III.

Theories of Natural Selection

When Darwin returned to England in 1836, he was a mature scientist. His letters and packages of specimens sent to Sedgwick, Henslow, and others during his voyage had established his reputation at home. He immediately threw himself into the work of preparing his share of an extensive report of the scientific discoveries made during the Beagle voyage, and editing his own travel diary for publication. Darwin’s Journal of Researches (1839) achieved popular as well as scientific acclaim, and it was followed in 1844 and 1846 by further volumes on volcanic islands and on the geology of South America.

None of this published work by Darwin challenged the assumption that biological species are immutable. However, in July 1837 Darwin opened a private notebook entitled “Transmutation of Species”, in which he recorded observations and speculations bearing on the question (which he subsequently called “that mystery of mysteries”). His thinking on how organisms evolve was brought into sharp focus in September 1838, when he read An Essay on the Principle of Population (originally published in 1798) by the British economist Thomas Robert Malthus. Malthus observed that all biological species, including human beings, possess a far greater reproductive capacity than can actually be realized. For human beings, there was always a potential disparity between the means of subsistence and the number of mouths to feed. Human population growth was thus limited by dire checks, such as famine, disease, and war.

Darwin immediately saw the relevance of Malthus’s work for his own thinking: if all the offspring of a plant or animal cannot survive to reproductive maturity, there must be biological reasons why those that survive, do so. This constant press, which he called natural selection, was the motor of biological change over time. In 1838, 1842, and 1844 he produced increasingly elaborate private versions of his evolutionary theory; the latter is virtually a précis of the famous book that he eventually published in 1859.

In the meantime, Darwin had married, in 1839, his first cousin, Emma Wedgwood, and they soon afterwards left London for a small estate, Down House, in Downe, Kent. His father had left him independently wealthy. At Down, he and his wife had ten children, three of whom died in infancy. By then, too, Darwin was beginning to suffer from an illness that was to plague him intermittently for the rest of his life. It produced shaking, nausea, dry retching, and great prostration. It left him, by his own testimony, unable to work for days and weeks on end, although his output of scientific books and correspondence continued to be prodigious. The source of his illness will probably never be completely unravelled, although it possibly had a large psychosomatic component. It also relieved him of many social, professional, and domestic obligations, thus enabling him to concentrate on what mattered most to him: his work.

In 1856, after eight years of sustained work on fossil and living barnacles (published in two large volumes), Darwin at last began work on a volume that he intended to call Natural Selection. It was interrupted in 1858, when Alfred Russel Wallace, a young naturalist then working in Malaysia, sent Darwin his own brief sketch of evolution through natural selection. Lyell, who had been privy to Darwin’s own evolutionary thinking, arranged for a joint presentation of Wallace’s sketch and a brief essay by Darwin at a meeting of the Linnean Society in London. Darwin was then stimulated to write On the Origin of Species by Means of Natural Selection. It sold out on the first day of publication in November 1859 and remains one of the greatest scientific treatises ever written. It went through five further editions in Darwin’s lifetime.

Darwin’s was by no means the first treatise to argue for the change of biological species over time. His grandfather, Erasmus Darwin, had developed his own evolutionary ideas in a series of medical writings and poems. The French naturalist Jean-Baptiste Lamarck, in his Philosophie Zoologique (1809), expounded a comprehensive evolutionary synthesis, based on the commonly held notion that characteristics acquired in an organism’s lifetime could be passed on to the offspring. He famously argued that the giraffe’s long neck was the result of generations of stretching to reach leaves higher in the trees. This form of inheritance was described as “Lamarckian”. In 1844 the Scottish publisher Robert Chambers anonymously published his own evolutionary synthesis, entitled Vestiges of the Natural History of Creation. Darwin knew all these works, and in later editions of The Origin of Species provided a historical introduction. Their influence on him was general rather than particular, however, as revealed by the differences in his theory.

Darwin’s theory of evolution by natural selection is essentially that, because of the population pressure as described by Malthus, the young born to any species compete intensely for survival. Those young that survive to reproduce tend to embody favourable natural variations (however slight the advantage may be)—the process of natural selection—and these variations are passed on by heredity. Darwin recognized that his understanding of the mechanisms of heredity was limited, but he insisted that as long as inherited variation does occur, his theory would work. Therefore, some members of each generation will be able to adapt themselves to changing environmental conditions (changes in food supply, predators, or climate, for example), and this gradual and continuous process of adaptation is the source of the evolution of species. Within Darwin’s vast conceptual scheme, extinct and present-day species of plants and animals were represented as a kind of “tree of life”, in which closely related modern organisms are descended from common ancestors. Moreover, he provided additional support for the older concept that the Earth itself is not static but evolving.

In a deliberate attempt to make his ideas more acceptable, Darwin did not discuss human evolution in The Origin of Species, confining himself to a single sentence: “Light will be thrown on the origin of man and his history”. Nevertheless, his private notebooks make it clear that he recognized from the beginning that human beings were also part of the evolutionary process. He elaborated his views on human evolution in two later works (see below), but the popular idea that he argued that human beings are descended from apes is false: within his scheme, human beings and other primates, such as modern apes and monkeys, are all descended from common, more primitive ancestors.