The Voyage of Charles Darwin S01E06 - Suppose That All Animals and All Plants Are Represented by the Branch of a Tree, the Tree of Life

On the Galapagos Islands, Darwin not only encountered creatures found nowhere else on earth, but also observed subtle differences between the inhabitants of one island and those of another.
But he nearly missed the significance of the little brown finches that could be seen everywhere. Some of these had beaks thin enough to pick up seeds from the hard ground; others had beaks thick enough to crack a nut. Here was the clue which led him directly to the theory of natural selection.