Cities are not separate from the natural world on which they depend. In a north Indian city, men and women build a new apartment structure largely by hand. They carry tiles in wooden hods on their heads, tiles formed of earthen clay that have been baked by burning wood and charcoal, which is partially oxidized wood, from the shrinking forests on hillsides far to the north. The scaffolding is of bamboo that grew in the same forests, tied with ropes of hemp from fields that can be seen in the hazy distance from the top of the building. Like all cities, this one uses resources transported from the land near at hand or far away.
In Shanghai, I visited a marketplace where an astonishing variety of stalls lined the lane, stocked with every staple for the kitchen: vegetables and fruits from gardens just outside the city, live ducks from a nearby lake, lotus and water chestnuts and snails that country people brought to sell. What is often described as a series of economic transactions also can be seen as humans manipulating and using other species of animals and plants.
An easy walk from the center of Avila in Spain took me along the crowded streets of a thriving provincial capital, out a gate in the massive walls, through wheat fields, vineyards, and olive orchards, to a viewpoint where, looking back over the city, I could glimpse the pineclad heights of the Sierra. In this short distance, I saw examples of many different ways in which the land is used to meet the preferences and needs of an urban population.
Each of these scenes has something important in common with the early cities that arose in the river valleys of Mesopotamia, the Nile, and the Indus, or on the loess plains of north China. The state with its religious and political institutions, the specialization of human occupations, the stratification of society into classes, and the development of arts such as monumental architecture, writing, and the measurement of space and time, appeared first and developed most fully in these large, densely populated human centers. The city is a structured human relationship with the natural environment. Although it is an artificial creation of human culture, it can also be seen as an ecosystem related to other ecosystems. Every activity of humans in it requires some resource from the surrounding environment. The city is not a truncated phenomenon, but has a natural context consisting of the many cycles of organic and inorganic substances that constantly affect it. Cities are part of the ecosystems within which they exist, although they make extensive changes within them and reorganize nature for their own benefit. Too often cities are studied only as a series of human social relationships and economic arrangements, and their intimate, constant, and necessary connections with the natural processes of the Earth are forgotten. 1
A more productive agriculture was the necessary condition for the genesis of cities, since they were larger, more densely populated, and organized in a more complex way than the