It is generally agreed that at a period which, though not fixed, was anterior to 1000 B.C.[109] a body of invaders known as Aryans and nearly akin to the ancient Iranians entered India through the north-western mountains. They found there other tribes not deficient in civilization but unable to offer any effective resistance. These tribes who retired southwards are commonly known as Dravidians[110] and possibly represent an earlier invasion of central-Asiatic tribes allied to the remote ancestors of the Turks and Mongols[111]. At the time when the earlier hymns of the Rig Veda were composed, the Aryans apparently lived in the Panjab and did not know the sea, the Vindhya mountains or the Narbudda river. They included several tribes, among whom five are specially mentioned, and we hear that a great battle was fought on the Ravi, in which a confederation of ten kings who wished to force a passage to the east was repulsed by Sudas, chief of the Tritsus. Still the south-eastern movement, across the modern United Provinces to the borders of Bengal, continued and, so far as our records go, it was in this direction rather than due south or south-west, that the Aryans chiefly advanced[112]. When the Brâhmaṇas and earlier Upanishads were composed (c. 800-600 B.C.) the principal political units were the kingdoms of the Pancâlas and Kurus in the region of Delhi. The city of Ayodhyâ (Oudh) is also credited with a very ancient but legendary history. The real history of India begins with the life of the Buddha who lived in the sixth century B.C.[113] At that time the small states of northern India, which were apparently oligarchies or monarchies restricted by the powers of a tribal council, were in process of being absorbed by larger states which were absolute monarchies and this remained the normal form of government in both Hindu and Moslim times. Thus Kosala (or Oudh) absorbed the kingdom of Benares but was itself conquered by Magadha or Bihar, the chief city of which was Pataliputra or Patna, destined to become the capital of India. We also know that at this period and for about two centuries later the Persian Empire had two satrapies within the limits of modern India, one called "India," including the country east of the Indus and possibly part of the Panjab, and the other called Gândhâra (Peshawar) containing Takshaśilâ[114], a celebrated university. The situation of this seat of learning is important, for it was frequented by students from other districts and they must have felt there in early times Persian and afterwards Hellenistic influence. There are clear signs of Persian influence in India in the reign of Asoka. Of Magadha there is little to be said for the next century and a half, but it appears to have remained the chief state of northern India.