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The Rhetorical Structure of Darwin's Origin of Species

by John Angus Campbell

 Darwin faced a steep persuasive challenge in his masterwork the Origin of Species. As his notebooks (1837-1839) amply show, from the earliest stages of his theorizing Darwin thought long and hard about the problem of persuasion. The Origin can usefully be sectioned into five parts: 1) The introduction explains how he came upon his theory and previews its structure; 2) The first four chapters explain the elements of his theory: selection, variation, competition and the resulting differential adaptation; 3) A fifth chapter explains inheritance; 4) chapters 6-13 comprise the bulk of the book and simultaneously rebut objections and confirm Darwin's case; and 5) chapter fourteen summarizes his argument. With a little leeway for chapter five the Origin roughly follows the five part pattern of a classical oration with an exordiam, to place the judge in a favorable state of mind, a narration, to give the background necessary to the argument; a confirmation/refutation designed respectively to support one's thesis and rebut one's opponents' (the order of these elements being variable and may be intermixed as circumstances require) and a peroration to summarize the argument and drive it home.

M.J.S. Hodge has argued that, though the pattern is clearer in Darwin's 1842 Sketch and 1844 Essay–the earliest drafts of what became the Origin–Darwin's masterwork follows the vera causa (hereafter vc) logic established by Newton and restated by John Herschel in his Preliminary Discourse (1830). According to the vc principle to establish a "true cause"–and this is what Darwin wished to do with natural selection–one must show three things, that: 1) the cause exists independently of the phenomenon in question; 2) the cause has the competence to bring about the effect; 3) the cause is responsible for the effect. Though the elements are not sharply delineated one can see the vc pattern in the chapter divisions of the Origin. The first chapter "Variation Under Domestication" establishes the existence of selection separately from nature in the practices of the domestic breeder. The second and third chapters establish variation and struggle for existence as active in nature. The fourth chapter, aided by the material on inheritance in the fifth, argues that the cause is capable of producing descendants modified from their parents. The remainder of the book argues that it is more plausible to accept natural selection as the true cause of species variation, diversity, and divergence than the received theory which offers no proper material explanation.

As even this brief sketch indicates Darwin's task is not just to convince his reader's of what we call "evolution." The concept Darwin called "descent with modification" had been known since antiquity, was advocated by his grandfather Erasmus (Zoonomia, 1794-96), by the French Scientist Lamarck (Philosophie zoologic, 1809) and known to the larger public partly through Charles Lyell's refutation of it in the second volume of his Principles of Geology (1831-33). Most literate mid-Victorians, (including Florence Nightingale and Abraham Lincoln) knew of evolution not through these technical sources but through the Vestiges of the Natural History of Creation, an overnight sensation published anonymously in 1844 by the Scottish publicist Robert Chambers. Darwin, however, was arguing not just that species changed over time but that natural selection and sexual selection–with a variable dose of other factors such as the inheritance of acquired characteristics–offered a scientific explanation of how it occurred.

Considered in relation to our ordinary expectations of scientific exposition it is evident that the Origin, even in its most demanding sections (for instance the middle of chapter four), is rhetorical in a sense more popular than professional. For starters the book is written in colloquial language, is only occasionally abstract, is often highly figurative and is hardly value free. Consider the title On the Origin of Species by Means of Natural Selection, or the Preservation of The Favored Races in the Struggle for Life. The very term "selection" implying as it does consciousness is a personification; "favored races" sounds uncomfortably close to racism to post-modern ears; and "struggle for life" sounds like war, competitive sport or both. One must bear in mind that there were few "professional" scientists in Darwin's day, that by today's standards Darwin himself was an amateur. The very word "scientist" was coined by Darwin's Cambridge mentor William Whewell in 1840. (Philosophy of the Inductive Sciences, Vol. 1, p. 113)

It cannot be stressed too strongly that the background to the Origin is natural theology–the belief that the universe manifests the kind of order one would expect from mind rather than from material self-sufficiency. The Origin first invokes this tradition on its fly-leaf in a citation from William Whewell (Bridgewater Treatise, 1833) A second citation is from Francis Bacon's Advancement of Learning (1605) urging readers to be versed equally in the Bible and in philosophy. A third citation from Butler's Analogy of Religion (1736) distinguishes natural law from miracle on the basis of speed and regularity of occurrence. This final citation was added in the Origin's third edition as was a notice of Asa Gray's pamphlet, following the table of contents, and which Darwin financially underwrote, reconciling natural selection with natural theology. In these opening examples, as throughout the book, Darwin takes pains to urge that explaining via "secondary" causes is no more impious in biology than in physics, geology or chemistry.

But how is Darwin to make good on his claim that natural law can produce the "contrivances" that we associate with intelligent agency? St. Augustine, himself no mean rhetorician, observed that no one believes a thing without first regarding it as believable. An excellent example of this principle is provided by the first four chapters of the Origin: 1) "Variation Under Domestication;"2) "Variation Under Nature;" 3) "Struggle for Existence;" and 4) "Natural Selection." Together these chapters provide a suasory stair-step leading from the familiar to the unfamiliar–from the unquestionable to the debatable. The psychological key to Darwin's persuasive effort is to enable his reader, before any of the truly technical parts of his argument have begun, to locate the possibility of accepting his views through a series of premises both familiar and relatively uncontroversial. While Darwin could have illustrated his first chapter with exotic plants and animals, in effect, he takes the reader to a British farm. Mixing patriotic celebration of the skill of animal breeders and nurseryworkers with detailed examples of their work Darwin shows that domestic plants and animals are often far removed from or have few close equivalents in nature. He stresses that the difference is owed to the skill of the breeder, practiced methodically over generations. With his observation that what his countrymen and other Europeans have raised to high art was practiced unconsciously from time immemorial by "savages" who favored their best animals and plants without much thought, Darwin has positioned himself to explain how mindless processes can produce consequences that seem designed.

Chapter two argues that even as domestic breeding rests on naturally occurring variation, variation in nature is similarly ubiquitous. Darwin notes that so persistent is variation in nature that distinguished taxonomists often cannot agree where a variety leaves off and a species begins. Using an industrial analogy Darwin urges that since the largest genera have the most species a large genus may be regarded as a "manufactory" (p.56) of species. He also changes the meaning "variety" and "species" by redefining a "species" as a "more or less permanent variety" and a variety as "an incipient species." (pp. 51-54)

 Chapter three, "Struggle for Existence," presents Darwin's exposition of Malthus, and is one of the hardest sells of the book. On Darwin's success in locating its thesis in the experiential repertoire of his reader rests the possibility of the reader making the transition between seeing organisms as the product of mindfulness to the result of unguided material processes. After a host of intriguing examples from nature and domestication Darwin ends the chapter asking the reader to imagine what kind of variation would be required to extend a plant beyond its known range. Having rehearsed a number of suggestions from the chapter Darwin concludes that the result of this thought experiment should teach us how little we know of variation and inheritance. Darwin's confession of ignorance might equally be read as a summation of his thesis. Given the quantity of Darwin's examples, to say nothing of the charm of his exposition, a reader who began the chapter with no idea how biological novelty might have originated ends with abundant clues.

Chapter four, "Natural Selection," which Darwin called "the keystone of my arch" develops the maxim that whatever man can do nature can do better. The key passage, one of the most colorful and celebrated of the Origin, is Darwin's famous personification "natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good…." (p. 84) In this description natural selection can be read two ways. If one reads his personification as purely figurative one can see that Darwin is describing an undirected process based partly on natural occurring variation, changing environmental conditions, Malthus' laws of population, heredity and unimaginable reaches of time. Read another way Darwin's figure suggests, as some readers mistakenly inferred, a force (a divine hand?) guiding the process by a wisdom greater than human. However one reads it the complex figure brings together what the reader has learned of the breeder operating under domestication and what is known of nature's own operations. In various passages Darwin tries to turn the traditional association of science with natural theology to his own advantage. In chapter five Darwin characterizes special creation as making "the works of God a mere mockery and deception." (p. 167) In the final chapter he affirms "There is grandeur in this view of life with its several powers having been originally breathed into a few forms or into one…." (p. 490) In the third edition he added "breathed by the Creator." Though Darwin admits to having "truckled" to "penteteuchal language" in this final section, aside from a Cheshire cat-like grin hovering over an occasional passage the argument from design has vanished from Darwin's world.

Though space precludes further elaboration the main elements of Darwin's persuasive appeal are already in place by the end of the opening pages of the Origin's fourth chapter. In the body of the work when the going gets difficult–and to Darwin's everlasting credit he includes in his work every objection he could think of–Darwin characteristically appeals to the example of the breeder to convince his reader that the work he requires of natural selection is conceivable, possible or likely. Given the tremendous achievements of the domestic breeder–in but a few hundred years–and considering the omnipotent scope of nature and the enormous time at its disposal–Darwin repeatedly urges his reader to consider what might not nature have achieved on its own? Well before the end of the Origin, if not every reader, then at least for the minority who would carry forward Darwin's legacy–the question was no longer rhetorical. 

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