From São Paulo, the Bandeirantes, adventurers mostly of mixed Portuguese and native ancestry, penetrated steadily westward in their search for Indian slaves. Along the Amazon river and its major tributaries, repeated slaving raids and punitive attacks left their mark. One French traveler in the 1740s described "hundreds of miles of river banks with no sign of human life and once-thriving villages that were devastated and empty." In some areas of the Amazon Basin, and particularly among the Guarani of southern Brazil and Paraguay, the Jesuits had organized their Jesuit Reductions along military lines to fight the slavers. In the mid-to-late 19th century, many Amerindians were enslaved to work on rubber plantations.