Sooner or later, in life as in the theater, there's a final curtain. On June 8, it will come down on "The Fantasticks." By then, Harvey Schmidt and Tom Jones' vest-pocket musical will have racked up a staggering 10,864 performances in New York.

I'm sorry to see it go, although it's far from my favorite musical. Oh, I loved it when we first met -- sometime in the 1960s during a college break. Over the years, as I was called upon to review any number of subsequent productions in and around Washington, I discovered I liked it somewhat less. Now I suspect further exposure would only do injury to fond memories. But I can still mourn its passing. Long runs, you see, reassure us that the theater, that most ephemeral of arts, has a measure of permanence. It's an illusory notion, of course, but "The Fantasticks," in its humble way, has been a bulwark against the flux.

And I can't deny my initial reaction, when the musical and I were both younger and I floated out of the tiny Sullivan Street Theater off-Broadway, proclaiming the experience "outta sight." Things, I recall, were either "neat" or "outta sight" back then. Strictly speaking, "The Fantasticks" was neither. It was a rather modest show, really, with some sweetly hummable tunes like "Try to Remember" and "Soon It's Gonna Rain." But shortly afterward I ran out and bought the original cast album, which I played constantly for about six months on my record player. (We had record players back then, too.)

What thrilled me as much as anything was the show's seeming spontaneity. Improvisational comics like Elaine May and Mike Nichols were all the rage and "The Fantasticks," which was performed by a handful of actors working out of a trunk on a bare platform under a tattered cutout moon, looked as if it was being hatched on the spot. Indeed, an arthritic old Shakespearean actor, accompanied by a dour Indian, climbed out of that trunk at one point, while a character SPECS called only The Mute held up a baton and an imaginary wall was born.

The story was beguilingly simple: a boy and girl falling in love over the apparent objections of their parents, who secretly couldn't have been more delighted by the match. But there was just enough cynicism to help make the sugar go down. "Without a hurt, the heart is hollow," sang El Gayo, the dashing narrator. So the starry-eyed lovers were propelled out into the wide world, got singed and limped home, chastened but still in love. In those days of unraised consciousness, no one thought to balk about "Rape," a comic fandango dedicated to the many varieties of abduction and assault.

The initial reviews were mixed -- Brooks Atkinson in The New York Times proclaimed the first act delightful, but thought the second act paid off in rapidly diminishing returns. No matter. "The Fantasticks" prevailed. It didn't hurt that Barbra Streisand, Harry Belafonte and Ed Ames were singing some of the songs over the airwaves. A subsequent television special in 1964, starring the late Bert Lahr, acted as a further spur to box-office. By one account, there have been no fewer than 8,913 productions in the United States alone and another 472 abroad.

Maybe it's just overfamiliarity that has dulled the show's charms for me. What seemed so innovative now strikes me as precious; what was wise is cloying. Did I really think the fable was saying something deep about "why we all must die a bit before we grow again"? I guess I did; maybe it was. Twenty-six years changes a lot.

Nonetheless, I'm sad. "The Fantasticks" folding up its tent for good is just another reminder -- not that reminders are lacking -- that the theater is perpetually engaged in a great vanishing act. Its preordained fate is to disappear before our very eyes. I'm not just talking about plays that open one night and close the next, or musicals that go belly up in previews, although that is increasingly the rule on Broadway, at least. I'm talking about the very purpose of the endeavor itself -- which is to create a moment for the moment. Each performance you see is passing through on its way to oblivion, unique, never to be repeated exactly the same way. Each production is colored by the whims and dictates of the instant. If it lives afterwards, it's only in the mind's eye or the heart's recesses.

I think that's why we hold onto our long runs. They reassure, at least temporarily, that the theater is not totally transitory. That the best plays do not always achieve the longest runs is not the point. Granted, you can make a case that the enduring "A Chorus Line" is the best of breed, but does anyone look upon "Abie's Irish Rose," which persisted on Broadway for 2,327 performances, as anything but a relic from the 1920s? Agatha Christie's "The Mousetrap" is a baldly manipulative piece of claptrap; more important is that after 33 years, it's still holding forth in London, a West End fixture and one of the few reference points on the ever-shifting theatrical map.

It was always comforting to me, whenever I went to Paris, which I did with some regularity once, to know that if the old neighborhoods were undergoing radical changes, I could still find Ionesco's "The Bald Soprano" at the minuscule Theatre de la Huchette on the Left Bank. An ever-renewable wave of exchange students learning French has kept it going for 30 years. And while the theater seats no more than a bus does, Ionesco put it in proper perspective, observing once that "A big success in a little theater is worth more than a little success in a big theater and decidedly more than a little success in a little theater."

In Los Angeles a couple of decades ago, I used to take my dates to a hoary old melodrama, now departed, called "The Drunkard." It was then in its 24th or 25th year. You sat around tables, drank beer and hissed the villain and cheered the hero. One of the actors had started out playing the dashing swain and, as the years ticked by, had graduated from role to role. He was playing the sage old uncle by the time I stumbled in. I relished that sense of continuity, suspecting even then that it was not the rule.

Indeed, the one way to arrest the eternal ebb and flow of the theater is the repertory system -- which puts plays up on the stage for a while, then tucks them away only to bring them back later. But repertory is a costly proposition and despite a number of noble stabs in the past, it has never taken root in America. In its absence, we rely on our long runs, telling ourselves that what's here today will be here tomorrow.

In rare cases, that certainly seems to be true. The principle of inertia comes into play and a show runs a sixth year simply because it's already run five, and then the sixth year spawns a seventh. Spectators turn up for the same reason that some mountain climbers are said to scale treacherous peaks -- because they're there.

Once the province of a few select metropolises, the long run has begun to crop up everywhere. In Seattle, "Angry Housewives," a show about four discontented suburban women who enter a punk-rock contest, has been selling out since April 1983. Boston has a participatory murder mystery called "Shear Madness" (it's set above a hair salon and a six-inch pair of scissors is the murder weapon) that's been luring crowds since January 1980. For 11 years, San Franciscans have rallied to successive editions of "Beach Blanket Babylon." And on our home ground, "Banjo Dancing," Stephen Wade's congenial exploration of tall tales, country music and clog dancing, is into its fifth merry year in Arena's Old Vat.

We tend to view such shows as phenomena -- and when we talk about them, it's more with statistical wonderment than artistic appreciation. There will be, if there have not already been, accounts of how many actors played in "The Fantasticks" over the years, how many pairs of shoes they wore out, how many pounds of confetti were tossed in the air. Nothing surprising about that. Tour guides are eternally pointing out the height of the Washington Monument -- not its elegantly simple proportions -- to tourists who nod appreciatively and then ask how much it weighs. And these shows are monuments in their fashion.

And then one comes down, like "The Fantasticks," and we are forced to confront the truth: Even the seemingly indestructible productions are perishable. Plays are forever destined to come and go. All that really endures is the urge to make theater.

You had 26 years to catch "The Fantasticks" on its home turf. That allows for a certain measure of procrastination. Normally, the theater doesn't. The carnival invariably moves on; the season ends; the actors pack their bags.

In the inconstant kingdom of the stage, the moral is: Catch it now or catch it never.