In his Autobiography, written toward the end of his life, Darwin (1958a, 140) wrote that the Origin consists of “one long argument.” Let us start there. The argument came in three main parts. In a letter written a year or two after the Origin was first published, Darwin outlined his strategy (Fig. 9.1).

Note Darwin’s use of the term “vera causa.” Verae causae, or “true causes,” were things insisted upon by Isaac Newton, a demand endorsed by those writing on science in Britain in the 1830s. This was just the time when Darwin was thinking creatively about evolution, and it is clear that the young scientist took the exhortation to heart. He wanted to produce an evolutionary theory that would live up to the standards of the best science, meaning the best Newtonian science.