On the Origin of Species by Means of Natural Selection

Citation

Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. John Murray. Free official edition: https://www.gutenberg.org/ebooks/1228

Chapter Summary

Context and Argument

Darwin calls the book an "abstract". He says he published it because his health was poor and because Alfred Russel Wallace had reached almost exactly the same conclusions; Wallace's memoir was published together with brief extracts from Darwin's manuscripts. The Introduction tells how, as naturalist on the Beagle, he was struck by the distribution of South American species and by the relation of living to extinct forms; how he began collecting facts in 1837 and wrote a sketch in 1844. It also warns that the book gives few references and that for almost every point facts could be cited that appear to point the other way.

The argument is that species are not independently created and unchangeable: those of the same genus are descendants of earlier, usually extinct, species, as varieties are descendants of their species. Natural selection is "the main but not exclusive means of modification". The Introduction lays out the order of the chapters, which moves from domestic variation and natural variation to the struggle for existence and selection, then to difficulties of the theory, then to geology, geographical distribution and classification.

Chapter I: Variation under Domestication

Darwin begins with domestic plants and animals, where variation and inheritance can be watched. He discusses causes of variability, correlation of growth and the many breeds of domestic pigeon, which he argues descend from the rock-pigeon. The key idea is selection by people, methodical and unconscious, which he calls by far the predominant power in producing domestic varieties.

Chapter II: Variation under Nature

Individual differences and doubtful species show that the line between varieties and species is arbitrary. Widely ranging, common species of large genera vary most, and species of large genera resemble varieties in being closely, but unequally, related and in having restricted ranges. Darwin treats this as evidence that species begin as varieties.

Chapter III: Struggle for Existence

Following Malthus, Darwin applies the geometrical increase of populations to the whole living world. The "struggle for existence" is used in a wide sense; checks include climate, competition and the complex relations among organisms. The struggle is most severe between closely related forms, and the relation of organism to organism is the most important of all.

Chapter IV: Natural Selection

The central chapter. Darwin argues that selection, powerful in the hands of people, can act in nature on slight, useful variations and on all ages and both sexes. He illustrates with an imagined wolf preying on deer, discusses sexual selection, intercrossing and isolation, extinction, and "divergence of character", and explains with a diagram how descendants of one parent diversify. The chapter closes with the image of the Tree of Life.

Chapter V: Laws of Variation

Darwin admits that "chance" only expresses ignorance of the cause of each variation. He surveys the effects of external conditions, use and disuse, correlation and compensation of growth, the variability of rudimentary and unusually developed parts, analogous variation and reversion, and concludes that natural selection accumulates the beneficial differences.

Chapter VI: Difficulties on Theory

Darwin sets out the strongest objections: why transitional forms are not seen everywhere, how an organ of extreme perfection such as the eye could form, and how animals could change their habits of life. He replies with graded series in related species, and develops the canon "Natura non facit saltum" and the claim that unity of type and the conditions of existence are both embraced by the theory.

Chapter VII: Instinct

Instincts, he argues, vary and form graded series like bodily structure. He treats the cuckoo, the ostrich, slave-making ants, the hive-bee's cell-making and neuter insects, and shows how graded behaviour in related bees and ants could lead to the complex forms.

Chapter VIII: Hybridism

Darwin separates the sterility of first crosses from that of hybrids and argues that sterility is not a special endowment for keeping species distinct but incidental to other differences. The fertility of varieties when crossed is not universal, and he concludes that there is no fundamental distinction between species and varieties.

Chapter IX: On the Imperfection of the Geological Record

He meets the objection from missing intermediates by arguing that the record is very incomplete: intermittent formations, poor collections, and the vast lapse of time. He compares the record to a history of which only the last volume, with a few lines of a few pages, survives, and he faces the sudden appearance of groups.

Chapter X: On the Geological Succession of Organic Beings

The chapter argues that new species appeared slowly and successively, that extinct forms are linked to living ones by affinity, and that the same types succeed each other in the same areas. These patterns are explained as inheritance with modification.

Chapters XI–XII: Geographical Distribution

The first chapter argues that distribution is not explained by physical conditions alone and discusses barriers, "centres of creation", means of dispersal and the Glacial period. The second treats fresh-water productions and oceanic islands, including the absence of batrachians and terrestrial mammals and the relation of island inhabitants to the nearest mainland.

Chapter XIII: Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs

Classification into groups under groups reflects descent. Darwin takes morphology, embryology and rudimentary organs as evidence for the same view.

Chapter XIV: Recapitulation and Conclusion

A summary of the difficulties and the case in favour, with a prediction that psychology will be rebuilt on gradation and light thrown on the origin of man. The closing paragraph of the first edition, on the "entangled bank", ends on the "endless forms most beautiful".

Key Concepts

Introduction and Chapters I–V

• Selection under domestication: methodical and unconscious selection by breeders, offered as the model for natural selection.
• Struggle for existence: competition that follows from the geometrical increase of populations.
• Natural selection: the preservation of slight, useful variations, with divergence of character and extinction.
• Variation: individual differences whose cause Darwin admits is largely unknown.

Chapters VI–X

• Transitional forms: the intermediate gradations that the theory requires and that Chapters VI and IX discuss.
• Natura non facit saltum: nature does not make jumps; modification is gradual.
• Imperfection of the record: why fossils are not expected to show all stages.

Chapters XI–XIV

• Descent with modification: the view that classification, embryology and rudimentary organs express common descent.
• Dispersal and barriers: the means by which species spread or are isolated.
• Tree of Life: the image that connects extinct and living species in branching groups.

Critical Analysis

Strengths

• A cumulative argument. Darwin moves from domestic breeds to nature, from variation to selection, and then tests the idea against geology, geography and classification. The evidence is gathered across fields rather than resting on one case.
• Frank treatment of objections. Four chapters (VI–IX) are devoted to difficulties, and the Introduction concedes that facts could be cited against nearly every point. Darwin states, for example, that supposing the eye formed by natural selection seems "absurd in the highest possible degree" before explaining why the difficulty is not real.
• Concrete cases. The pigeons, the hive-bee's cell-making, the wolf and deer illustration and the diagram give readers something specific to follow.

Limitations

• Mechanism of heredity unknown. Darwin writes that variation is governed by unknown laws and that "chance" only expresses ignorance. Readers will see where later genetics supplies what he lacked.
• An abstract without references. Darwin himself says the book gives no authorities and promises a fuller work. Some sections are dense with natural-history detail, especially Chapters VIII, XI and XII.
• Period language. The text uses terms such as "savages", and "higher" and "lower" forms, and treats some inherited effects of use and disuse as real; these need reading in historical context.

Where It Fits in the Course

In the evolution course, the Origin is the primary source. Zimmer's The Tangled Bank and Charlesworth and Charlesworth's Evolution: A Very Short Introduction are modern introductions that make good companions, the first richly illustrated and the second a concise account by two population geneticists. Bergstrom and Dugatkin's Evolution and Futuyma and Kirkpatrick's Evolution are full textbooks; the latter assumes prior biology and genetics. Charlesworth and Charlesworth's Elements of Evolutionary Genetics is a graduate text on the genetic processes that Darwin could not yet explain.

A sensible order is a modern introduction first, then Darwin, then a textbook, so Darwin's reasoning is read as an argument and not as a source of current facts.

How to Use This Book

The book is accessible to a motivated undergraduate, but the sentences are long and the natural-history examples are many. Read the Introduction and Chapters I–IV closely, since they contain the theory. Then read the opening and summary of Chapters V–X, taking the objections in Chapter VI and the geological argument in Chapter IX in full. Chapters XI–XIV reward reading together: distribution and classification are the evidence for descent. Finish with the last paragraphs of Chapter XIV.

Keep a list of Darwin's objections and his replies, and compare each with what a modern textbook says. Note where he is cautious and where he is bold. The Project Gutenberg plain-text edition does not reproduce the Chapter IV diagram, so consult a facsimile for that figure.

Edition and Sources

This guide is based on the first edition (1859) in Project Gutenberg eBook #1228 (https://www.gutenberg.org/ebooks/1228), the text of the John Murray first edition. Its contents, the Introduction, the Chapter IV passages on selection and the diagram, Chapter VI on the eye, Chapter VII on the hive-bee and the closing paragraphs were read, along with the opening, summaries and conclusions of every chapter in the second-edition text (eBook #22764). In the first edition, the closing sentence says life was "breathed into a few forms or into one"; the later edition adds "by the Creator".