After nine years of poring over blood stains, fingerprint samples and stinky corpses, Gil Grissom, the forensics whiz played by William Petersen on “CSI: Crime Scene Investigation,” bids farewell in tonight’s episode.

Apparently, some people aren’t very happy about it.

“No Grissom, no interest,” wrote a disgruntled commentator on a fan blog. “CSI without Grissom is CSDEAD,” moaned another, echoing a widespread sentiment.

Their sense of fatalism is understandable. Petersen, who is being replaced by Laurence Fishburne, was the linchpin of television’s top-rated scripted show. His quirky and deadpan science geek had a magnetic quality to him. And he was arguably a chief reason why the original “CSI” remained more see-worthy than its two inferior offspring.

On the other hand, “CSI” fans might want to reserve judgment and sift through the trace evidence of TV history. They would find ample proof that the departure of a key star does not always signal the demise of a show.

Exhibit 1A: “Cheers.” In 1987, the sitcom ranked No. 3 in the Nielsens. But Shelley Long, harboring visions of big-screen glory, decided to bolt. Uh-oh. The witty wordplay and sexual tension between Long and co-star Ted Danson was a vital component of the show. Big trouble for “Cheers,” right?

Wrong. In stepped Kirstie Alley and the show shifted from a hysterical relationship between an odd couple to a more equitable blend of all the characters without losing any of its fizz. “Cheers” thrived for six more years and climbed to No. 1 in the 1990-91 season.

And then there was “M*A*S*H.” When the medical dramedy launched, it was tethered to a pair of lead actors, but Wayne Rogers gradually realized that co-star Alan Alda was beginning to hog more of the spotlight.

Rogers left the show in a contract dispute in 1975 and was replaced by the relatively unknown Mike Ferrell. The popularity of “M*A*S*H” only grew over ensuing seasons, and its finale in 1983 was seen by the largest audience ever to watch a scripted show.

But wait, there’s more: Remember how “Bewitched” pulled a Darren switcheroo (Dick Sargent for Dick York) midway through its run and retained its magic? Bratty Shannen Doherty left “Beverly Hills 90210” but there was still enough soap to go around. And David Caruso made one of the dumbest showbiz career moves ever by blowing off “NYPD Blue” after one season. The groundbreaking cop drama simply focused more on Dennis Franz, who became an Emmy magnet.

No shows have been more impervious to cast defections than “ER” and “Law & Order,” which have kept the revolving doors on constant whirl during their distinguished runs. The former survived the loss of George Clooney and Anthony Edwards, among others, while managing to integrate compelling replacement characters along the way. The latter has said goodbye to a number of familiar faces, including Jerry Orbach’s beloved Lennie Briscoe.

Perhaps “Law & Order” provides the best lesson plan for “CSI.” Both shows are procedural crime dramas that tend to be more about the cases than the individual characters. As someone once wrote, “the most interesting people on ‘CSI’ are dead.”

With that in mind, “CSI” might do just fine without Grissom as long as its writers can keep coming up with darkly intriguing stories. That doesn’t mean we won’t miss our favorite bug-loving sleuth. It just means that what our bosses have been telling us for years is probably true:

“No one is irreplaceable.”

Tune in to …

WHAT: “CSI: Crime Scene Investigation”

WHEN: 9 p.m. Thursday


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