When we deal with cities we are dealing with life at its most complex and intense. Because this is so, there is a basic esthetic limitation on what can be done with cities: a city cannot be a work of art.
. . . the life of the planet began the long, slow process of modulating and regulating the physical conditions of the planet. The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms.