The “principles of domestication are important for us,” Charles Darwin (1868b, 3) wrote in his Variation of Animals and Plants under Domestication, to illustrate “that the principle of Selection is all important” in producing evolutionary change (Fig. 8.1). The work of breeders, he explained, might be seen as “an experiment on a gigantic scale” that provided empirical support for his claims about analogous processes in nature. For instance, centuries of artificial selection of small heritable differences (variations) among domestic dogs had produced breeds as different as the bulldog, the greyhound, and the spaniel, each of them specialized to perform a specific task in the human household. In similar fashion, natural selection, by acting on the variations of wild animals and plants, had created the stunning diversity of the living world, in which every species was characterized by adaptations enabling it to survive and reproduce under the circumstances given by its natural surroundings.

Historians and philosophers of science agree that the analogy between artificial and natural selection was a vital element of Darwin’s argument in the Origin of Species (1859). Philosophers have argued that he deployed the analogy to show that natural selection was a vera causa, a true cause, in nature. Darwin proceeded by arguing, first, that domestic races can be produced by sustained selection of individual variations. He then claimed that both these elements, the variations as well as selection, are also present in nature and can in an analogous way, on a much longer time scale, produce new species (Waters 2003). There is some debate on how essential the analogy really was (Ruse 1975a; Gayon 1998). Darwin (1859, 457–59) himself claimed that, even without it, the available evidence spoke convincingly in favor of descent with modification. Nevertheless, he made good use of the analogy in his effort to structure the Origin as “one long argument,” as he called it.