Please list the assumptions, reasoning and conclusion for the summary below.

Already in October 1835, while they were on their way from the Galapagos toTahiti, he had made a note on coral islands:we saw several of those mostcurious rings of cora land, just rising above the water's, which have been called Lagoon Islands ... These low hollow coral islands bear non proportion to the vast ocean out of which they abruptly rise; and it seems wonderful, that such weak invaders are not overwhelmed, by the all-powerfuland never-tiring waves of that great sea, miscalled the Pacific.
He had asked himself, it was found that these reefs went down a very long way, and that all the corals below the 120-foot mark were dead - would not that be a proof that the floor of the ocean had been gradually sinking, and that the coral polyp had kept pace with this sinking by building the reefs up to the surface? This was a theory that he could now put to the test.

He went out with FitzRoy in a small boat to the outer reef and carefully took numerous soundings on the steep outside of Keeling atoll. This suggested to Darwin that coral formations were the end products of aeons of slow reciprocal processes: the up lifting fan island by submarine volcanic action, the colonizing of its slope by myriad coral polyps, and finally the gradual subsiding of the island into the sea.

He worked out that there were three different varieties of coral formations: barrier reefs and fringing reefs, all part of the same evolutionary process stretching over millions of years.The growth of the coral must keep pace with the subsidence beneath it, and so form first a barrier reef and then an atoll: Mountains of stone accumulated by the agency of various minute and tender animals.The following illustrations show the three stages of coral development by means of section drawings of the same island.