Darwin's Origin of Species

The fantail has thirty or forty tail feathers where an ordinary pigeon has twelve or fourteen. The pouter inflates an enormous crop, a pouch in its throat. The short-faced tumbler has a tiny beak. Darwin argues that these strikingly different breeds descend from the rock-pigeon, Columba livia, rather than from separate wild species. He points to fertile crosses between breeds, the reappearance of ancestral markings, and the absence of the supposed wild ancestors.

Breeders choose which birds reproduce, accumulating features that were initially slight. Some work toward a specified result; others keep the birds they prefer without planning the breed’s eventual form.

The breeds show how inherited variation can support substantial change. They remain domestic varieties. Darwin still has to explain why some variants leave more descendants outside the pigeon loft. Chapter I, pp. 20-36.

A plant may release a thousand seeds, yet only a few establish themselves. Even the slow-breeding elephant’s descendants would multiply enormously if all survived and reproduced. Darwin uses these examples to show potential increase, not to establish a death rate for every species.

Darwin applies Malthus’s argument about population growth and limited resources to plants and animals. Population numbers cannot keep increasing unchecked. Drought or disease can check them, as can competition for food.

His mistletoe example makes the broad meaning of struggle clear. It needs a host tree and birds to carry its seeds. Success depends partly on attracting a bird. Living a long time without offspring is not enough. Chapter III, pp. 62-65.

Suppose individuals with a particular inherited difference leave more offspring, on average, than others in the same population. Their descendants can make up a growing share of later generations. Further variation permits further change. Selection works on existing differences; need alone cannot supply a useful feature.

Darwin sometimes writes as though natural selection examines and chooses. He is personifying a process with no intention or knowledge of a future advantage. A coat that helps in one climate can be a burden in another.

Immediately after the definition, Darwin leaves neutral variations outside selection. In chapter VI he denies that selection necessarily produces absolute perfection. The advantage is relative to other available forms, under existing conditions. Chapter IV, pp. 81-84; chapter VI, p. 206.

Darwin calls the larger process descent with modification. Descendants differ from their ancestors; lineages can diverge and others disappear. His diagram explores how diverging descendants could occupy different places in the same environment. Isolation can preserve differences; interbreeding can work against them.

Naturalists disagree over borderline cases between species and varieties, and reproductive sterility is not an infallible test. Darwin argues that their differences need not belong to wholly separate kinds of change. Chapter II, pp. 48-52; chapter VIII, pp. 245-248.

Common descent explains why species fall into groups within groups. A human hand, a bat’s wing, and a porpoise’s paddle use corresponding bones for different tasks. Darwin reads their shared arrangement as an inheritance from a common ancestor. Classification can therefore record relationships. Chapter XIII, pp. 420-421 and 434-435.

He later extends the possibility to all life, calling a single primordial ancestor an inference from analogy and warning that analogy can mislead. The diagram represents species within one genus. Chapter XIV, pp. 483-484.

The second sentence sets conditions: intermediate structures must work and vary heritably, and a change must provide an advantage. Selection cannot preserve an otherwise useless intermediate because of a future advantage.

Darwin compares simpler light-sensitive structures with more elaborate eyes in living animals, particularly crustaceans. These suggest that useful gradations are possible. The actual history would lie in an animal’s ancestors, which are usually unavailable. A living species with a simpler eye is not thereby an ancestor.

A modern optical model by Dan-E. Nilsson and Susanne Pelger (1994, pp. 53-58) makes a narrower test. Starting with a light-sensitive patch, it constructs small changes toward a lens eye, with each step improving spatial resolution. The model demonstrates a possible route for the optics under its assumptions. It does not reconstruct a particular lineage or explain the initial origin of light-sensitive cells.

Darwin estimates how long erosion took to remove rock from the Weald in southern England: 306,662,400 years, rounded to three hundred million. The apparent precision exceeds what his assumptions can support. He offers lower alternatives in 1860 and removes the calculation in 1861. The changes at p. 287; third-edition corrections.

Where are all the intermediate forms? Darwin calls the lack of a finely graduated fossil sequence a grave difficulty. Sediment accumulates intermittently, much is lost, and little has been examined. His explanation for an incomplete record still leaves many transitions unobserved. Chapter IX, pp. 280 and 287-302.

Later, William Thomson, known as Lord Kelvin, challenged the available time using Earth’s cooling. His model omitted heat transport by convection in the interior. The familiar story that radioactive heat alone overturned it is too simple; England, Molnar, and Richter explain the distinction (2007, pp. 4-9).

Modern radiometric dating puts Earth’s age at about 4.54 billion years. Darwin could not measure that timescale. It does not validate his estimate for the Weald.

Darwin observes inheritance but lacks a satisfactory mechanism for it. A difficulty follows if heredity always blends parental differences into an average: repeated crosses can dilute a rare useful difference. But his observations also include traits that reappear after generations, and he does not consistently treat inheritance as simple blending.

In 1868 he proposes pangenesis: body parts shed tiny hereditary units, called gemmules, that gather in the reproductive organs and can remain dormant. He presents this as provisional. This proposed mechanism has not survived. Variation of Animals and Plants under Domestication, volume II, chapter XXVII, pp. 357 and 374-375.

Gregor Mendel’s pea experiments, published in 1866, showed patterns of inheritance in which a character can disappear from view and reappear in later offspring. There is no evidence Darwin knew his work. Mendel’s results did not immediately supply an explanation of every continuously varying trait. Fairbanks’s research on their relationship and the NHGRI account of Mendel’s findings explain the limits.

Modern genetics distinguishes a trait from the inherited DNA variants, called alleles, that help produce it. The variants can persist when parents’ contributions combine. Many traits depend on several genes and the environment. Evolution also includes genetic drift, changes in the frequency of variants through chance. Those findings belong to later science. Darwin also accepts inherited effects of use and disuse, a further difference from the modern account. Chapter V, pp. 134-139.

The Origin leaves its human implications largely undeveloped. Darwin addresses them directly in The Descent of Man (1871). His later social views complicate any defense of Darwin against political uses of his name. In chapter V, pp. 168-169, he fears harmful hereditary consequences from preserving weak people, while insisting that deliberately neglecting the helpless would cause a certain and great present evil. He also hopes that those he calls weak will refrain from marrying. Both views are his.

Turning reproductive success into a moral entitlement requires an additional argument. Natural selection supplies no instruction to abandon the poor or sick.

The bank brings together two parts of the argument: organisms have histories, and their ways of living depend on one another. Darwin proposes that ordinary processes, extended across generations, can account for their adaptations and relationships.

Useful structure can develop without an intended end. Descent and selection, however, begin with reproducing organisms. This book leaves the first origin of life unexplained.