The mosaic of subcultures in a city results in hundreds of different cultures living, each in its own way, next door to one another. Such subcultures have their own ecology. They can only live at full intensity, unhampered by their neighbours, if they are physically separated by physical boundaries. This is important because where there is insufficient separation between subcultures, they tend to oppress or subdue the life style of their neighbours, or in turn to feel oppressed and subdued. In general, whenever one subculture in a city is very different from another next to it, people are afraid that the neighbouring area will encroach on theirs, and upset their values. Thus they will do everything to make the next door area like their own, and destroy it as another subculture. In effect, for unhampered existence, such subcultures should be separated by swathes of open land, workplaces, public buildings, water, parks or other natural boundaries.
This need for separation of subcultures is reinforced by ecology. In nature, the differentiation of species into subspecies is largely due to the process of geographical separation. It has been observed by a multitude of ecological studies that members of the same species develop distinguishable traits when separated from other members of the species by physical boundaries like a mountain ridge, a valley, a river, a dry strip of land, a cliff or a significant change in climate or vegetation. In just the same way, differentiation between subcultures in a city will be able to take place most easily when the flow of those elements which account for cultural variety - values, style, information and so on - is at least partially restricted between neighbouring subcultures.
Whenever there is an area of homogeneous housing in a city, its inhabitants will exert strong pressure on the adjacent areas to make them conform to their values and style. For example, the 'straight' people who lived near the 'hippie' Haight Ashbury district in San Francisco in 1967 were afraid that the Haight would send their land values down, so they put pressure on City Hall to get the Haight 'cleaned up' - to make it more like their own area. Other studies in California confirm that it is necessary to have a physical barrier between subcultures to prevent this happening. For example, in San Francisco the two most distinctive areas are Telegraph Hill and Chinatown. Each of these is surrounded by 'natural boundaries' - on two sides by docks, on another two sides by the city's banking area.