Enter the philosopher's cave and behold the metaphor of the dirty drawers: From within a sacred circle of underwear scattered around a Stone Age easy chair emerges the theory of men and women as, well, different. Man doesn't mind the mess he has left on the floor. Woman tries to change man's thinking. But woman has difficulty penetrating man's absorption with the TV.

Comedian Rob Becker made a killing by sharply packaging such shopworn observations in the 1990s as his one-man "Defending the Caveman" toured the country and then became the longest-running solo show in Broadway history. Becker's stage persona was pretty irresistible; the writer-performer melded stereotypes and archetypes in a way that captured the country's fascination with the pop psychology of Mars-Venus bestsellers, always cheerfully making himself the butt of the joke. It was an extended "Take my wife" set for the Oprah generation.

"Caveman" lives on, but it struggles without Becker. Actor Chris Sullivan headlines the tour that's at the Rosslyn Spectrum Theatre this week, and though he's a skilled performer and reasonably adept with a punch line, he's almost too good. Sullivan has a football player's build and a Shakespearean actor's ripe voice; the microphone pinned to his shirt is overkill in the smallish Spectrum. Even wearing the sloppy Caveman outfit of faded jeans and an untucked black T-shirt, Sullivan comes across as toned and polished -- more sophisticated than he'd like you to believe. So despite the wry grins and his facility with frat-boy language, you get the impression that this is someone who would make a capable Caesar or Macbeth.

That makes him slightly implausible here, for Becker's modern-day Caveman is a major boob, an apparently insensitive, inarticulate slob. He can't help it; it's in his nature. Explaining that unrefined nature, and defending it, is the whole show. Becker crudely sketches men as hunters and women as gatherers, and these characteristics are enough to get the jokes going. It explains, for instance, the essential physiological differences: men, as hunters, are endowed with spears, while women, as gatherers, are blessed with baskets.

Some of the material remains sure-fire, and Becker's explorations are not without droll insights. Yes, he renders guys as monosyllabic potty mouths obsessed with power tools and junky old cars, and the line drawing of women as adept shoppers and gossips is primitive. But primitive is the advertised point, and Becker refines his sensitivity a little by learning that women aren't just gabbing for the heck of it. "If you bond through conversation," goes one of the wide-eyed script's better lines, "then details are like little gifts you give each other."

Sullivan nicely illustrates this by turning his hands into a pair of lovingly chattering figures and watching obtusely from the outside; throughout the show, his small gestures come off well. The bigger stuff is tougher -- the steady diet of Holy Cow! expressions to the audience, broadcasting his Neanderthal astonishment at myriad distaff mysteries. He has the self-deprecation bit down pat, and under Sullivan's care the piece is still a gentle, nonthreatening artifact of affection. But his give-and-take with the Spectrum audience is stiff and unnatural; indeed, too much of his shtick has the unsettled air of appropriation. The laughs come in bunches, with arid flat spots in between, and the show never really gets rolling.

Becker, tousled and laid back, was hardly an inimitable talent onstage, so watching Sullivan go through Becker's paces isn't as unthinkable as watching someone try to do a monologue by Lily Tomlin or Spalding Gray. But it's not a clean fit: The actor is too plainly dressed in borrowed robes.

Defending the Caveman, by Rob Becker. One hour and 45 minutes. Through Nov. 28 at the Rosslyn Spectrum Theatre, 1611 N. Kent St., Rosslyn. Call 202-397-SEAT or visit www.ticketmaster.com.

Chris Sullivan is a skilled performer, but he's too polished to be a believable "Caveman."