Time was when Holland and England were joined together - when the Rhine, with its tributary the Thames, joined the sea on a course roughly parallel to the present English coast.
The Dutch, more than the English, are aware of this. But then, more than most, the Dutch know the power of the sea and its ability to change the landscape. They have seen it happen all too frequently.
When the last Ice Age came to an end, a turbulent North Sea was born that began cutting a broad path between Britain and the Continent. This erosion process has continued ever since, particularly at the expense of what is now Holland. When the Romans occupied the lowlands in 34 BC, the present islands off the north coast of Holland were not islands at all but a part of the mainland, and the Zuiderzee, which changed Amsterdam from an insignificant hamlet into a prosperous port city, did not exist. In its place was a small freshwater lake (Lake Flevo). In a dramatic reshaping of the land, the North Sea roared in via a series of storms between AD 1200 and 1300 to take posession of the area for some seven centuries before the Dutch began the slow process of repossession in the 1930s.
If the Dutch - with little more than shovels in their hands and a stubborn refusal to capitulate - had not challenged the sea with dikes, dams, and polders in a ceaseless game of give-and-take, much of the more densely populated part of present-day Holland would no longer exist. In fact, since 1200, the Dutch have reclaimed close to 1.7 million acres from the sea.
At first dikes were designed merely to hold back the advancing high tides - to keep in a dry and permanently usable state the vast acreage that existed at the low-water mark. Later, more ambitious projects even won back lands that had previously been permanently underwater.
The Dutch call their land Holland, literally hollow land, or more correctly the Netherlands, meaning bottom lands. The names are particularly apt for a nation that has created so much new living space by adding one polder to another. A polder is not land that has been taken from the sea by filling it in, but rather land that is maintained in a dry state by surrounding it with a broad , high embankment that keeps the sea out. The polders, then, are a collection of dishlike hollows in which 65 percent of the present-day Dutch live, work, and play.
Today fully 20 percent of Holland is reclaimed land. Schiphol Airport, where most visitors to this country first touch down, is 151/2 feet below sea level. Even 700-year-old Amsterdam is 41/2 feet below the average high tide of the North Sea. So, while other people go down to the sea, a majority of the Dutch go up to it.
A country that is so low that it has, in effect, walled itself off from the sea is ever vulnerable to that sea. Dutch farmers, in particular, refer to the sea as ''the waterwolf that devours our lands.''
Holland's problems start with storms that develop in the far north and send rising seas racing toward the south, where they are checked by the bottleneck between Dover and Calais. At this stage the British and French coasts are only 20 miles apart - too close by half in the Dutch view - and the sea backs up as a result. These storm surges, as they are called, threaten lowlands everywhere, even London on occasion, but nowhere more than Holland.
Over the centuries the sea has breached the dikes in major flooding some 130 times. But seldom has it done so with more devastation than on the night of Jan. 31, 1953.
Jan Heusdens now lives in retirement in The Hague and remembers vividly that stormy night almost 30 years ago. He stood on a bridge that spanned the Rhine and watched the incoming tide - propelled on by hurricane-force winds - threaten the dike that protected his town of Vlardingen and ''the heart of Holland'' that lay behind. ''We must never allow it to recur,'' Dr. Heusdens says with quiet intensity.
At the time he was burgomeester of the small industrial city alongside Rotterdam, where he directed efforts to shore up the dike. He watched the water rise to within three inches of the top, knowing that if it were breached there was nothing after that to stop the sea from racing through Rotterdam, The Hague, and Leiden, almost to the doorsteps of Amsterdam. Every Dutchman knows that once a storm-driven sea tops the dike it quickly gouges out the land behind, and soon thereafter the sea wall collapses with dramatic suddenness.
As it was, Zealand to the south - an area with so many inlets to the sea that it has been likened by the Dutch to a sieve - was breached in 89 places. Gallant skippers in some instances even sailed their ships broadside into the gaps in an attempt to block newly opened breaches. And in one providential incident the storm itself tossed an abandoned sloop into a breach as accuratey as if a computer had programmed the incident.
When at last the winds were calm and the ultrahigh waters had dissipated through the English Channel or retreated back toward the far north, this most attractive and prosperous farming area of Zealand had been devastated. Almost 2, 000 had perished; most livestock had disappeared; and nearly half a million acres of much-needed farmland was underwater and out of commission for years.
The only answer was to start all over again. But how could similar devastation be averted in the future? The dikes all along the hundreds of miles of peninsula and estuary coastline could not be raised more than two or three feet because the soil below would sink under the increased weight. In addition, many homes and churches were built on top of the dikes. This meant the dikes could not be made high enough to resist the exceptional storm that might come along no more than once or twice in a century.
So the Delta Plan, envisioned 400 years earlier by Andries Vierlingh, an engineer in advance of his day, was put into effect. It was an ambitious project that called for the damming of three major inlets, just as the Zuiderzee had been dammed off from the North Sea some 50 years ago, converting that once-stormy salt sea into a relatively placid freshwater lake - IJssel Meer. The effect of the Delta Plan is to reduce the Dutch coastline (and its accompanying need of dikes) by 700 kilometers (about 435 miles).
Work began in the late 1950s and will be completed by 1985 - some seven years behind schedule because of alterations resulting in less environmental dislocation. Where two of the inlets (Haringvliet and Brouwershavense Gat) were blocked with the solid sea walls originally envisioned, the Eastern Scheldt, largest of the three, was changed from a dam to a storm-surge barrier. It has gates that allow the tides to rise and fall almost as freely as before but which can lock tight to exclude marauding storm waters whenever the need arises.
When the Zuiderzee was dammed in the 1930s, its salinity slowly faded as rivers discharged fresh water into the sea to push out the salt water through sluice gates at every low tide. The effect was both predictable and unexpected. While the salt-loving herring slowly disappeared, the flounders, remarkably, were able to make an adjustment to their changed surroundings, while the eel population exploded. Now vendors in the little fishing towns along the IJssel shore sell eel as freely as they did herring in pre-World War II days.
Something similar to the flounder adjustment has taken place behind the Greyvelingen dam at Browershavense Gat. The seawater there is now tideless, and it was assumed that the thriving oyster population would die out, because oysters eat by sucking in food that constantly flows by them on the currents. Harvesting stopped when the dam was completed.
But now it turns out that the oysters have reacted to the stagnant situation by oscillating their shells to create minicurrents of their own. So successfully have they done this that in the decade-long absence of harvesters they have multiplied many fold to an estimated 50 million.
The Dutch have not yet decided whether or not the Greyvelingen shall slowly become a freshwater lake, as happened to the Zuiderzee. The unexpected evolution of the oysters may well make them retain it as a saltwater lake.
Environmental concerns today are a major part of any debate over land reclamation in this tiny country in a way they never were before the 1960s. Whenever man changes the landscape with a dam or a dike, the environment is altered, although never ''destroyed,'' as some seem to think. If fresh water replaces sea, freshwater species take over from their saltwater counterparts. Where new land has been taken from the sea, wind-blown seeds soon take root. Hares are quick to appear, which in turn attract hawks and owls. Other species quickly follow. The polders of IJssel Meer, for instance, support a greater variety of birds and a more concentrated population than anywhere else in Holland.