Darwin’s Literary Models
It may not be structured like a journal paper, but On the Origin of Species was written according to classical rules of rhetoric
The Pattern Within
Darwin specifies the pattern of his argument in the introduction to the Origin. First he establishes a key premise: Artificial selection of varieties of plants and animals would be its central analogy (chapter 1). He then documents variation in nature (chapter 2) and follows with the mathematical imperatives of population biology and the “Struggle for Existence,” which combine to produce natural selection (chapter 3). He brings all of these topics together in chapter 4 and introduces the concept that transmutation of existing varieties of organisms into new species “almost inevitably causes much extinction of the less improved forms of life, and induces what I have called the Divergence of Character.”
These four chapters contain his idea in a nutshell. He extends it in chapter 5 with discussion of “little known laws of variation and of correlation of Growth.” But then, instead of pressing on directly with his case, he makes a detour into four chapters in which “the most apparent and gravest difficulties on the theory” are admitted and dealt with. Only after putting the whole affair into a geological context (chapter 10) is he ready to deal with corroborating elements of his theory—such as geographical distributions, homology, vestigial organs and (one of the strongest parts of his argument) the fact that a pattern of descent explains the patterns of similarity long since formalized in systems of classification (chapters 11 through 13). At last, there is a final summing up and some rather lyrical “concluding remarks.”
In the synthetic mode of deductive rhetoric, the most effective argument is often syllogism: If all A are B, and all C are A, then all C are also B. Darwin’s “long argument” has the character of an extended syllogism: Variation can be artificially selected by breeders to produce new forms; variation exists in nature; the mathematically driven struggle for life is comparable to the ruthless hand of the breeder; there have been vast eons of time within which natural selection could operate; therefore natural selection explains both origins and extinctions within organic diversity. The order in which Darwin developed his subjects is, however, different from the patterns originally laid out in his Sketch of 1842 and Essay of 1844, particularly with respect to the position of the “difficulties” discussion in chapters 6 through 9.
Darwin was quite self-effacing in his introduction, explaining to his readers that the book would necessarily seem imperfect, and that he could not “give references and authorities for my several statements; and I must trust to the reader reposing some confidence in my accuracy.” This was a remarkable assertion for someone who was proposing to lay out a revolutionary idea that he could not definitively prove. In the same vein, in his first chapter, on “Variation in Nature,” he announced that there would be no space to give “a long catalogue of dry facts.”
Missing from the whole book is any introductory section laying out in detail the problem to be addressed. Darwin seems to have assumed that his readers would be perfectly familiar with the notion of transmutation of species, “the mystery of mysteries, as it has been called by one of our greatest philosophers” (referring to John Herschel). On his third page he cited Robert Chambers’s Vestiges of the Natural History of Creation (1844), which suggests that this assumption was the case. But Darwin did not use his vast knowledge of plant and animal diversity and distribution to establish initially, through examples, that transmutation was a phenomenon seeking an explanation. Instead, he steadily built up an “argument” almost, as it were, from the other end, carrying the reader along until even the most recalcitrant must (he hoped) be persuaded.