On the surface of things, a two-toed sloth doesn’t look much like its closest fossil kin. The tubby, pug-nosed mammal is not quite as imposing or majestic as Megalonyx – the “great claw” Thomas Jefferson discovered and mistakenly identified as an enormous lion over two centuries ago. But the two are relatively close relatives. In fact, today’s shaggy two-toed sloths are more closely related to Megalonyx than that other variety of upside-down mammal, the three-toed sloths. Together, the two- and three-toed flavors of sloth represent a remarkable case of convergent evolution.

The distant...

ZIP codes have been around for less than fifty years. In addition to allowing future historians to ascertain that the television show Beverly Hills 90210 must have been created no earlier than 1963, ZIP codes have become part of our culture, organizing our locations and determining the flow of mail.

But ZIP codes are not created randomly. There is an order and a structure to this mail system. Let’s use this nearly-fifty-year anniversary as an opportunity to examine the quantitative aspects of ZIP codes.

One quick way to look at ZIP codes is by seeing how each part of a ZIP code defines a part...