There are places where frogs could be — but aren't.
And places where frogs could be — and are.
Ninety years ago, scientists were debating the question of animal dispersal. How come there are kangaroos in Australia, and none in southern Africa --which seems, environmentally, very kangaroo-friendly? Certain frogs show up in warm ponds in one part of the world, but warm ponds a thousand miles away have none. Why?
Scientists in the 1930s hadn't yet come up with the notion of plate tectonics (that continents break up and move, carrying animals with them).