But around the time of Tyson’s reign of terror, mixed martial arts, which permits a wide variety of striking and grappling techniques, was slowly making inroads in combat sports. In the beginning, the roster of this professional fight club circuit was made up of unscientific bouncer types like David “Tank” Abbott. Shortly thereafter, however, the Gracie clan, a family of Brazilian jiu-jitsu masters, invaded the sport. They were good-looking, non-imposing Clark Kent types. But they quickly laid bare the fact that any brawler, big or small, and no matter how hard a puncher, could be taken down and dismantled by an array of “arm bars” and other specialized holds.

Little by little, Americans began to understand that when it came to street fighting, the multidimensionality of mixed martial artists rendered them more pernicious than pure pugilists. Maybe 8 out of 10 times, take it out back, and the boxer ends up on the ground. In 2010, James “Lights Out” Toney, one of the toughest and most technical boxers of our era, ventured into the octagon (the mixed martial arts equivalent of the boxing ring) against an older Randy Couture. Couture, a former wrestler, easily dispatched Toney with a submission hold in the first round. Boxers are not competitive in the octagon.

But now, the tables have been turned: A top-notch M.M.A. fighter is entering the boxing ring. Conor McGregor, the reigning lightweight and former featherweight champion of the Ultimate Fighting Championship organization, who has never boxed professionally, is poised to challenge the greatest boxer of our time, Floyd Mayweather Jr., in Nevada on Saturday night. What an insult! What a head butt to boxing!

As a longtime boxing trainer and writer, my initial reaction to this event, for which each fighter could earn more than $100 million, was a wince and a moan. What won’t “Money” Mayweather do for more money?

To be fair, there is a history of such spectacles. For example, in 1976, Muhammad Ali fought a bizarre contest against the Japanese wrestler Antonio Inoki. In a similar fight the day before, the heavyweight boxer Chuck Wepner was picked up and flung out of the ring in a free-for-all match with the wrestler Andre the Giant. But the difference between those sideshows and the bout on Saturday is that McGregor is a world-class athlete who is deadly serious about beating Mayweather at his own game.