Colonia Obrera is a way station on the underground railroad that leads up from villages thousands of miles south of here to the orchards and factories of the United States.

Slums are flung like garbage on the hillsides, squatters' shacks pieced together from anything that came to hand, without plumbing or electricity, along dry and dusty trails. Permanency comes slowly, out of necessity, as the men stay north or come back only to realize that once again they are out of money and have to return, illegally, to the United States.

Yet a culture has developed here. What were first only shadow villages have taken root. Now they are a fixed element in a pattern of migration so deeply ingrained in the life of rural Mexico that it may be virtually impossible to change.

Even village priests from the town of San Jeronimo in the far southeastern state of Oaxaca now come to Tijuana. The language of the Mixtec Indians, common there, now is spoken here. Food is cooked among heated stones in the ancient fashion, and as new arrivals come up from their village 2,000 miles away they can feel sheltered until they decide to move north across the border. Coming back, they will be able to find their wives here, their children, their parents.

Maria Corona is one of those left behind. She has built a little grocery for herself and her 10 children while she waits for her husband to come back from his work in the orchards of the United States. She says she wants to return to Oaxaca, but the weeks have turned to years and she has come to feel at home here. To return now, for good, would be moving against the tide. f

Anthropologists Michael Kearney and Richard Mines at the Univeristy of California in San Diego have done extensive research on the developement of migratory patterns in rural Mexico. San Jeronimo and its satellite community in Tijuana are relatively new to the network. But research on their towns suggests that once a village falls into the migrant stream, it is not likely to get out.

Mines spent three years studying the people of Las Animas, a village in the Mexican state of Zaczatecas. The town has been sending people north since the 1880s, before there were even U.S. immigration laws to break.

The predominant way of making a living is to head for El norte. "It's a disease," said Mines. "There is a saying, 'Why would anybody stay here? We all go north. Nothing but north.'"

What Mines discovered was that the entire society of Las Animas is structured around the migration and the benefits received from it. The social composition of the town breaks down roughly into those families that have succeeded north of the border and those who couldn't make it.

The former send money back and perhaps return to Las Animas for vacation. The latter are left with the task of teaching agricultural survival to their sons, who will leave as soon as they get the chance.

Mines' research suggests a process of development in the Mexican villages that sent people north. It begins with a few men who cross the border illegally, work for low wages in the United States and often come from families of about average income in the village.

Then it becomes a mass phenomenon, said Mines. There is a large component of legal aliens and better jobs become more common.

Eventually one can discern several types of migrants: those who have only gone north one or two times; those who shuttle regularly but illegally back and forth; men who have been able to obtain legal permanent-resident status in the United States, and long-term legal permanent residents north of the border.

The longer a village has been sending people north, the larger its core of legal permanent residents in the United States.

"The only asset they have is their social network, and it comes through for them," said Mines. "They devote so much energy to it. Their world is their network. Even when you are settled in the United States, if you don't buy land in the village when you can, if you don't loan 'corn money' for planting, people will talk about you."

The smugglers, called "coyotes," plug into these social networks and can depend on the loyalty of the people they serve.

But among the more well-developed networks the need for smugglers appears to diminish. About 50 families from Las Animas have settled in Tijuana, some of them grown quite wealthy in the vegetable wholesaling business. They stand ready to help newcomers from their village with whatever resources they can bring to bear.

One owns a boarding house not far from Tijuana's main street where scores of villagers from Las Animas can be seen waiting for the right moment to cross the border.

Once the villagers arrive in the United States, they find they are again more secure than illegal immigrants from less extensive networks.

"The crucial thing is job advice," said Mines. "It's much more difficult to exploit a guy who's complaining to his legal uncle."

San Jeronimo's people here in Colonia Obrera are still at the early stages of the pattern's development. Virtually all of those who cross the border are illegal. But their village in Oaxaca is already experiencing an affluence undreamed of 20 years ago. The addiction to El Norte is already with them, and no one knows if there is a cure.