The idea of evolution by natural selection formulated by Charles Darwin and Alfred Russel Wallace is a cornerstone of modern biology, yet few biology students or professionals are familiar with the processes of discovery behind the idea. Focusing on Darwin, I draw on letters, notebooks, and other resources to trace key insights and put them into historical context, illustrating how major elements of Darwin's theory came to him over many years. I further consider how Darwin came to formulate the logical argument structure of his Origin of Species, discussing the philosophical arguments inherent in the book's structure and how this and Darwin's other works can be seen as part of a larger argument and way of looking at the world. I suggest that in teaching evolution today, educators could profitably draw on both Darwin's personal intellectual journey in coming to his ideas, and the compelling argument structure he devised in presenting his theory.