The world seems to have been half empty about the middle of the last century. To be sure, population wasn't down to what it was at the beginning of things, say at the start of organized agriculture, about 8000 BC. Then, ethnologists figure, total population was around 800 million. But let's skip that; with a half-empty world in modern times (say AD 1850), global population was leaping forward. My ancestors were helping it along. One of them up at South Lee, N.H., was one of 20 children. A good-size family. As an unworthy descendant I can't match that. The fact is, however, that since 1900 or so there has been a population explosion, and that it continues, and presents one of the world's greatest dangers; yes, as big in its way perhaps as the nuclear bomb threat.
The public is just becoming aware of it, but the population problem intrudes more and more. Where is tomorrow's meal coming from? Former World Bank president Robert S. McNamara has a piece in the current Foreign Affairs saying that even firewood is getting scarce in some parts of the overpopulated developing countries. China (population: a billion or so) is taking the most sweeping, and controversial, precautions. It is trying to regularize the one-child family, and there seems danger of coercive measures, government intrusions, forced sterilization, and maybe even infanticide.
World population is at present around 4.7 billion. It may not stabilize till 11 billion at the rate it is growing, round about AD 2100. Will population slow by voluntary means - or by war? Here's an arresting figure to mull over (purveyed by Mr. McNamara): ''Whereas it had taken mankind more than a million years to reach a population of one billion, the second billion required only 120 years, the third billion, 32 years; and the fourth billion, 15 years.'' It's like going downhill on a sled.
There seems to be a people-surplus everywhere we look, particularly in extra-poor countries. Yes, the rate of increase is coming down in some areas, but nowhere near fast enough to suit Mr. McNamara, and not at all in big areas of the earth. We are going to hear a lot more about it in August when the World Population Conference meets in Mexico City. Mexico City is a good object lesson: People are pouring into Mexico City from all parts of Mexico. Mexico City in a few years may be the largest city on earth: 31 million by 2000, according to one count. Sao Paulo, Brazil, will have 20 million. Where will they sleep at night?
This comes at a time when Congress is trying to grapple with the first big immigration bill since 1952. Immigration enforcement on parts of the Mexican border has all but broken down. My impression is that America lacks the will to meet the situation, though I may be wrong. The public has not really faced up, I think, to the enormous cost and discipline of the enforcement undertaking proposed. The mood in Congress is something like the mood in America after the 18th Amendment - declaring that ''one year from the ratification of this article , the manufacture, sale, or transportation of intoxicating liquors . . . is hereby prohibited.'' That took effect Jan. 29, 1919. With a sigh Congress turned to other matters. What a relief that after all these years the matter was ''solved'' . . . .
Of course, the problem was not solved, but just set aside. The 18th Amendment was finally repealed Dec. 5, 1933, in the curt 10-line 21st Amendment.
So now Congress wavers over whether to tackle the problem of immigration control: the object being to preserve the territory of the United States from the less favored citizens of poorer lands. Last week the House narrowly passed a complex bill, by five votes. A different version has passed the Senate. The two bodies must reconcile their difficulties. President Reagan has his own ideas. A presidential election is under way. Few of the public really understand, I think , the intricate mechanism needed to protect the border. Sam Bernsen, a one-time assistant commissioner of immigration, says, ''The Immigration and Naturalization Service is hardly able to carry out its responsibilities under present law.'' The pending House measure, he thinks, would lead to a mammoth increase in the volume of work.
One proposal makes it illegal for employers to hire illegal aliens. There is something almost as artless and innocent in the approach to immigration control today, I think, as there was in this capital over prohibition 65 years ago.