Similarities, Ancestors and Relatives: How Evolutionary Thought Took Shape
It is not a recent idea that living beings resemble one another. Not all do so in the same way or for the same reasons, but there is a certain family resemblance that had already attracted attention in antiquity. To formulate the foundations of so much similarity with any precision required not one but many ideas about the dynamics of the transformation of living beings to come together.
The ideas inherited from classical antiquity concerning biological kinship should be left aside as evolutionary interpretations, since they had little continuity with modern thought. A more decisive moment came in eighteenth-century France, where a remarkable mixture of natural history, philosophy, geology, and speculation prepared the ground for nineteenth-century evolutionary theories.
German Naturphilosophie also contributed indirectly. Its advocates often searched for an underlying plan behind the enormous variety of living forms. Although they did not necessarily accept evolution, their insistence on structural similarities among organisms helped encourage the idea that related forms might somehow be connected through change.
The Ideas That Prepared the Way for Evolution
Several intellectual developments helped make evolutionary explanations increasingly conceivable.
Deism became influential among Enlightenment thinkers, reducing reliance on miracles and encouraging the idea of a natural world governed by its own laws. The traditional concept of the Great Chain of Being—a continuous hierarchy extending through different forms of existence—also blurred strict boundaries between species and even between living and nonliving matter.
Fossils acquired a new meaning as they came to be interpreted as the remains of organisms that had actually lived in the past. The succession of fossil plants and animals suggested a history of the Earth very different from the present one and increasingly challenged simple interpretations of a single biblical flood.
Naturalists also began questioning the immutability of species. French investigators such as Maupertuis, Adanson, and Duchesne observed variations in plants, while Georges-Louis Leclerc, Comte de Buffon, entertained the possibility that species might descend from others through a process he described in terms of “degeneration.”
At the same time, the new systems of classification—above all that of Carl Linnaeus—revealed hierarchies and morphological affinities among organisms. Although these classifications were not originally evolutionary, they made relationships among living things increasingly visible and placed human beings within nature rather than entirely outside it.
These eighteenth-century ideas were important, but they remained fragmented and speculative. They lacked a general mechanism capable of explaining biological transformation. Erasmus Darwin, grandfather of Charles Darwin, even proposed evolutionary ideas in works such as Zoonomia, but without developing the kind of explanatory system that would emerge in the following century.
Lamarck and the First Great Transformist Theory
At the beginning of the nineteenth century, evolutionary thinking crystallized in the transformist theory of the French naturalist Jean-Baptiste Lamarck.
In Philosophie zoologique, published in 1809, Lamarck attempted to integrate several earlier ideas into a broad theory of biological change. He accepted the spontaneous and continuous generation of simple living beings and imagined life progressing along a hierarchy of increasing complexity.
Environmental circumstances, however, could divert organisms from this regular progression. Lamarck argued that changes in conditions produced changes in habits and behavior, which in turn altered the use of particular organs.
His famous principle of use and disuse proposed that frequent and sustained use strengthened an organ, while continued disuse weakened it and could eventually cause it to disappear. A second principle held that modifications acquired in this manner could be transmitted to offspring.
Lamarck therefore offered something earlier evolutionary speculation had generally lacked: a mechanism connecting the environment, the organism, and heredity.
His theory was never universally accepted, even in France. Georges Cuvier, the dominant figure in French comparative anatomy, strongly opposed transformism. Yet interest in the origin and diversification of life survived Lamarck’s death.
In 1844, Vestiges of the Natural History of Creation, published anonymously and later revealed to have been written by Robert Chambers, presented a vision in which species gradually developed from one another according to natural laws. The book became enormously popular in Britain, even though scientists criticized many of its arguments.
Darwin, the Beagle, and a Changing Earth
Charles Darwin was born in 1809, the same year Lamarck published Philosophie zoologique.
Darwin initially studied medicine at Edinburgh, where he encountered the naturalist Robert Grant, an admirer of Lamarck’s ideas. After moving to Cambridge, his friendship with the botanist John Stevens Henslow helped secure him a place aboard HMS Beagle as a naturalist.
Between 1831 and 1836, the voyage took Darwin around much of the world. His observations of the geographical distribution of organisms, the relationship between fossils and living species, and the similarities and differences among island faunas became fundamental to his later thinking.
Darwin also read Charles Lyell’s Principles of Geology. Lyell argued that geological features could be explained by processes operating gradually over immense periods of time. Darwin applied a similar perspective to living organisms: small changes, accumulated over enormous spans of time, might produce major biological transformations.
Malthus and Natural Selection
Another decisive influence came in 1838, when Darwin reread Thomas Robert Malthus’s An Essay on the Principle of Population.
Malthus had argued that populations possess the capacity to grow faster than the food supply required to sustain them. Population growth is therefore continually checked by scarcity, disease, competition, and other pressures.
Darwin recognized the biological implications. Organisms generally produce more offspring than can survive. Yet individuals of the same species are not identical: populations contain small variations.
If some of those variations provide an advantage in a particular environment, their possessors will tend to survive and reproduce more successfully. If the advantageous characteristics are heritable, they will become more common among descendants. Over long periods, natural selection can gradually reshape populations and contribute to the formation of new species.
Darwin worked on these ideas privately for about two decades. Then, in 1858, he received an essay from the naturalist Alfred Russel Wallace, who had independently reached a strikingly similar explanation of evolutionary change while working in the Malay Archipelago.
Charles Lyell and Joseph Hooker arranged for writings by Darwin and Wallace to be presented together before the Linnean Society of London on July 1, 1858.
The following year, Darwin published On the Origin of Species.
What Darwin’s Theory Actually Proposed
Darwin’s work contained several interconnected ideas: species change through time; new species arise from preexisting ones; evolutionary change is generally gradual; living organisms share common ancestry; and natural selection provides a major mechanism for adaptation.
The last of these was the conceptual key.
Species have the potential to increase rapidly, yet their populations remain limited because resources are finite. Individuals differ from one another, and some differences affect survival and reproduction. Heritable advantageous variations therefore become more common over generations.
The result is not an organism consciously adapting because it “needs” to change. Rather, the environment continually favors some inherited variations over others.
Darwin’s theory nevertheless had serious gaps. He did not know the origin of biological variation, nor did nineteenth-century biology possess an adequate theory of heredity.
The experiments of Gregor Mendel, published in 1866, provided fundamental principles of inheritance, but their significance was largely overlooked until around 1900.
August Weismann also helped undermine the traditional inheritance of acquired characteristics by distinguishing reproductive cells from the cells making up the rest of the body.
From Darwinism to the Modern Evolutionary Theory
Darwinism quickly acquired ideological meanings extending far beyond biology.
Herbert Spencer promoted what came to be called social Darwinism, applying ideas of competition and “survival of the fittest” to human societies, economies, and nations. Such interpretations often confused biological natural selection with political or moral prescriptions.
Even more consequential was the rise of eugenics, associated initially with Darwin’s cousin Francis Galton. Eugenic movements claimed that human populations could be “improved” by controlling reproduction, and these ideas became entangled with racism, compulsory sterilization, and theories of supposed hereditary superiority.
In the United States, more than 30 states eventually enacted sterilization laws, and more than 60,000 people were forcibly sterilized under such programs. Nazi Germany later carried racial eugenics to vastly more radical and murderous extremes.
By the end of the nineteenth century, evolution itself had become widely accepted among naturalists, but natural selection temporarily lost ground to competing explanations.
Only during the modern evolutionary synthesis of the 1930s and 1940s, when Darwinian natural selection was united with Mendelian genetics and population biology, did natural selection regain its central position.
Darwin’s great achievement was therefore not simply the claim that species change. Variations of that idea had existed long before him. The decisive breakthrough was providing a natural mechanism capable of explaining how inherited variation, competition, reproduction, and immense stretches of time could together produce the diversity of life.