AT HARVEST TIME in the strawberry fields of San Diego County, California, hundreds and hundreds of stooped-over migrant workers pick fruit under the hot sun. They are the poorest of America's working poor. Many migrants have recently traveled north across the Mexican border in search of agricultural jobs. Many are Mixtec Indians, from the southern Mexican state of Oaxaca. In addition to strawberries, they also harvest the yellow grapes that are dried and turned into raisins. "Raisin grape harvesting is probably one of the worst jobs you would ever want to do," Don Villarejo, a professor at the California Institute for Rural Studies in Davis, California, told the New York Times. "It's a job where you need a very large number of people for a very short period of time, and most employers I have spoken to prefer the Mixtecs. They work very, very...