One of the truly strange and most amazing aspects of evolution is that certain designs,

certain forms seem to just work. On at least five occasions independent of each

other, something very much like a crab evolved. Half the crabs you see in the

world today are not actually crabs they're just stealing the design

This habit of evolution of always making crabs is called carcinization, but it shows up in all sorts of ways

Sometimes it's just a great design feature, like eyesight.

What a killer feature to evolve. And so it has evolved independently multiple times. But my favorite, the sleekest, most glamorous

of all convergent evolutionary traits has got to be flight. The ability to fly is amazing and it shows up again and again.

The terraosaurs had it. Obviously the dinosaurs who became birds are the big game these days, but even us mammals have gotten in the game.

I am talking about bats, beautiful little hairy flying creatures.

So today on the third day of creature week,

we join producer Abby Pearl and go to Portugal

to a beautiful ancient library that is home not just to incredible books but to a staff of

winged library helpers who do their best work after the lights go out.

Hope you enjoy it. The library at the National Palace of Mafra looks like the kind of library where something

magical could happen at any given moment. Think long hallways, bookshelves lining the walls all the way up to these arched

ceilings. It's an old library that's part of an old palace built in the 1700s and its books are even

older. There are more than 35,000 volumes here, including what's considered the first

Encyclopedia, a whole collection of books that were forbidden during the

Inquisition, and even in Canabula books that were printed before the 16th century.

The collections here are extraordinarily rare but the books aren't the only unusual things about this library.

At the end of the day, the lights are turned off. The building closes and out come the wind a daily celebration of the world's, incredible, and wondrous places.

...