During the opening scene of "Martin Guerre" at the Kennedy Center, an enormous cannon is wheeled to the front of the stage, aimed at the audience and fired: Blam! A large noise and a smoke ring are emitted. Across town at Arena Stage, another musical introduces itself more gently, as the "Guys and Dolls" of Damon Runyon's Broadway saunter across the stage and then sing of a horse--Paul Revere? Valentine? Epitaph?--that can win it by a half.

Fifty years separate these two shows, time in which man landed on the moon and invented the Cuisinart. Local theatergoers have had the chance recently to observe this time lapse in person, through productions that illustrate by their differences what musical theater was and what it has become, from babes to blam.

It might be useful to begin our comparison with ways in which these two shows are similar. For starters, they are both based on other sources--"Guys and Dolls" on Runyon's short stories, and "Martin Guerre" (which closes today) on an old French legend. And two movies.

Both contain numerous references to agriculture. In "Guerre," the success and failure of the crops is a key factor in the acceptance and then demonization of the putative Guerre. "May the Lord be praised/ For having raised this corn/ From the earth through sun and rain/ This wheat was born," the inhabitants of the rural village of Artigat sing during one of the rare upbeat episodes in the show.

In "G and D," the chanteuse Adelaide and the Hot Box girls also sing about crops and farm life.

"The cows and chickens are going to the dickens," they lament as they measure their love in typically agrarian terms: "A Bushel and a Peck." This is followed by a plaintive yet hopeful chorus: "Doodle, oodle, oo-doo, doodle, oodle, oo-doo, doodle, oodle, oo-doo, doo."

Religion, in particular the saving of souls, is another common theme. "Guerre" is set during 16th-century hostilities between French Catholics and Protestants, with an overlay of Catholic witch-hunting. In the song "The Exorcism," two characters exhort the teenage Guerre to mount his bride for fear the crops will mimic the couple's infertility:

Martin, confess to God and tell

The truth or else you'll burn in hell! . . .

The flesh of our savior deliver his soul

The blood of our Lord God to make a man whole . . .

And then the priest whips the lad to a bloody pulp.

Sarah Brown, the Salvation Army babe in "Guys and Dolls," takes a more pacific route to redemption. "Put down the bottle and we'll say no more," she sings. "Follow, follow the fold." She is torn between her desire to "save" Broadway and the gamblers, thieves, sluts and drunks she sees there, and her attraction to the high-roller Sky Masterson, himself something of a biblical scholar. ("There are two things been in every hotel room in the country: Sky Masterson and the Gideon Bible," he explains.)

Due to the delivery of 12 sinners whose presence has been won by Sky in a game of craps, Sarah is able to have a prayer meeting at which the lowlifes confess to numerous misdeeds and indiscretions. Nicely-Nicely Johnson, in particular, tells of dreaming he was "on a boat to Heaven" but finding himself trying to get the other passengers to gamble and drink, while they told him "Sit Down, You're Rockin' the Boat." They sing:

And the Devil will drag you under

By the sharp lapel of your checkered coat

Sit down, sit down, sit down, sit down,

Sit down you're rockin' the boat.

Well.

Of course, we all know that "Guys and Dolls" and "Martin Guerre" are about as much alike as cheesecake and strudel. It isn't just that one show is bright and the other dark, or that the former is 103 percent American and the latter a British-French collaboration. Somewhere between the Save-a-Soul Mission and the village of Artigat, musicals mutated.

What was a lighthearted art form descended from operetta has become a serious spectacle just a trill away from opera. What was expected to be an evening that sent you out of the theater happy and humming is now akin to doing penance for your sins, except you don't have to hit the kneelers. The best of the new musicals can be entertaining, moving and exciting, and they have brought forth amazing singers and dancers. But they also often overwhelm, and not with joy.

"Guys and Dolls"--said by many astute critics to be the most perfect musical ever written--makes it clear from the first prance that authors Jo Swerling, Abe Burrows and Frank Loesser do not want you to take it seriously. But "Martin Guerre"--as well as previous efforts by lyricist Alain Boublil and composer Claude-Michel Schonberg,"Les Miserables" and "Miss Saigon"--practically crawls out and begs you to hail it as an Important Work of Art. This even while you cringe with fear that the starving masses will die of the machine-made smoke or the helicopter will explode or, in this production, that the barn is truly burning us all out of the theater and the stage blood will clot into slippery clumps onstage.

The supremacy of spectacle is not what really separates these eras of the musical. "The King and I," with its dozens of dancing children and wives, had plenty of spectacle, for one example. It's the self-importance that distinguishes the modern musical from the old, the mission to deal with subjects like revolution, racism, human misery, totalitarianism and war. One of the latest (and now deceased) musicals in New York is "Marie Christine," a modern reworking of "Medea," complete with offstage child-murders.

Plenty of the old musicals tackled tough subjects--"Lost in the Stars" showed us the racism of South Africa and by implication of our own country, and "West Side Story" portrayed gang warfare and juvenile delinquency, to name two. But the new musicals preen with significance, seeing themselves as not just big musicals, but Big Important Musicals.

If "Guys and Dolls" were opening today, the synopsis would probably be "an inside look at criminals and their struggles to go straight."

Today the audience has been separated from the magic of live singing by enormous moving sets, amplification and thundering orchestras, but it doesn't seem to mind very much, despite numerous predictions of revolt. "I have a feeling that the day of the big musical is ending," predicted British critic Michael Billington in 1996. "And although 'Cats,' 'Les Miz' and 'The Phantom' will be with us for a long time to come, they may well be the first and last of their kind." Four years later, all of those shows, as well as "Miss Saigon," are still running in New York, and other Big Important Musicals like "Rent" and the Disney-concocted "Beauty and the Beast" and "The Lion King" have joined them.

But a closer look at the 21 musicals playing on Broadway today is instructive. Four hits are revivals: "Annie Get Your Gun," "Kiss Me Kate," "Cabaret" and "Chicago." Four are either reviews or homages: "Putting It Together," "Smokey Joe's Cafe," "Fosse" and "Swing." Half are BIMs. I'm not sure what all this means, but it's worth noting.

But let us close with a look at love songs. In "Martin Guerre," it's "Live With Somebody You Love," a stirring tune sung by Martin's friend Arnaud, who subsequently poses as Guerre and assumes his husbandly duty.

Live with somebody you love

Let your love bless each day

As the years slip away

And you never can say goodbye.

And so on. It's a really good song, intense and moving. But it's not a great song.

"I've Never Been in Love Before" is a duet between Sky Masterson and Sarah Brown, and surely one of the sweetest songs ever sung.

I've never been in love before

Now all at once it's you

It's you forever more

I've never been in love before

I thought my heart was safe

I thought I knew the score

But this is wine that's all too strange and strong

I'm full of foolish song

And out my song must pour

So please forgive this helpless haze I'm in

I've really never been in love before.

Who'll bet a coupla G's on which show will be playing in 2050?