This summer, 160 years ago, it dawned on Charles Darwin that he might have to go public with the theory of evolution. He had been working on his theory slowly, gradually building it out for decades. And Darwin probably would have kept working on it, if not for a letter he received from English naturalist Alfred Russel Wallace, which outlined Wallace’s own ideas about natural selection; ideas that, unfortunately for Darwin, were very similar to his own. Iain McCalman, author of “Darwin’s Armada: Four Voyages and the Battle for the Theory of Evolution, walks us through the complicated origins (no pun intended) of the theory of evolution, and how that theory changed everything from biology to religion to politics.