The Origin has a three-part structure: analogy of artificial selection; struggle for existence leading to natural selection and hence explaining the tree of life; consilience of inductions confirming evolution. Relevance of “paradigms” from Thomas Kuhn’s Structure of Scientific Revolutions.

Several versions of ‘social Darwinism’ flourished in the late nineteenth and early twentieth century, along with ideologies derived from non-Darwinian evolution theories. They exploited discoveries of fossil hominids including Neanderthals and the Piltdown fraud to construct rival explanations of the emergence of human characteristics that might shape social development. The linear hierarchy of races erected in the nineteenth century remained the basis of many popular accounts, even though professional anthropologists began to turn their backs on it. Ideologies based on national or racial competition were advocated even by writers who did not accept the Darwinian theory of competition within populations. Fear of racial degeneration fuelled the eugenics movement’s calls for the elimination of ‘harmful’ characters, although the input from genetics encouraged an analogy with artificial rather than natural selection.

Newspapers expanded around 1900 to reach a wider readership, often reporting sensationalized stories about science. Attacks on the Darwinian theory of natural selection intensified, leading to claims that the theory was on its deathbed. Lamarckism remained active along with the theory of directed variation (orthogenesis), both presented as less materialistic than Darwinism. New alternatives appeared, including the ‘mutation theory’ (evolution by jumps) and genetics, which was at first presented as a threat to Darwinism rather than a supporting factor. In the 1920s a new surge of creationism in the United States intensified the attack on Darwinian materialism, culminating in the widely reported trial of J. T. Scopes. The same critiques appeared in a less muted form in Britain. The Darwinian ‘struggle for existence’ remained a source of anxiety for those who feared a potential threat to moral values and social stability.

From the very outset Darwin’s extensive use of metaphor in the Origin has proved controversial, with some people thinking Darwin was thereby committed to ascribing intentions or even consciousness to nature, and others fearing that readers would be misled into thinking that he was. Also, some have argued (e.g. Gillian Beer) that Darwin should be regarded as much as a poet as a scientist. We argue that, on the contrary, his metaphors have a substantively scientific role, and do real work in the development of his argument. Firstly, as Darwin himself stresses, ‘such metaphorical expressions… are almost necessary for brevity’. Secondly, they provide a method for forming new concepts (as in the case of ‘struggle’). Thirdly, and, most significantly, the use of metaphor enables Darwin to explore further the analogy between NS and AS and directly compare the achievements of human breeding and those of the struggle for existence.

Against this background, we turn to Darwin himself. We first look at the selection analogy in his theorising before writing the Origin. Darwin arrived at his theory of natural selection before contemplating such an analogy. We cannot, then, understand the analogy as what led him to the theory. Its role was to support a theory already arrived at. The evolutionary process takes place over millions of years at an imperceptibly slow pace, and so is inaccessible to direct observation. However, here and now we can observe the selective human breeding of domestic animals and cultivated plants. Darwin can then use an argument by analogy to give his theory indirect empirical support. The struggle for existence in the wild and the human breeders are not intrinsically similar agencies, but are relationally comparable in having the same kind of causal relation to the animals and plants that they are acting on with effects similar in kind but not in degree.

There are conditions satisfied by successful analogical arguments which Darwin’s argument satisfies. Darwin first establishes that breeding practices are an analogical model of the struggle for existence in the wild: just as humans discriminate in favour of animals and plants with desirable traits, so the struggle for existence discriminates in favour of creatures with traits best enabling them to cope with that struggle. Domestic breeding creates new varieties because it is systematic – there will be a tendency always to discriminate in favour of the same set of traits. The struggle for existence will have the same systematic tendency to favour certain traits at the expense of others. Therefore it is possible for it also to create new varieties. Darwin now alternates the analogy and its proportionality: if natural selection (NS) is to new wild varieties as artificial selection (AS) is to new domesticated varieties, then NS is to AS as new wild varieties are to new domesticated varieties. Thus if NS is a massively more efficient selector than AS, and the greater the cause, the greater the effect, then, a fortiori, NS should produce not only new varieties but new species.

Humans are the end point of almost four billion years of evolution. The generally accepted explanatory theory is Charles Darwin’s theory of evolution through natural selection, brought on by a struggle for existence. Darwin bolstered his case through a “consilience of inductions,” showing how evolution through selection explained in many areas right across the life sciences – behavior, fossil record, geographical distributions, morphology, taxonomy, and embryology. The missing element was an adequate theory of heredity, supplied by the Moravian monk Gregor Mendel. A major problem was why random change in the units of heredity, mutations of the genes, should provide enough suitable material for selection to work on. Theodosius Dobzhansky solved this problem by hypothesizing that all populations contain much variation, quite enough for selection to work on. His student, Richard Lewontin, using molecular techniques – gel electrophoresis – showed that Dobzhansky’s hypothesis was right.