Robert Krulwich

When a species gets rare, its market value rises. The higher its price, the more it's hunted. The more it's hunted, the rarer it gets. Not a happy cycle, and this keeps happening ...

Add all of us up, all 7 billion human beings on earth, and clumped together we weigh roughly 750 billion pounds. That, says Harvard biologist E.O. Wilson, is more than 100 times the biomass of any large animal that's ever walked the Earth. And we're still multiplying. Most demographers say we will hit 9 billion before we peak, and what happens then?

I guess everybody, even the smartest people who ever lived, have days when they feel dumb — really, really dumb. Oct. 1, 1861, was that kind of day for Charles Darwin.

In a letter to his friend Charles Lyell, Darwin says, "I am very poorly today," and then — and I want you to see this exactly as he wrote it, so you know this isn't a fake; it comes from the library of the American Philosophical Society, courtesy of their librarian Charles Greifenstein. Can you read it?

It says:

Whoah! You know the feeling, right?

Oh, dear.

First off, this lizard? It's not really a lizard. It's an almost vanished species, a reptile like no other.

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This is the moon as Morse code.

Here's a question you probably didn't know was a question: Why is the sky dark at night?

Look at this map, and notice that deep, deep in the Republican South, there's a thin blue band stretching from the Carolinas through Georgia, Alabama and Mississippi. These are the counties that went for Obama in the last election. A blue crescent in a sea of red.