4 ideas
Reef-building corals can only live in shallow, sunlit water, yet reefs rise from great depths. Darwin argued that a fringing reef around a volcanic island keeps growing upward while the island slowly subsides. The reef becomes a barrier reef separated from land by a lagoon, then an atoll once the peak disappears, so the shape of a reef records movement of the ocean floor that no one can see directly.
In 1835 the vice-governor of the Galápagos, Nicholas Lawson, told Darwin he could tell which island any giant tortoise came from by its shell. Darwin then noticed that mockingbirds also differed from island to island, although the islands sat only 50–60 miles apart under the same climate. He admitted he had mixed his finch specimens without labelling their islands, missing the pattern at first. It became his key case of related but distinct forms living in neighbouring places.
After the 20 February 1835 earthquake at Concepción, Chile, Darwin saw beds of dead mussels lifted above the high-tide line. FitzRoy measured the land around Santa María island raised by about eight to ten feet. Darwin connected this to fossil sea shells he found high in the Andes. He read the mountains as the sum of many such ordinary, observable uplifts over immense time, not as a single catastrophe.
Darwin argued that people who defend slavery judge it from the master's position and never imagine themselves as the enslaved. As evidence he told how a Brazilian slave flinched and lowered his hands, expecting a blow, when Darwin simply waved his arm while giving directions. He concluded that where misery comes from human institutions rather than from nature, the moral guilt belongs to the society that keeps those institutions.