Water is an element essential to the existence of living things. It also conditions human life, and humans are always striving to exploit its use to the maximum. Before the development of railways and road networks, the major cities were built close to the sea or near navigable rivers, on the major maritime and river routes. Up to the 19th century, the major economic alliances were determined on the basis of water routes. At the end of the 18th century in France, for example, the only towns with a population of more than 50,000 that were not near sea ports were located on navigable rivers—Paris (between the confluence of the Seine with the Marne and the Oise), Lyon (at the confluence of the Rhone and the Saone), and Strasbourg (on the Rhine).