It was about 4000 year ago that wandering bands crossed the Himalaya passes into India from the great plains of central Asia. These people were called Aryans, from a Sanskrit word meaning “noblemen” or “owners of land.” Many of these remained and others wandered into Europe and formed the root of the Indo-European stock. The influence of the Aryans gradually extended over all India. During their first 1000 years they perfected both written and spoken Sanskrit. To them is also due the introduction of the caste system. In the sixth century B.C., the Persian armies under Darius entered India but made no permanent conquests. To this period belong two great early Indians, the grammarian Panini and the religious teacher Buddha. This probably is also the approximate time of the Sulvasutras (“the rules of the cord”), some religious writing of interest in the history of mathematics because they embody geometrical rules for the construction of altars by rope stretching and show an acquaintance with Pythagorean numbers.