On Charles Darwin's On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1859 with the final edition published in 1872), ch. 1–4, 6, and 14.
What are the philosophical ramifications of Darwin's theory of evolution? Our last reading ended with David Hume saying that given the order of nature, the idea of a designer was natural and inescapable. Less than a century later, Darwin provided an alternative that undermines the common intuition Hume is talking about. Apparent design, Darwin claims, is just the result of random variation (Darwin actually had no idea why this occurred) plus differential survival of variants depending on the survival or reproductive advantage of the variations. If pollution darkens the bark of trees, then the dark moths who blend into those trees get eaten less and propagate more widely than light-colored moths.
Though the book is light on philosophical argumentation, Darwin was certainly aware of the implications of his theory, and so he sneaks the reader toward it, starting off in chapter one with examples of the enormous varieties within a species produced by domestic breeding of plants and animals. In chapter two he argues for the claim that such variation happens in the natural world too, and that there's no hard-and-fast line between the concept of a species and that of a variety within a species. Chapter three covers the struggle for existence, which causes any naturally occurring variations that provide a survival advantage to become more prevalent with succeeding generations. Finally in chapter four we get his theory of natural selection. Chapter six responds to objections to the theory, and chapter fourteen summarizes and concludes the book.
Mark, Wes, Dylan, and Seth go through the book's arguments, put Darwin's theory in historical context with prior theories of evolution like Lamarck's, and talk about how an evolutionary way of looking at things has influenced philosophers.
Buy the book or read it online. The two relevant Stanford encyclopedia of philosophy articles are this one about pre-Darwinian evolution and this one about Darwinism, covering both his thought and his successors.
Darwin picture by Olle Halvars.