"I need a haircut, Dad."

"A haircut! You just had a haircut!"

"That was last month. I can't keep it combed when it gets this long."

"Look, kid -- at 10 bucks a whack, your barbershop visits are breaking me up. I can't afford to underwrite monthly haircuts. If you insist on staying that well groomed, you'll have to pay the tab yourself."

This conversation actually took place in my house not long ago. I think of it as a sea change, a fundamental shift in the evolution of the planet. A decade ago, when the abovequoted son was 7 and his brother was 12, both of them had long, straight locks that fell below the necks of their T-shirts. Now the younger one looks like a college freshman from the 1950s: button-down collars, finger-thin ties, narrow jacket lapels -- and short hair.

I'm not qualified to interpret this phenomenon, but I do have some hunches about it that may partially explain what is happening:

First, the lad has a girlfriend.

Second, he is an active member of a successful track team whose coach encourages his athletes to look like winners.

Third, he is in passage from adolescence to early adulthood.

Fourth, he is an easy rider on a social pendulum swung by forces beyond his understanding, let alone his control.

And, finally, the writer of Ecclesiastes spoke an eternal truth when he said there is nothing new under the sun.

If I had to choose one of these as the most likely explanation for the changes I am witnessing in my son, I think it would be track -- not because the sport itself holds any special virtue, but simply because it happened to become the first real passion of his life.

The sort of passion I'm referring to has nothing to do with sex or suffering; I'm speaking of passion as intense caring about something, anything. The more fortunate of us develop positive passions -- for music, let's say, or auto mechanics, or healing the sick, or growing flowers. The less fortunate suffer from passions that become destructive addictions: alcohol, drugs, gambling.

But least fortunate of all, in my opinion, are the ones who never encounter any passion at all. Never to be obsessed, not once to be absorbed or consumed by some force outside yourself, is to me a tragic loss, a total waste.

I have no idea where the passions come from, or how they enter our head, or why it is that some of us get "good" ones, some "bad" ones, and All I know is that when I see a young person committed to a constructive passion, I marvel at the fickleness of fortune.

With this youngster, it's track; with his older brother, it was chess. Neither their mother nor I had any previous experience with either of those activities. We are the lucky beneficiaries of developments we can't begin to explain.

It's not the short hair that pleases me -- I don't even like short hair. It's not even the superb physical conditioning, the healthy diet, the stamina, the graceful stride -- as much as I admire all of that.

It's the caring I like, the enthusiasm, the determination to excel, the self-disciplined pursuit of a distant goal, the intense commitment in victory and defeat. Long after he is too old to clear a hurdle or a highjump bar, those qualities will still be there to serve more important passions -- justice, perhaps, or beauty, or fatherhood, or peace.

For the moment, though, he has a one-track mind. He follows the fortunes of Edwin Moses and Carl Lewis and Dwight Stones, memorizes pages of fine-print statistics in Track & Field News, eats and drinks judiciously, goes to bed week nights at 10 and dreams of attaining heights and speeds and distances heretofore beyond the reach of mortals.

He also speaks a jargon understood only by those who share the passion.

"Get this," he says, reading from some esoteric source of knowledge. "Coe ran splits of 52.4, 56.8, 59.1 and 43.65 in the 1500."

"That's blinding speed," I respond, trying to get the hang of it. "Was it hand-held or electronic?"