Darwin & Coral Reefs

On Darwin's voyage on the Beagle, he observed several kinds of coral reefs. Coral atolls are circular coral islands surrounding a shallow lagoon. Fringing corals growing in shallow water on the edge of an oceanic volcano. Barrier reefs form around a lagoon with a volcanic island at the center. Darwin, because of natural selection, decided that the reef types had to be part of a single sequence of development.

So, thinking like Darwin, what sequence would you put these reefs in, how could they change into another type? Coral reefs only grow in shallow water, but all three types can be seen in the central Pacific.

I would say that the the coral atolls began to develop around the shallow lagoon, and then in order to expand/spread, the fringing corals began to grow around the edge of oceanic volcanoes because around the very edge of the volcanoes would be more shallow water. And then barrier reefs began to form around lagoons by volcanic islands because they expanded from the edge of the volcanic islands to the lagoons. That is my view, and it's similar to his theory of evolution in that life adapts to it's surroundings as it evolves, and this is the case for the coral reefs because in order to expand each type of reef had to adapt to a slightly different environment.

What do you think?