over his people. He was occasionally referred to as samrat (supreme ruler). The rajan's increasing political power enabled him to gain greater control over the productive resources. The voluntary gift offering (bali) became compulsory tribute; however, there was no organised system of taxation. Sabha and samiti are still mentioned in later Vedic texts, though, with the increasing power of the king, their influence declined. By the end of the later Vedic age, different kinds of political systems such as monarchical states (rajya), oligarchical states (gana or sangha), and tribal principalities had emerged in India. According to Michael Witzel's analysis of the Kuru Kingdom, it can be characterized as the earliest Vedic "state", during the Middle Vedic Period. However, Robert Bellah observes that it is difficult to "pin down" whether the Kurus were a true "state" or a complex chiefdom, as the Kuru kings notably never adopted royal titles higher than "rājan," which means "chief" rather than "king" in the Vedic context. The Middle Vedic Period is also characterized by a lack of cities; Bellah compares this to early state formation in ancient Hawaii and "very early Egypt," which were "territorial states" rather than "city-states," and thus "it was the court, not the city, that provided the center, and the court was often peripatetic." Romila Thapar characterizes Vedic-era state formation as being in a condition of "arrested development," because local chiefs were relatively autonomous, and because surplus wealth that could have been directed towards state-building was instead used for the increasingly grandiose rituals that also served to structure social relations. The period of the Upanishads, the final phase of the Vedic era, was approximately contemporaneous with a new wave of state formations, linked to the beginning of urbanization in the Ganges Valley: along with the growth of population and trade networks, these social and economic changes put pressure on older ways of life, setting the stage for the Upanishads and the subsequent śramaṇa movements, and the end of the Vedic Period, which was followed by the Mahajanapada period. According to George Erdosy, archaeological data for the period of period from 1000 to 600 BCE shows a two-tiered settlement pattern in the Ganges Valley, with some "modest central places," suggestive of the existence of simple chiefdoms, with the Kurukshetra District itself displaying a more complex (albeit not yet urbanized) three-tiered hierarchy. Subsequently, (after 600 BCE) there are four tiers of site sizes,