Immigrant populations are another factor. They are disproportionately entrepreneurial, exhibiting the same initiative that brought them to this country. Brazilian enterprises in Union and Magoun Squares occupy otherwise-vacant storefronts. Latino, Brazilian, and Haitian-owned businesses are revitalizing commerce on Lower Broadway. Many of Union Squares' food purveyors are Asian.

States are adopting smart-growth policies that concentrate infrastructure investments in population centers. Roadway and streetscape improvements are laying a foundation for future growth in Assembly Square and along Somerville Avenue. The advent of the Green Line and the Assembly Square Orange Line station will give surrounding neighborhoods the boost that T stations gave to Davis Square and East Somerville's portion of Broadway.

One factor that kept people away from the latter commercial district was the (false) perception of crime. But national crime rates have steadily declined over the last 15 years, and this fact has entered public perception as well.

Americans are increasingly attracted to neighborhoods like those of Somerville's squares. The University of Michigan's Jonathan Levine finds that one in three homeowners would prefer to live in walkable urban neighborhoods like ours.

A National Association of Homebuilders survey found that 20-to-35 percent want destinations within walking distance, sidewalks, and workplaces closer to home. A remarkable 47 percent of respondents in a Public Policy Institute of California study would "choose to live in a mixed-use neighborhood where you can walk to stores, schools, and services" compared to 50 percent that would choose "a residential-only neighborhood," even if it means one must drive a car to stores, schools, and services. Studies in Atlanta, Seattle, and elsewhere yield the same results.

A neighborhood's commercial district is its "face." It's the basis on which people who are interested in residential, institutional, and light manufacturing uses will judge the neighborhood. Somerville's squares are one factor attracting households with more disposable income.

While these new arrivals provide the buying power to expand squares' range of goods and services, they also erode the relationship networks that constitute Somerville's precious human community. Many people who grew up in the city, and residents who contributed their time in ways that enrich community, have been priced out.

This process is already well advanced. City government must act forcefully and creatively to ensure that a significant portion of Somerville's housing stock remains affordable.

Building new housing units that are net losers for city finances and increase density in already too-dense neighborhoods is not the way to do it. Instead, nonprofit and public housing developers should acquire and make affordable already existing units. An ad hoc commission convened during the last days of the Capuano administration produced a report that still offers many effective tactics for accomplishing and financing this.