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.