From the time Tokugawa Ieyasu became shogun in 1603, he and his progeny ruled Japan until 1867. Among the shogun's vassals were about 260 daimyo (territorial lords). As a token of their submission to the shogunate, the daimyo were required to leave their wives and children permanently in Edo (now Tokyo) and to alternate their own residence between Edo and their domains. Their family were detained in Edo to prevent the daimyo from rebelling against the shogunate, which wielded great power over them: It could reduce the size of their domains; move them to another domain; or even execute them. To perform their obligations under the sankin kotai system, the daimyo had to maintain residential estates (yashiki) in Edo, which housed their families and a number of their retainers. Besides the daimyo, direct samurai retainers of the shogunate, called hatamoto and gokenin, also resided in Edo. These samurai and their families and a population of servants (shiyonin) reached 500 thousand by beginning of the 18th century. Adding to that another 500-thousand merchants and artisans, the population of Edo at that time exceeded one million. A census taken in 1801 showed the population of London, then the largest capital in Europe, to be around 860, 000. Edo, therefore, was the most highly populated capital in the world during that period. In Edo, the daimyo's estates contained spaces for doing research and for editing and compiling manuscripts. The daimyo themselves wrote manuscripts while their retainers often conducted much of the research. These activities exerted a substantial influence on scholarship and research in the Edo period. To accommodate the annual travel of the daimyo and their sizable retinues to Edo, a network of roads (gokaido), including bridges and ferries, was created and maintained, as were systems of lodges (hatagoya) and of baggage conveyance, including packhorses, barges and porters. As merchants and farmers were also able to use the hatagoya, by the beginning of the 19th century they made a travel very easily more than a month. The sankin kotai system imposed a large financial burden on the daimyo in requiring them to journey to and upkeep estates in Edo, which consumed a large part of their income. Through the system's execution, however, a considerable degree of political and cultural similarity was achieved throughout Japan. This made it possible to establish without major obstacle a unified national system under the Meiji Restoration, after the collapse of the bakufu (Tokugawa shogunate). This ability to bring the country together quickly distinguished Japan's unification effort from those of the period that took much longer to achieve due to entrenched internal economic and cultural differences in such European countries as Italy and Germany.