The world welcomed its 6 billionth soul on Oct. 12 and kept adding some 350,000 little goo-gooing reasons to celebrate the gift-giving season every day thereafter, according to U.N. calculations. Hard to top that 1999 development for long-term significance. It is also reason to gaze on the arriving new millennium with trepidation and awe.

While we cannot know with certainty which blessed event was Little Mr. or Ms. Billionth, the child was almost certainly born in the developing world, where 95 percent of population growth occurs today. And we know that his or her chances of surviving infancy and going on to live at least four or five decades are better than any other humans have ever had.

That's where awe is appropriate: The world today's adults are shaping for future generations is, for the most part, in better shape demographically and economically than the experts predicted as recently as a quarter-century ago. Much of the systemic pessimism of the 1970s has been shown to have been at best premature.

Population growth has slowed, even in the developing world, while globally, death rates have plunged. The world's net population growth has declined from 86 million a year at the beginning of the decade to 78 million this year. The fertility rate, 6.2 percent in 1950, is half that today.

Global consumption, education levels and per capita income have as a statistical matter risen along with longevity--the average human life span has grown from 46 to 66 years in the past half-century, according to the statistics of well-being and progress contained in the recent annual reports of the United Nations Population Fund and UNICEF.

But their complete statistical tables show that it is a very uneven world that today's adults create and tolerate. There is cause for trepidation that over the long term the misshapen world of today is an unstable one.

World 2000 is a misshapen creation demographically, economically and culturally. It is a world Charles Dickens would recognize: the best of times available for the lucky many, and the worst of times (at least in relative terms) for the even more numerous unlucky, most of whom live in the Third World.

Consider a statistic that I find hard to get over or around: Primary school enrollment in sub-Saharan Africa has been declining every year for 20 years as the great land mass between Tangier and Cape Town has become the Forgotten Continent.

We recoil at the stories of many of Africa's children being forced into combat, of an estimated 1.2 million Africans under the age of 15 being infected with AIDS, of a widespread lack of clean water or decent housing. But the systematic shutting off of educational opportunities--the closing down of the roads out of today's hell--may eclipse all of those horrors in significance for Africa over the course of the next century.

The lack of money, teachers and administrators to run schools provides much food for guilt.

The world's leading industrial countries share some responsibility. While they have increased their collective gross national products 30 percent over the past decade, their aid to developing countries has declined by about the same amount in the same period.

Bearing even more responsibility, however, are the post-independence governments of the Third World that have squandered or stolen aid and national income from their own children. "Of 27 developing countries in a recent survey only 5--Belize, Burkina Faso, Namibia, Niger and Uganda--allocate virtually 20 percent of their budgets to basic social services," UNICEF reports in its "The State of the World's Children 2000."

Wherever the fault lies, this is not a tragedy that will stay confined to Africa, Asia, Latin America, the Balkans or to the other developing countries that account for that 95 percent of current population growth.

In 62 Third World countries more than 40 percent of the population is now under 15. The graying of the rich in the industrial north and the runaway growth of the young in the poor south are powerful forces that will reshape global work, investment and migration patterns in the decades ahead.

It will take a major effort and many years in both the industrial and developing worlds to prepare for the south-to-north population movements today's statistical imbalances suggest will inevitably occur. That is all the more reason for an urgent start in the new millennium on cooperative programs that improve the odds that the young foreigners crossing our frontiers in the future will be coming with skills and friendship rather than materials for terrorist bombs.