Scientific Proof That Cities Are Like Nothing Else in Nature - Emily Badger - The Atlantic Cities

A little bit on the planner/geek end of the scale, but this is an illuminating article on the function and commonalities all cities share. And these functions and qualities all vary with size. The article also talks about density and its role in sustainability.  Click title for link. From the article:

“We tend to look at things by the way they look, by form,” he says. And this is why most of our existing metaphors fail. “All the successful theories of science are not about form at all – they’re about function. They’re about how things develop, how things change. They’re about process.”
If we take that point seriously, he says, we have to ask what cities do, not what they look like, or even how they grow. At their most fundamental, cities are not really agglomerations of people; they’re agglomerations of connections between people. All of their other properties – the roads we build to reach each other, the density required to do that, the economic products and ideas we create together – derive from this fact.

The "he" refers to the author of a thorough study on the topic which is the basis for the article. The study itself is not required reading and there will be no pop quiz.