There are places where frogs could be — but aren't. Ninety years ago, scientists were debating the question of animal dispersal. How come there are kangaroos in Australia, and none in southern Africa ...

Two Harvard professors. One on a rooftop with a bucket of frogs. The other in the front yard, down below. Ready? Get set. Throw! There are places where frogs could be — but aren't. And places where ...