If everybody in the whole world moved to the United States, it would be no more crowded than Taiwan, which has about 1,500 people per square mile.

I used to cite that odd fact in support of the thesis that the United States could absorb virtually unlimited immigration, and should, at least in principle, be moving in that direction.

Now that I've been living in Los Angeles for a few months, though, I'm a little less romantic about how the theory works out in practice than I was when I lived in Minnesota, although Minnesota is also a magnet for immigrants, especially from Southeast Asia.

Figures released by the Census Bureau Tuesday show that 307,000 immigrants settled in California from August 1991 through July 1992. That's more than 30 percent of the nation's new residents, as well as more than half of new Californians.

As the example of Taiwan shows, crowding is not really the problem. Los Angeles is about four times as densely populated, at 6,500 people per square mile [-] or, to put it in more suburban terms, about 10 people per acre. Hong Kong, which is somewhat smaller in area than Los Angeles, has vast and beautiful parkland and about 14,000 people per square mile.

I still think the United States should be encouraging immigration, but even I can see that we couldn't cope if a billion people arrived next week. Even spread out, that many people [-] a million new people a week every week for the next 20 years [-] would stretch our social and physical resources beyond their limit.

But perhaps five million immigrants a year would be manageable. The question is, who should they be and how can the negative impact be minimized?

One can believe that immigrants can be a great advantage to the country that welcomes them and yet recognize that the short-term costs are burdensome. A report prepared for the Los Angeles County Board of Supervisors earlier this year estimated that the county spent about $800 more on services for immigrants than it collected from them in taxes, partly because most of the taxes they pay go to the federal government.

Illegal aliens (not a slur; an alien is someone living in a country of which he is not a citizen) are more of a problem than legal immigrants. That's not because they are an inherently different population, although that may be true, but because there's no way to choose who they will be or to plan for their presence.

And, reasonably enough, they may not wish to be found. Los Angeles officials believe the city will lose as much as $150 million in federal funds because illegal residents were undercounted in the census. Street vendors in the Pico-Union district are reluctant to cooperate with police trying to stamp out extortion.

I don't believe there's any effective way to eliminate illegal immigration without injuring everyone's civil liberties. Trying to make the nation's employers immigration policemen has scarcely caused a ripple in the flow of illegal entrants to the United States, while significantly increasing discrimination against legal residents who look or sound foreign, especially Latinos. And I resent having to prove to my employer that I have a right to be here, when I have done nothing to warrant the imputed suspicion that I don't.

But we could, I believe, reduce at least the relative impact of illegal immigration by sharply increasing the number of legal immigrants. At present, we go to a lot of trouble to keep out people we ought to be glad to have.

A few years ago, our family tried to help the daughter of a Chinese friend get a visa to go to college in the United States. That's how we found out that the State Department requires some American consular offices overseas to deny student visas to anyone who is likely to be thinking of becoming a permanent resident. Besides China, Mexico City and the Dominican Republic are mentioned as ``high-fraud posts.''

That makes no sense to me, besides carrying a nasty sniff of racism. Higher education, in fact, is one of our most successful exports, and there are hundreds of thousands of international students in the United States. If they want to stay, we should let them. If they might want to stay, all the more reason to encourage them to come in the first place.

People who already have jobs waiting in the United States should be welcome, without the charade of pretending to prove that no American is qualified for the job. People who come to work here not only occupy jobs, they make jobs, through the goods and services they buy.

Above all, we should accept immigrants as individuals rather than as members of national groups. In 1990, Congress authorized a green-card lottery for 40,000 visas a year. Under pressure from Sen. Ted Kennedy, 40 percent of them were reserved for people from Ireland. There were probably some unknown and distant cousins of mine in that group, because three of my great-grandparents came from Ireland last century, but I still think it is the wrong way to do things.

President Kennedy enjoined us to ask not what our country could do for us, but what we could do for our country. The world has a great many people who can answer in the spirit he meant, and I would be happy to claim them as fellow-citizens.