On the Origin of Species

Contents

1. Introduction 13 min 10

   Darwin explains how the facts of distribution and fossils he saw on the Beagle led him to the question of the origin of species in 1837, and why he is publishing an “abstract” now: Wallace has reached the same conclusions. A theory of descent is not enough unless it explains adaptation; he will start from variation under domestication, apply the doctrine of Malthus to all living things, and argue that species are not immutable and that natural selection has been “the main but not exclusive means of modification”.

2. I Variation Under Domestication 1 h 19 min 10

   Domestic plants and animals vary far more than wild ones, probably because changed conditions act on the reproductive system; variation follows complex and unknown laws of correlation and inheritance. Taking the domestic pigeon as his special study, Darwin argues that all its strikingly different breeds descend from the rock-pigeon. The key to domestic breeds is man’s power of accumulative selection, methodical or unconscious: nature supplies slight variations and the breeder adds them up over many generations.

3. II Variation Under Nature 33 min 9

   Wild species also vary, and individual differences supply the material for selection. Naturalists cannot agree where varieties end and species begin, which is what we should expect if a well-marked variety is an “incipient species”; Darwin treats “species” as a term of convenience. His tables of floras show that common, widespread species and the species of large genera produce the most varieties: where species have been manufactured, the manufactory is still at work.

4. III Struggle for Existence 40 min 11

   Because every species produces far more offspring than can survive, there is a struggle for existence, which Darwin uses in a broad, metaphorical sense that includes dependence and success in leaving offspring: the doctrine of Malthus applied to the whole of nature. Numbers are kept down by food, enemies, climate and disease, through webs of relations so complex (cats, mice, bumblebees and clover) that we can rarely predict them. The struggle is most severe between individuals of the same species and between closely allied species.

5. IV Natural Selection 1 h 47 min 17

   Natural selection preserves favourable variations and destroys injurious ones, acting on every organ and at every stage of life; sexual selection adds the struggle between males for mates. Darwin illustrates it with imaginary cases (wolves, flowers and bees), discusses intercrossing, isolation and the size of areas, and shows how selection causes extinction. His principle of divergence of character, set out with the book’s one diagram, explains how varieties grow into species and genera and why all living things fall into groups within groups, like the branches of a great tree of life.

6. V Laws of Variation 1 h 21 min 12

   Darwin admits that the causes of variation are unknown, and reviews what can be said about them: the direct effect of climate and food, which he thinks small; the inherited effects of use and disuse, as in wingless island beetles and blind cave animals; acclimatisation; and “correlation of growth”, by which a change in one part brings changes in others. Parts that are repeated, rudimentary or unusually developed are especially variable, and specific characters vary more than generic ones, which he explains by how recently they arose. He closes with stripes on horses, asses and their hybrids, which he reads as reversion to a striped common ancestor.

7. VI Difficulties on Theory 1 h 14 min 13

   Darwin sets out four objections to his theory: the absence of transitional forms, organs of extreme perfection, instinct and hybrid sterility. He answers the first two here. Intermediate varieties are rare because they live in smaller numbers and are supplanted by the forms they connect. Organs as complex as the eye can be built up by graded steps, each useful, often through a change of function. His theory would break down if any organ could not have been formed gradually, or if any structure existed purely for the good of another species. Natural selection makes organisms only as perfect as their competitors, and it explains both unity of type and the conditions of existence.

8. VII Instinct 1 h 17 min 13

   Instincts, Darwin argues, vary and are inherited like bodily structures, and natural selection can accumulate slight variations of them; domestic breeds such as pointers and tumbler-pigeons show instincts shaped by selection. He traces how three remarkable instincts could have arisen gradually: the cuckoo's laying in other birds' nests, slave-making in ants, and the hive-bee's comb, which he derives from bees digging equal spheres and building flat walls where they meet. The sterile castes of ants, which he calls the gravest special difficulty for his theory, are explained by selection acting on the family, and they disprove the idea that instincts are inherited habits.

9. VIII Hybridism 1 h 8 min 13

   The sterility of species when crossed was taken as proof that species were specially created. Darwin argues that it is not a special endowment but a by-product of other differences, chiefly in the reproductive system. Fertility between species comes in every degree, varies between individuals, differs in reciprocal crosses, and follows the same erratic rules as the capacity of trees to be grafted. Varieties are usually fertile together, but not always, and breeders select only visible traits; apart from fertility, hybrids and mongrels follow the same laws of inheritance, so there is no fundamental distinction between species and varieties.

10. IX On the Imperfection of the Geological Record 1 h 5 min 14

    The missing transitional forms are, Darwin concedes, the gravest objection to his theory, and he answers that the geological record is extremely imperfect. He shows the vastness of geological time from rates of erosion (including his estimate, later withdrawn, of 300 million years for the denudation of the Weald), the poverty of fossil collections, and the long gaps between formations, which are laid down mainly while the sea floor subsides. The sudden appearance of whole groups and the absence of fossils below the Silurian remain unexplained; he likens the record to a history of which only scattered lines survive.

11. X On the Geological Succession of Organic Beings 1 h 10 min 14

    Darwin argues that the main facts of the fossil record fit descent with modification better than separate creations. New species appear slowly and one after another; different groups change at different rates; a species or group once extinct never reappears; and extinction is the gradual, almost inevitable result of competition from improved forms, not of catastrophes. Using the diagram from chapter IV, he explains why the oldest fossils differ most from living forms yet often fall between living groups, and why the extinct and living animals of each continent, such as the marsupials of Australia and the sloths and armadillos of South America, are related (the “law of the succession of types”).

12. XI Geographical Distribution 1 h 16 min 12

    Geographical distribution cannot be explained by climate: regions with similar conditions have unrelated inhabitants, barriers mark the boundaries between faunas, and the species of each continent are related to one another. Darwin argues that each species arose in one place and spread from there, and he examines how. He rejects Forbes’s land bridges between continents, tests by experiment how long seeds survive in salt water, in birds’ crops and in mud, and shows how the Glacial period drove arctic and temperate species south and then left them stranded on distant mountain-tops, even across the equator.

13. XII Geographical Distribution—continued 57 min 11

    Freshwater plants and animals range widely because birds carry their seeds and eggs from pond to pond, as Darwin’s experiments with a duck’s feet and with pond mud suggest. Oceanic islands have few species but many found nowhere else; they lack frogs and land mammals, which cannot cross the sea, but have their own bats. Above all, island species resemble those of the nearest mainland: the Galápagos fauna is American and that of the Cape Verde Islands African, though their conditions are alike. Colonisation from the nearest source followed by modification explains this; independent creation does not.

14. XIII Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs 1 h 38 min 15

    The grouping of all organisms into groups within groups is explained by branching descent with extinction, and the “natural system” naturalists were seeking turns out to be genealogy. Descent explains why classifiers value trivial and rudimentary characters, and characters of the embryo, above adaptive resemblances such as that of a whale to a fish. Homology, the same pattern of bones in a man’s hand, a bat’s wing and a porpoise’s paddle, shows modification of a common ancestor. Embryos of related animals resemble each other because variations usually appear, and are inherited, late in life. Rudimentary organs, like silent letters in a word, are relics of ancestors.

15. XIV Recapitulation and Conclusion 1 h 4 min 19

    Darwin recapitulates “one long argument”: the difficulties of the theory (gradation of organs, hybrid sterility, distribution and the imperfect fossil record), then the case for it, from variation under domestication and the struggle for existence to the facts of succession, distribution, classification, homology, embryology and rudimentary organs. He explains why naturalists have resisted it, predicts a revolution in natural history in which classifications become genealogies, and suggests that all life descends from a few forms or one. The book ends with the tangled bank and the “grandeur in this view of life”.