By Tom Fordyce BBC Sport Online

On Saturday, two months after his 40th birthday, Evander Holyfield goes back into the ring.

The heavyweight legend takes on Chris Byrd in Atlantic City for the vacant IBF heavyweight title.

But there are many in boxing who fear that 'Van the Man' is tarnishing his great legacy by fighting on when the magic has long gone.

What do the careers of Holyfield's heavyweight heroes say about the wisdom of his decision to keep going?

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Muhammad Ali

The Greatest, for sure - but also a man who fought on when he should have called it a day.

Ali prepares to take on Larry Holmes in 1980

Ali should have called it quits in October 1975, when the Thrilla in Manilla had left him, in his own admission, close to death.

But that cruel victory over Joe Frazier persuaded him to keep going - and the final fights of his career succeeded only in tarnishing his great legacy.

He may have regained the title he lost to the steady Leon Spinks, but the defeats to Larry Holmes and then, most painfully, Trevor Berbick, at the age of 39 in 1981, saddened many of his fans.

Lesson: QUIT NOW

George Foreman

A fearsome face-crusher when he first arrived on the professional scene, Foreman had the anger pummelled out of him by Ali in the Rumble in the Jungle.

A battered Foreman after defeat by Shannon Briggs

He retired for the first time after losing to Jimmy Young in 1977 - but on that very night found God and became a minister.

He returned to the ring at the age of 39, ten years later, ostensibly to raise funds for his church, but strung together a run of 24 straight wins, all bar one coming by knock-out.

This won him a shot at Holyfield's world title in 1991, and although he was well beaten in both that fight and a subsequent joust against Tommy Morrison, Foreman kept going.

Vindication came in November 1994 when, way behind on points, he knocked out Michael Moorer to take the WBA and IBF heavyweight crowns as a 46-year-old grandfather.

Lesson: FIGHT ON

Larry Holmes

A wonderful fighter and, in many people's eyes, the true successor to his old sparring partner Ali, Holmes suffered the same fate as his hero.

Holmes after defeat by Holyfield in 1992

He first retired at the age of 37 in a burst of anger after his second defeat to Michael Spinks, but returned to face - and be knocked out by - a young Mike Tyson in January 1988.

Three years later the 42-year-old Holmes was back to (ironically) fight Holyfield and lose.

He came back again in 1995 to be out-pointed by Oliver McCall when challenging for the WBC crown, and continued fighting for another four years until his 50th birthday.

Lesson: QUIT NOW

Archie Moore

Moore was a man to whom great things came at an age when most fighters are finished.

The "Old Mongoose" was 39-years-old and in his 175th fight when he first challenged for the world light-heavyweight title in 1952, but still outscored Joey Maxim over 15 rounds in St Louis.

The step up to full heavyweight never quite worked for him - Rocky Marciano beat him when he was 42 and Floyd Patterson did the same a year later.

But he never lost his light-heavyweight title in the ring, retiring at the age of 50 after being stripped of the title through inactivity.

He went on to coach that other old warrior, Foreman, and was revered to the end as an intelligent man who never allowed himself to be exploited in the name of a quick buck.

Lesson: FIGHT ON