it a chance. No use in killing even an eel.

A look of disgust comes into the sensitive face. There is a natural shrinking from anything so snake-like. It is hardly fair. We owe some return. Before I have done I hope to invest the wriggling form with a wonder, absent from its brighter stream-mates. So that next time we may look with a curious and more kindly eye. The eel will be avenged.

Further fishing is hopeless. The burn swarms with such. Had we not watched them coming up in the spring-myriads of wriggling needles? So many that when they grew bigger the water would not hold them. And what enemy had they, save a solitary heron, or an otter journeying from the water beyond the ridge?

A long-time wonder was where the elvers came from. If the hatching is done amid the gravel, or the mud, why should they not appear singly, or in small shoals, as the trout do? Why crowd up the current in an almost unbroken phalanx from the sea, like travellers who were eager to be at their journey's end? The puzzle was to make a life cycle of the big eels in the burn, and the elvers which appeared so strangely and suddenly in the spring-to fill up the gap between. From elver to eel was a matter of growth; between eel and elver lay the rub.

The rustic had his theory, as he has of all things recondite. So arose folk-lore, and countryside natural history, so childlike, so past belief, and yet so attractive. Hid from the wise and prudent, these charming half-truths or whole fables are revealed unto babes. If an explanation holds the field, so long as it is alone, then the rustic's view had its day. Science might pooh pooh, but could find nothing to take its place. Say it is unlikely that elvers came from horse hairs, and that the link of connection must be sought in the tail of the steed grazing on the rich streamside grasses. Nor is there any such thing as spontaneous generation. Life comes out of life, and a break means death. That is all negative. How much more can the savant tell?

That the big eels went down stream in the autumn was known. Men forecast the time, and prepared traps which they set facing up current, whence the rush of migrants would come. Before they left, they were seen to pass through certain changes. The eyes grew bigger, so did the ears; as though they were preparing for a place harder to hear and see in; going to deeper and dimmer wastes than the shallow spring burn.

A certain sheen came over them, the promise of something brighter, a faint glory of attire, such as other creatures don on the eve of wedding. Plainly they were bound on a fateful journey, fraught with a faint sense of bliss, increasingly dawning; every stage of which brought them nearer to a promised land. Many wildlings set themselves for the same land ; but not so steadfastly, nor over so dim or mysterious a way. So far can we follow, to the kiss of burn and salt wave, where the eel vanishes in the autumn, and the elver appears in the spring. They were going to spawn. But where? And when? Not near, nor soon.

Spawning was distant, where neither male nor female was ripe. Why, then, start so early, if it be not, that the mating and the spawning were far off, many weeks and long miles away. Thus unripe, they pass out, where the brown of the autumn flood broadens and finally loses itself in the blue of the deepening sea.

The North Sea seemed tenantless. Amid its multitude of eggs and larvae was no egg nor larva of eel. A little light was thrown elsewhere. The warring currents in the Straits of Messina cast up ripe eels on the shore. It was concluded that they spawned at depths of not less than 500 metres, under great pressure of water, and died in the spawning. But it is a far cry to Messina.

Further progress has been made. A second area has been discovered. To the west of the British Isles, where the water lies to the depth of several thousand feet, was evidence of a great company of spawning eels. The larvae are ribbon-shaped, and named, from the smallness of their heads, Leptocephali. In one sweep of the net as many as seventy were caught. To the discoverer of this promising area, it seemed as though he had come upon the breeding ground of the eels of Northern Europe.

From the Baltic, from the rivers that rush, the burns which trickle into the North Sea, the descending eels launch out with a confidence that allows of no hesitation. Through the English Channel, they seek their way to the westerly goal. Those from the more northerly streams may choose the shorter route round the north of Scotland.

Onward they go, with the precision of an army on the march, or a flight of migrant birds, which follow the same route, year by year. By ocean valleys, or along water-dimmed slopes they go. Possibly they join forces by the way, to swell the number of the main body. Of the marked specimens caught at the various stages of the journey, one was found to have covered upward of four hundred miles in fifty-one days.

This little flooded burn is an unconsidered item in the drainage system. Still it has the beginning and the end of the story. So that, whatever takes place elsewhere is gone through here. The eel just put back, if it survived, would-with all the other eels-leave by and by. When they reached where the current met the waves, they would turn to the right, or to the left. Being so far north, they would be likely to choose the left. At the rate of fifteen or twenty miles a day, they would pass up the east coast. I n three weeks they would round Cape Wrath. A month later they would reach the deep water and the breeding ground.

There they would shed their now ripened eggs, to float midway between the bottom and the surface-and, having done so, would die. In due time and order, would come the hatching, and the early larval changes. The needle-like and transparent elvers would start on the momentous journey. They would retrace the way the adults had come-would repass Cape Wrath-coast down the North Sea. And-assuming the young revisit the scene their elders left-arrive some day in March or April, at the mouth of the burn. Thence they would crowd up the current to complete the cycle.

All this is very interesting. But, if a theory is to be tried by its weakest part, then there are one or two weaknesses. There is a marked tendency in some observers to push their little discoveries unduly. Scientific imagination is an excellent thing, meant to span gaps and connect what stand apart, but it must be skilfully used. Were engineers to span chasms so, the structure would collapse. Were business men so to speculate, they would be bankrupt in a week. In the library of science is no place for an Arabian Nights.

The adult eels from the North Sea burn die far out in the Atlantic, and leave their young in the waste of waters. By what means do these young find their way back? Do they come, these fifteen miles a day through the unknown, in the directest route to an unknown destination, by reason of something within that no one understands? Of course, anything is possible. But this does not differ from the many baseless things which modern thought killed out. Science, it seems, may be a mystery-monger. For my part, I should be extremely hard to convince that an elver could find its way out of the Atlantic, much less round the north of Scotland and into the mouth of an obscure burn.

When the elver grows into an eel, and the autumn flood bears it down to the sea, it may seek the deep waters where it was hatched: it is just possible. More probable is it that a single journey in the elver stage would prove insufficient.

If the breeding ground for Northern Europe is thus out in the Atlantic, some filling up is needful. The story is not quite told. Between the elver and the eel some chapters are wanting. In any case, a mystery does not solve a mystery.

Common sense comes to the aid of a somewhat wild imagination. Possibly all the breeding eels do not die. Enough may be left to guide the young back. Or immature eels may accompany the breeders, and so come and go more than once. Anything seems reasonable, compared with finding a way never before traversed.

Migration is no more than unbroken habit. It is the same in eels as in birds: it is not a mystery. Did all the breeding sedge - warblers die, the young would never leave our land, or, if they did, would not know which way to turn, and surely be lost. The old birds guide; the young learn the way against the time when they in their turn will be guides. And so the lesson is handed down from generation to generation. If the elvers come back to the eddy, it will be as the sedge-warbler comes to the long grass under the willow, where it has its nest.