But Darwin was also dedicated to extrapolating from what he could directly observe, which ultimately led him to the third idea: the principles we now call the theory of evolution by natural selection. Darwin (and his coauthor Alfred Russel Wallace, who’d arrived at the same theory on his own) argued that life’s variety wasn’t the product of a master plan; it had sorted itself, over time, by passing on the traits of varied individuals to their descendants. Life on Earth, they concluded, will never arrive at a fixed state.
