The evidence of the topography, the climate, and the products of the country thus shows that the people by whose poets the Rigveda was composed were settled in the north-west of India, from the Kabul to the Jumna. But they were still engaged in conflict with the aborigines, for many victories over them are referred to. Thus Indra is said to have bound 1000 or slain 30,000 of them for his allies. That the conquerors were bent on acquiring new territory appears from the rivers being frequently mentioned as obstacles to farther advance. The invaders, though split up into many tribes, were conscious of a unity of race and religion. They styled themselves Aryas or "kinsmen," as opposed to the aborigines, to whom they gave the name of Dasyu or Dasa, "fiends," in later times also called anarya, or non-Aryans. The characteristic physical difference between the two races was that of colour (varna), the aborigines being described as "black" (krishna) or "black-skins," and as the "Dasa colour," in contrast with the "Aryan colour" or "our colour." This contrast undoubtedly formed the original basis of caste, the regular name for which in Sanskrit is "colour." Those of the conquered race who did not escape to the hills and were captured became slaves. Thus one singer receives from his royal patron a hundred asses, a hundred sheep, and a hundred Dasas. The latter word in later Sanskrit regularly means servant or slave, much in the same way as "captive Slav" to the German came to mean "slave." When thoroughly subjected, the original inhabitants, ceasing to be called Dasyus, became the fourth caste under the later name of Çudras. The Dasyus are described in the Rigveda as non-sacrificing, unbelieving, and impious. They are also doubtless meant by the phallus-worshippers mentioned in two passages. The Aryans in course of time came to adopt this form of cult. There are several passages in the Mahabharata showing that Çiva was already venerated under the emblem of the phallus when that epic was composed. Phallus-worship is widely diffused in India at the present day, but is most prevalent in the south. The Dasyus appear to have been a pastoral race, for they possessed large herds, which were captured by the victorious Aryans. They fortified themselves in strongholds (called pur), which must have been numerous, as Indra is sometimes said to have destroyed as many as a hundred of them for his allies. The Rigveda mentions many tribes among the Aryans. The most north-westerly of these are the Gandharis, who, judged by the way they are referred to, must have been breeders of sheep. They were later well known as Gandharas or Gandharas. The Atharva-veda mentions as contiguous to the Gandharis the Mujavats, a tribe doubtless settled close to Mount Mujavat; evidently regarding these two as the extreme limit of the Aryan settlements to the north-west.