Central to this book is a trio of chapters (4, 5 and 6) on Darwin’s Origin of Species in its first edition of 1859. Darwin called his book 'one long argument'. These three chapters clarify how this long argument is conducted; how Darwin’s analogical reasonings about natural and artificial selection support the argument; and how his various metaphors are grounded in those reasonings. The conclusions from these chapters support the claims in our chapters 1 to 3 about the decisive antecedents, from ancient times on, for Darwin’s conception of analogical reasoning and what it can do for his one long argument. Equally these conclusions support the claims made in our chapters 7and 8 as to how that reasoning should be analysed and evaluated by philosophers, biologists and historians today. Our writing combines throughout narratives that are often not overtly normative with judgements that often are so.

Here we examine how the Origin deploys the selection analogy. Darwin works in a vera causa tradition. To discover the true cause of some phenomenon, one first establishes that the cause exists (an existence case); then shows that it has the power to produce the effect (an adequacy case); and finally that it has in fact done so (a responsibility case). After examining how breeding practices produce new varieties, Darwin establishes the existence of natural selection. In the wild, resources are limited, and so there is a struggle for existence, with the individuals with the most favourable traits most likely to survive. Next comes the argument showing that natural selection should produce new species, that it is causally competent and adequate. This argument includes reasoning a fortiori: natural selection, being far more comprehensive, precise and prolonged than artificial selection, is a much greater power and so able to produce the adaptive diversification of distinct species over eons of time. Darwin, in later chapters, offers evidence that natural selection has in fact been responsible for producing new species, new genera and even new classes of animals and plants.