The second part of the book focuses on diversity - a key ingredient for a successful city - and the conditions that make it possible. The single most important point she makes is that the conditions she discusses must all be present; if any of them are missing, things will not work. Her point is that this runs contrary to the methods of urban planning common at the time, which tended to focus on individual uses rather than on the larger picture. Modern planners have, to some extent, learned this lesson, but in many cases where the necessary conditions cannot all be created easily or cheaply, they attempt to create one or two and hope for the best, resulting in either complete failures or projects with serious flaws (Battery Park City and Metrotech being two good local examples).
Why is diversity important, and why is it associated with cities? Diversity attracts people with unusual interests, and large cities provide enough people that businesses and events serving these unusual interests can survive. This is one point that urban planners miss; the mollification of places like Times Square turns a city into something no different than small-town residents can find at home. Meanwhile, the unique appeal of the city is damaged. "Wherever the lively and popular parts of cities are found, the small much outnumber the large." This is certainly not the case in Times Square, but then, that area is more a tourist destination than a place for residents; those areas that are locally popular (such as Soho, the Village, etc.) are indeed populated as much or more by small businesses as by big chains.
If large populations should encourage diversity, Jacobs asks, why are so many cities and areas of cities so bleak and devoid of life? "Huge city settlements exist without their presence generating anything much except stagnation and, ultimately, a fatal discontent with the place." 
The four conditions she lists are:
- The area must have more than one primary function:
- The primary reason for this is that successful areas of a city are populated throughout the day. Nothing's sadder or more desolate than the kind of 9-5 downtown you see in a place like Cincinnati. This not only keeps streets lively, it also provides a constant flow of customers that allow businesses to survive. She cites Wall Street as an example - in 1960 it was not a residential area and even as it started to become so in the late 1990s, it had a remarkable dearth of amenities. I remember a friend of mine who moved into a John Street apartment in 1997 complaining that he couldn't get pizza delivered. I was stunned there was anywhere in New York City you couldn't get a pizza delivered. But if you have a huge crush of customers for two hours a day, and absolutely no one for the rest of the day, it's hard to keep a business going.
Which speaks to another of her points: residential use is a result of vitality and diversity, not a cause of it. It took a very long time for downtown Manhattan to become livable, and the early residents there had to hike over to Battery Park City (a government-funded residential oasis with services, stores and restaurants) to get by.
So her point is that people need to use the same streets, at different times, sometimes patronizing the same businesses, to provide the economic lifeblood that will support the kind of stores and businesses that make a district good to live in and attractive to visit. The whole focus on "Central Business Districts" rather than downtowns misses this point, as does zoning that does not allow a mixture of residences and businesses. This encourages usage only at certain parts of the day, which is deadly.
This also speaks against the idea of taking certain important institutions and segregating them in one area, as New York City did with Lincoln Center. Not only does this turn a large area of the city from a working neighborhood into a wasteland populated only at specific times, it also takes the institutions themselves away from areas that could benefit from a different kind of usage. If the opera house is next to an office building and across from an apartment building, you have a vital neighborhood. If the opera house is isolated behind a bleak expanse of concrete, it won't work that way. The area around Lincoln Center is not blighted, but it's sure not pleasant (walk down Amsterdam Avenue behind the complex), and is less attractive for residents than the Upper West Side or other, more diverse, areas. And other parts of the city could have benefited from those cultural institutions being spread around more.
- Most blocks must be short:
- This never occurred to me before, but once she mentions it, it's obvious. Long blocks are isolated - it's too much walking to go to the next block - and also reduces the number of avenue blocks, where businesses are located. She cites Chelsea (which in the 60s was not a desirable area by any stretch of the imagination), which in theory should have benefited from the proximity of the Village. Yet it took years to start improving. Why? Long blocks that discourage pedestrian traffic. Traditional planning viewed streets as "wasteful," focusing on "superblocks" and big plazas and such, but her point here is that the opposite is actually necessary for a good neighborhood.
- The buildings must be varied in age and condition:
- Another point I had not thought of. When you walk around truly attractive neighborhoods, you see old buildings. They're pretty, and built differently, and attract people to the area. Jacobs recognizes this, but she makes another connection - new buildings are also necessary if the neighborhood is going to attract businesses and residents that can't or don't want to be in older buildings. If you only have older buildings, those people and businesses will avoid the area. But if you only have new buildings - and particularly if you tear down entire blocks of old buildings to build new ones - you will only have businesses that can support the higher costs of the new buildings. The result is that the used book stores, cafes and odd little stores that attract pedestrians will disappear. Again, look at Metrotech in downtown Brooklyn, which replaced blocks of low-rise apartments and stores with huge office buildings and new retail space. The businesses that left were bookstores, hardware stores, and other places that people still talk about more than ten years after they disappeared. Replacing them are chain restaurants, placed around an attractive plaza that people hardly ever go to except during working hours.
And contrary to the beliefs of urban planners, business doesn't just grow in spanking new office buildings, but often in old ones. Startup businesses can't afford large office buildings, and office complexes often don't provide the services and character that startups want and need. As Jacobs puts it, "Old ideas can sometimes use new buildings. New ideas must use old buildings."  Despite millions of dollars spent attempting to attract new-media businesses to the Wall Street area in the 1990s, "Silicon Alley" grew up in Tribeca and along Broadway between Union Square and Herald Square -- cheaper areas with lots of older, low-rent buildings (along with the kind of street life that twenty-something entrepreneurs want to work in). Admittedly, many of the downtown buildings that the city tried to encourage new media businesses were old ones, office buildings that had outlived their usefulness, but they were big and expensive, and located in a largely dead area, speaking again to Jacobs' point that all these factors are necessary for success.
She concludes this section by discussing "newness" in and of itself. Not only is the construction of huge complexes bad in that it limits diversity economically, but also in that it chains an entire area to a particular style of building. How many super-complexes still look vital ten or twenty years after being constructed? The World Trade Center was hideous. Battery Park City is showing its age in many places. Old buildings - often built organically, without planning, and having some combination of characteristics that allowed them to survive over years -- attract investment and renovation. New complexes, built all at once and carefully planned out, often don't age well. Malls built in the 1970s are being torn down or are struggling to keep anchor tenants while downtowns built in the 1800s thrive. "Newness, and its superficial gloss of well-being, is a very perishable commodity."  This amuses me most when I look at the white-flighters who fled brownstone Brooklyn in the 1960s and 1970s, moving to new housing developments in Staten Island. Those developments now are ugly, desolate expanses of cheaply built houses that aren't aging well, while the "blighted" neighborhoods they fled are getting more and more expensive every year. We can thank our lucky stars that a bunch of urban planners didn't tear down Park Slope wholesale in the 1960s and replace it with some hideous housing project or planned community.
This type of project, she points out, essentially kills a neighborhood. From an organic, growing, evolving area, it becomes a static place, incapable of adjusting to the times. "The neighborhood shows a strange inability to update itself, enliven itself, repair itself, or to be sought after, out of choice, by a new generation." 
- There must be a dense concentration of people:
- This is the most obvious point, although you have to note that she's not just saying you need a lot of people, but that you need a lot of people concentrated in small areas. Population density is an economic necessity for diversity - she quotes a 1959 New York Times article about an Arizona business professor who studied American suburbs, concluding, "decentralization produced such a thin population spread that the only effective economic demand that could exist in suburbs was that of the majority." 
Jacobs wrote this book at a time when decentralization was gospel, and dense urban populations were regarded as an inherent problem -- this despite the fact that some of the most successful cities and sections of cities are also the most densely populated. Was it not obvious to these people that there's a big difference between high-density dwellings, and overcrowded dwellings? More importantly, wasn't it obvious that tearing down a neighborhood and replacing it with larger dwellings occupying more space would actually increase overcrowding unless a significant proportion of the population was displaced?
The economic results bear her theory out -- overcrowding is bad, but it's worse in low-density areas than in high-density areas, and high-density areas are generally more attractive (and more expensive) than low-density areas. This is largely because high-density areas will inevitably generate urban problems, but without the other necessary success factors, they'll have none of the tools city neighborhoods use to combat these problems. In general, she says that city density should be as high as possible to stimulate diversity, without getting so high as to repress it (for instance, by necessitating standardized buildings as in housing projects). Greenwich Village, she notes, has densities varying from 125 dwellings per acre to more than 200 per acre, and works largely because almost all the available land is used for homes -- without big swaths of worthless parkland, large complexes of office or cultural buildings, or areas where residences are not permitted by zoning rules. This turns out to be a direct contradiction of the planners' belief in "open space." Open space can hurt a neighborhood, by reducing the concentration of people. Lower densities, particularly in the in-between ranges found in suburbs, can be absolutely fatal to diversity.
In addition, she says, it's not a good idea to try to increase density all at once - rather the natural market forces should be allowed to take their course, removing some old buildings and replacing them with new ones in a gradual process that allows for continued building diversity as the concentration of homes increases.
In a concluding chapter on the "myths of diversity," she argues that planned communities are not orderly in some senses, that they exhibit "the disorder of conveying no direction."  A lack of city diversity, she says, is "innately depressing on the one hand" (as in desolate housing complexes) or "vulgarly chaotic on the other," as in the ghastly strip-mall eyesores littering commercial roadsides in suburbs. Diversity and concentration don't necessarily cause traffic congestion, unless the community has other problems that force people into cars for every need. Zoning should be focused less on restricting the types of uses (and creating desolate unvaried areas) and more on restricting the scale of uses - a clothing store on a city street is a good thing, but a clothing store that takes up an entire block less so.
She also touches on what I feel is a core point of the book--that city planning (or anti-planning) has been driven by a particularly harmful mindset:
People gathered in concentrations of big-city size and density can be felt to be an automatic--if necessary--evil. This is a common assumption: that human beings are charming in small numbers and noxious in large numbers. Given this point of view, it follows that concentrations of people should be physically minimized in every way ... [and] by aiming at illusions of surburban lawns and small-town placidity. It follows that the exuberant variety inherent in great numbers of people, tightly concentrated, should be played down, hidden, hammered into a semblance of the thinner, more tractable variety or the outright homogeneity often represented in thinner populations.
On the other hand, people gathered in concentrations of city size and density can be considered a positive good, in the faith that they are desirable because they are the source of immense vitality, and because they do represent, in small geographic compass, a great and exuberant richness of differences and possibilities ... they should be enjoyed as an asset and their presence celebrated. 
Jacobs continually expresses wonderment at the foolishness of planners who believe so ardently in things that never work in reality. In some ways, she is remarkable in the calmness and factual approach she takes to dissecting the idiocy and blindness of planners. Their policies were so destructive that one has to wonder if the terrible results were unintentional.
Jacobs says, "the development of modern city planning and housing reform has been emotionally based on a glum reluctance to accept city concentrations of people as desirable." I believe she is understating the case here -- the issue is not "glum reluctance" but outright hatred. I believe that the people behind the destruction of American cities, then and now, are not unaware of the natures of big-city populations, but well aware of and actively hostile towards them. Call it the Robert Moses theory: They either had no intention of improving anything, or their notions of "improvement" were as odious as the motives of those seeking to purify the races.
The impulse to replace city neighborhoods with sterile plazas and complexes, to subsidize suburbs while destroying downtowns, to herd poor and minority residents into hideous housing projects, was not based on any altruistic beliefs, but on the same hatred of diversity and intellectual activity that burns books, bans teaching of evolution in schools, and clamps down on political protests. The disastrous policies of the second half of the twentieth century are essentially incomprehensible without understanding this; in this case, I believe we must ascribe to malice what cannot be explained by stupidity (because I simply don't believe that anyone could have been that stupid). Our failure to understand this leads us to continue allowing the same thing to happen again and again -- as in Bruce Ratner's deliberate gutting of downtown Brooklyn with ugly and hostile projects. More about that in a future post.