How to Read On the Origin of Species: Themes, Structure and Questions to Ask

What is On the Origin of Species, and why is it still read?

Charles Darwin (1809–1882) was an English naturalist who sailed around the world on HMS Beagle from 1831 to 1836, collecting specimens and studying geology, fossils and island species. Over the next two decades he developed his theory privately, until in 1858 Alfred Russel Wallace sent him an essay outlining a similar idea. Papers by both men were presented to the Linnean Society of London that year, and Darwin hurried out what he called an abstract of a larger planned work. John Murray published On the Origin of Species in London in November 1859.

It is read today for two reasons. It is the founding text of evolutionary biology, and its central claims, that living things share common ancestors and that natural selection shapes them, are the organizing ideas of modern biology. It is also a model of scientific argument written for a general reader: no equations, a single diagram, a mass of concrete examples from pigeons to honeybee cells to island species, and an author who states the strongest objections to his theory and answers them one by one.

How On the Origin of Species is structured

The first edition has an Introduction and fourteen chapters. Darwin does not label parts, but the chapters fall into four movements: build the mechanism, face the difficulties, show what the theory explains, then sum up. The sixth edition (1872) inserts a new seventh chapter, "Miscellaneous Objections to the Theory of Natural Selection," which brings the total to fifteen and moves every later chapter up one number (Instinct becomes chapter 8, the conclusion chapter 15). The chapter numbers in this guide follow the first edition, so check them against your copy when following a syllabus.

Main themes in On the Origin of Species

Five ideas run through the book. For each, know the chapter where Darwin develops it and one example he uses.

Key ideas to define, and what Darwin did not know

Be able to define each of these in a sentence, with the chapter it comes from. Keep one gap in mind as you go: Darwin had no workable theory of heredity. Gregor Mendel's work on inheritance was published in the 1860s but went largely unnoticed until 1900, so Darwin argues for selection without knowing how traits pass between generations.

• Artificial selection: breeders choosing which plants or animals reproduce, the model for the whole argument.
• Struggle for existence: the competition for survival and reproduction that follows when more are born than can live.
• Natural selection: the preservation of favorable variations and the rejection of harmful ones in that struggle.
• Sexual selection: competition for mates, which can favor traits such as display or weapons.
• Divergence of character: descendants spreading into different ways of life, which reduces competition and splits one species into several.
• Organs of extreme perfection: complex structures such as the eye, which Darwin explains through a graded series of simpler eyes found in living animals.
• Homology and rudimentary organs: shared structural plans and reduced, useless parts, read as traces of common ancestry.

How to read On the Origin of Species: a six-step plan

Read it the way you would read any scientific argument: find the claim, then weigh the evidence and the objections. Our guide to reading a research paper properly uses the same habits, and the methods in how to remember what you read help with a book this full of examples.

1. 1Read the Introduction and the final chapter first. The recapitulation gives you the whole argument in Darwin's words, so you know where each chapter is heading.
2. 2Read chapters 1–4 closely. Copy the branching diagram by hand and explain in your own words what the branches, gaps and endpoints represent.
3. 3Skim chapter 5, listing the questions about heredity Darwin cannot answer.
4. 4For chapters 6–9, write each difficulty as a one-sentence objection and Darwin's reply as a one-sentence answer, then judge the reply.
5. 5For chapters 10–13, list each kind of evidence with one example Darwin gives, such as island species resembling those of the nearest mainland.
6. 6Reread the final chapter and write a one-page summary of the long argument in your own words.

Questions to ask about On the Origin of Species

Use these to check your understanding or to start an essay; our guide to writing a critical analysis shows how to turn one into an argument.

• Why does Darwin begin with domestic pigeons rather than wild animals?
• How strong is the analogy between artificial and natural selection, and where does it break down?
• What does Darwin mean by struggle, and why does he insist the word is used broadly?
• What does the branching diagram claim about extinction?
• Which difficulty in chapter 6 does Darwin answer most convincingly, and which least?
• How does he account for complex instincts such as the honeybee's cell-building?
• Why does the incompleteness of the fossil record matter so much to his case?
• What do island species show that a theory of separately created species struggles to explain?
• How does the missing theory of heredity weaken the argument, and how does Darwin work around it?
• Why does the book say so little about humans?
• Does the closing image of the entangled bank answer the charge that natural selection makes nature cruel?

Which edition of On the Origin of Species to read

Darwin revised the book through six editions between 1859 and 1872, and the changes are substantial. Later editions add a historical sketch of earlier thinkers on species, adopt Herbert Spencer's phrase "survival of the fittest" (from the fifth edition), add a chapter of replies to critics and shorten the title to The Origin of Species. The word "evolution" itself appears only in the sixth.

For a first reading, the first edition is a good choice: it is shorter and shows the argument as it first appeared. Harvard University Press publishes a facsimile of it with an introduction by Ernst Mayr. The sixth edition shows Darwin's final responses and is the text of many inexpensive reprints, so check the front matter to see which you have.