Diversity (J. Jacobs)

The diversity, of whatever kind, that is generated by cities rests on the fact that in cities so many people are so close together, and among them contain so many different tastes, skills, needs, supplies, and bees in their bonnets.

Even quite standard, but small, operations like proprietor-and-one-clerk hardware stores, drug stores, candy stores and bars can and do flourish in extraordinary numbers and incidence in lively districts because there are enough people to support their presence at short, convenient intervals, and in turn this convenience and neighbourhood qualities are big parts of such enterprises' stock in trade. Once they are unable to be supported at close, convenient intervals, they lose this advantage. In a given geographical territory, half as ma-ny people will not support half as many such enterprises spaced at twice the distance. When distance inconvenience sets in, the small, the various and the personal wither away [...].

Smallness and diversity, to be sure, are not synonyms. The diversity of city enterprises includes all degrees of size, but great variety does mean a high proportion of small elements. A lively city scene is lively largely by virtue of its enormous collection of small elements [...]. 

To generate exuberant diversity in a city's streets and districts, four conditions are indispensable:

1. The district, and indeed as many of its internal parts as possible, must serve more than one primary function; preferably more than two. These must insure the presence of people who go outdoors on different schedules and are in the place for different purposes, but who are able to use many facilities in common.

2. Most blocks must be short; that is, streets and opportunities to turn corners must be frequent.

3. The district must mingle buildings that vary in age and condition, including a good proportion of old ones so that they vary in the economic yield they must produce. This mingling must be fairly close-grained.

4. There must be a sufficiently dense concentration of people, for whatever purposes they may be there. This includes dense concentration in the case of people who are there because of residence."

Jacobs, Jane (2011 [1961]) The Death and Life of Great American Cities. New York: Modern Library. pp 192-197.