Oceanic island chains built by volcanic activity are initially lifeless—dark spots on a lighter sea. But no matter how remote their location, eventually they are colonized. A bird flies in, bringing seeds in its gut or caught in its feathers. Other seeds arrive borne by wind or water. A lizard floats in on driftwood. A seal hauls itself onto the shore. Forests emerge and the islands slowly change color from gray-black to green.

On hundreds of islands, landscapes of rock have come to life in this fashion. The particular trajectories of the biota on any island depend on which species arrived first. In each set of islands, different lineages prosper. The radiation of species is happenstance, following Darwin’s rules, with an infinite variety of possible ends.