On Natural Selection

Charles Darwin's The Origin of Species presents his argument for evolution through natural selection, the process by which favorable variations in organisms are preserved and injurious ones rejected. Darwin builds his case methodically, moving from the competitive pressures all organisms face to the mechanism by which those pressures drive species to change over time.

Darwin opens with the concept he calls the "Struggle for Existence," a term he uses broadly to encompass not only direct competition between organisms for food and territory but also dependence on environmental conditions and success in leaving offspring. He observes that while this struggle is easy to acknowledge in the abstract, failing to keep it constantly in mind leads to misunderstanding patterns of distribution, rarity, abundance, and extinction. The struggle is often indirect and metaphorical: Two dogs may fight over scarce food, but a plant at the edge of a desert "struggles" against drought, and mistletoe depends on birds for seed dispersal and on host trees for sustenance.

The foundation of this struggle, Darwin argues, is geometrical increase, the tendency of all organisms to reproduce at rates far exceeding what their environments can sustain. He invokes the doctrine of Thomas Malthus, an economist who argued that human populations grow faster than food supplies, and applies it across the animal and vegetable kingdoms. The Swedish botanist Carl Linnaeus showed that a plant producing only two seeds annually would yield 1 million descendants in 20 years, and Darwin estimates that even the elephant, the slowest breeder among known animals, would produce 15 million descendants from a single pair in five centuries. He cites empirical evidence as well, including the rapid spread of European plants across the plains of South America and the unchecked multiplication of cattle and horses running wild in South America and Australia, attributing these population explosions not to heightened fertility but to reduced destruction.

Darwin then examines the checks that hold populations in balance. From his own observations, he reports that of 357 seedlings he marked on a cleared plot, 295 were destroyed mainly by slugs and insects. Food supply sets an upper limit on population, he argues, but predation often determines actual numbers more decisively. Climate also plays a role: The winter of 1854–55 destroyed four-fifths of the birds on his own grounds. Yet climate acts largely by favoring competing species rather than by killing organisms outright.

To illustrate the complexity of ecological relationships, Darwin traces chains of dependence linking seemingly unrelated organisms. Planting Scotch fir on a barren heath in Staffordshire, combined with fencing to exclude cattle, transformed the vegetation and introduced 12 new plant species and 6 insectivorous bird species. In Paraguay, a fly that lays eggs in the navels of newborn cattle and horses prevents those animals from running wild, while insectivorous birds check the flies and hawks regulate the birds, so that altering any single link produces cascading effects. In another chain, cats reduce mice, which destroy bumblebee nests; bumblebees pollinate red clover and heartsease, a type of wild pansy. The presence of cats in a district could therefore determine the frequency of certain flowers. Darwin emphasizes that competition is most severe between closely related organisms: When several wheat varieties are sown together, the best-suited supplant the others within a few years.

From this foundation, Darwin introduces natural selection, defining it as the preservation of favorable variations and the rejection of injurious ones. The principle follows logically from three premises: Organisms vary, more individuals are born than can survive, and any individual with even a slight advantage will have the best chance of surviving and reproducing. He contrasts natural selection with artificial selection, the deliberate breeding by humans for desired traits. Humans can act only on external, visible characters and select for their own purposes, whereas nature acts on every internal organ and constitutional difference, selects solely for the organism's benefit, and accumulates changes over incomparably longer time spans.

Darwin demonstrates that natural selection can act on seemingly trivial traits. Leaf-eating insects are green; bark-feeders are mottled grey; grouse match their habitats. These tints protect organisms from predators guided by eyesight. He also explains sexual selection as a related but distinct mechanism: Males compete not for survival but for access to females, through combat (as with stags and stag beetles) or through display (as with birds of paradise performing before watching females, who choose the most attractive partner).

To make his theory concrete, Darwin offers imaginary illustrations. If deer increased in a region, the swiftest wolves would survive, and over time a faster wolf variety would emerge. In a more complex example, he traces how plants excreting nectar attract insects that inadvertently carry pollen between flowers. Natural selection favors flowers with larger nectaries and better-positioned reproductive organs, while insects develop longer feeding tubes to access nectar more efficiently. Flower and insect gradually become adapted to each other through the continued preservation of mutually favorable variations.

Darwin discusses circumstances favorable to natural selection, including large populations, inheritable variability, intercrossing, geographic isolation, and large continental areas. He introduces "living fossils," organisms like the platypus and lungfish that have survived by inhabiting confined, less competitive environments.

Connecting natural selection to extinction, Darwin argues that as better-adapted forms increase, less-favored forms inevitably become rare and disappear. He introduces the principle of Divergence of Character: The most diversified descendants of any species can occupy the widest range of ecological niches, so natural selection tends to push varieties further apart. Using a detailed branching diagram, he illustrates how a single ancestral species can, over thousands of generations, give rise to multiple new species and higher groupings, while intermediate forms go extinct. He compares the relationships among all living and extinct species to a great tree, whose green and budding twigs represent existing species and whose dead branches represent extinct lineages.

Darwin then addresses difficulties with his theory. He explains the absence of transitional forms by arguing that natural selection itself causes the extinction of intermediates and that the fossil record is extremely imperfect, preserving organisms only under rare geological conditions. He addresses skepticism about highly specialized organs by presenting graduated series of living forms: Among squirrels, a continuum exists from species with slightly flattened tails to flying squirrels with full flank-membranes, suggesting how gliding animals could have evolved into bats. He confronts the example of the eye, freely confessing that its formation through natural selection seems at first utterly impossible to believe, but argues that graduated eye structures exist among arthropods such as insects and crustaceans, from a simple optic nerve coated with pigment to complex compound eyes. He emphasizes that organs originally built for one purpose can be converted to another, citing the swim bladder of fish, originally used for flotation, which in some lineages became a lung.

In his conclusion, Darwin identifies the chief obstacle to accepting his theory as the human difficulty in grasping the cumulative effects of slight variations over immense time. He extends his argument to propose that all members of the same biological class share a common ancestor, that all animals may descend from at most four or five progenitors, and that all life may trace back to a single primordial form. He forecasts that acceptance of his theory will transform natural history: Taxonomy will become genealogy, and terms like "affinity" and "rudimentary organs" will acquire literal meaning. He closes by contemplating an entangled bank of diverse, interdependent organisms and declaring that all these forms have been produced by laws acting around us, including Growth, Reproduction, Inheritance, Variability, a high Ratio of Increase leading to a Struggle for Life, and Natural Selection entailing Divergence of Character and Extinction, so that "from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved" (117).