A THUD FOLLOWED by terminal-type screams roused me somewhat and I peered out the Yucatan bus to see what I supposed would be the poor mortal remains of six Mayan tots. But no.

It was only a farmer who had got the bus and claimed his suitcase from the luggage bin, that dark peritoneum, and now was tugging a rope with a vast Hampshire pig at the end of it. Built for the ages, the pig had been checked with the suitcase and had no more intention of leaving his safe (if dark) site than a pyramid. His were the protest screams. They stopped once he was pulled from the bus to the road and, such is the optimism of man and beast, he forgave in a minute and trotted gaily, almost, down the road with the farmer.

"I don't see how that pig could have been comfortable," said a woman.

"But that was long ago," I said as pig and master reconciled down the road together, "and besides the pig's alive."

Travel, of course, is largely fantasy. You see only with your own eyes and register with your own sounding board, nothing more.

The children of Yucatan grow up without intervention, I think, and tumble out the open doors of the modest houses right onto the streets where the entitle life of the town takes place. Marvelous.

In fact, all the errors we make up here in raising children (as any book will assure you) they avoid in Yucatan. It must be paradise, I thought, to grow up like Mayan children.

The mongrels are uniformly friendly and available to any child for patting purposes, and the weather is suitable for playing throughout the year. Moreover there is not a car pool in all Yucatan. Every child is in the street - in some villages cars operate only on the perimeter of a core that has no vehicle.

As a result the children grow up joyous. As far as that goes, adult life can hardly be improved on. Palm trees, shrimp and endless clean shirts (for the Mayans are more fastidious than we are). If that's not paradise, than what is?

But paradise is not enough. Mark Twain used to say dogs should have a reasonable number of fleas, to keep them brooding that they were just dogs. We do not agree with that - dogs should not have fleas.

But people should. Dogs are not improved by frustation, anxiety, grief, and in any case their memories are too short. So they should be made as happy as possible.

But humans - this came to me in a flash in Mexico - probably do not grow very well without those shocks, upheavals and humiliations to well known to all of us in the United States.

There was that notable preacher who said no man has adversity enough that is not made better by it, and I used to think that was a sorry cop-out to justify the repression of the poor; but now I understand what he meant.

The failures, the disappointments - you don't have them much if your life runs in immemorial tracks and you never encounter advanced algebra or Moissac or the Iliad or the big wars.

It is probably impossible to get the full effect of wretchedness without advanced symbolism, especially language and music, and that is why we are always being urged (by the healers that blow into great cities with every wind) to keep both those symbolic systems to a minimum.

There is no doubt in my mind that life can be happier if symbols are kept to the simplest.

Despair, which can be so dangerous to the young who have not learned to go into neutral for a time, can be avoided if the mind is not well enough layered to admit it. And yet even the most dangerous things, like despair, or for that matter courage, may have their uses.

Should frustration, anxiety, yearning abound so wisdom and an unshakable center may at last come? God forbid. But I think that's the way it is.

I hardly mean that the Mayans of Yucatan, because they live in paradise and have loving, radiant happy faces, are stupid and dull. Not at all. Their lives are possibly superior to ours - certainly they are less hostile, more loving, and all the good things the books reproach us for not being.

But for me it would never do, and I think they have a whole different set of symbols in their heads and I like mine better.

Their streets, like their persons, are immaculate, but I am less sure about their language, their stimulants, and the resulting concepts in their heads. If you are happy and relatively undisturbed there is probably no incentive for more and more and more symbols. And yet without them in endless richness, there is no thought or cohesion.

And although they master all things admirably, no doubt, I could not handle their particular sort of happiness.

Of course the Scots are said to have the world's greatest aptitude for misery. If so, they may also have special pinnacles of horizon and joy of a complex sort.

Now a chauvinist is a man who goes abroad and learns nothing, except that foreigners do not speak his language well. I found much to admire in Yucatan which is not worth mentioning. I saw paradise and it flat don't work.

Eudora Welty once said something in a story in which kids were caught in a ramshackle school house by a tornado. Scared blind. The old teacher said, "We're in the best place right here."

It may be whistling in the dark. Or it may be the Mississippi teacher in that story knew more than any little kid ever dreamed of.