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Just a Theory? How Darwin Discovered Evolution (Part 3)

Part 1 of this article explored Charles Darwin’s early life and education, the voyage of HMS Beagle, and evolutionary thinking before Darwin. Part 2 covered various lines of evidence that shaped his thinking on evolution and his initial formulation of the theory of natural selection.

On the coast of Chile in 1835, Darwin had collected a tiny and very unusual species of barnacle. It was a parasitic barnacle that made its way in the world by boring inside a conch shell and setting up home there. Darwin anticipated that he would write some short, scientific papers describing this new species. He wrote to a friend: “I am going to begin some papers on the lower marine animals, which will last me some months, perhaps a year, & then I shall begin looking over my ten-year-long accumulation of notes on species.”1

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But he soon ran into a problem: In order to properly classify his one little barnacle species, he had to know something about the taxonomy of barnacles in general. And in 1846, the field of barnacle taxonomy (despite its obvious, inherent sexiness as an area of research) was in a state of complete chaos.

Barnacles were originally thought to be a kind of mollusk, like clams or mussels. But it was discovered in 1830 that they are actually crustaceans, built on the same general body plan as crabs and shrimp. Zoologists had a pressing need for an up-to-date, systematic study of these misunderstood creatures.

Barnacles, Barnacles, Barnacles

Darwin began collecting other barnacle species. He wrote to colleagues requesting specimens, and he began the painstaking, delicate work of dissecting them and comparing their anatomy. Very quickly, the project got completely out of hand. Word got out on the barnacle-enthusiast grapevine that Darwin was on the job, and next thing he knew he was up to his eyeballs in barnacles; people from all over the world were sending him specimens. Collecting fossil barnacles as well as living species, he began grouping whole families, orders, and genera of barnacle.

Instead of a few short papers describing his one little friend from Chile, Darwin ended up undertaking the Herculean task of organizing and classifying the entire subclass Cirripedia (the scientific term for barnacles).

By 1852, his project of “some months” had been dragging on for six years. He wrote to his cousin saying: “I hate a barnacle as no man ever did before, not even a sailor in a slow moving ship.”2

By 1853, he had been dissecting barnacles for seven years. At this time, four of his children were actually younger than seven years old, which means that all they ever knew him to do in his work life was sit hunched over his microscope studying barnacles. As one biography noted, they must have concluded that this is what all fathers do, because one of them asked about a neighbor’s father: “Where does he do his barnacles?”3

By 1854, the project was completed at last. After eight years of intense labor, he was finally through with barnacles for good. The result was four technical monographs on cirripedes, which were published to great acclaim.

Darwin estimated that in the eight years he spent on the barnacle work, he lost about two years worth of working time to illness. He suffered from a mysterious health condition, which slowed his work considerably. Symptoms first appeared in the years just following his return from the voyage on the Beagle and continued intermittently until his death in 1882. He suffered from frequent stomach cramps and vomiting spells, he had headaches and all sorts of skin disorders, and for long periods of time he was able to work for only a couple of hours each day. The exact nature of Darwin’s illness has been the subject of much scholarly debate, but it appears to have eluded scholars and remains somewhat of a mystery.

Even without the health delays, however, the barnacle project took far longer than Darwin had intended. He could be the patron saint of anyone who has begun a project only to see it expand out of control and become a monumental task taking way longer than anticipated. But, in the end, the barnacle work proved to be extremely useful to him.

For one thing, it was his public debut as a biologist, as opposed to a geologist (recall that prior to this work, his primary focus was on his geological discoveries from the Beagle voyage). The barnacle work expanded his expertise into the field of zoology, and the experience of working out in exhaustive detail the classification of an entire suborder of species gave him the knowledge and confidence to speak authoritatively on species.

More importantly, the barnacles provided crucial evidence for his theory of evolution. Barnacles exhibit a large degree of natural variation, a key element of Darwin’s theory. Natural selection can operate only if there are natural variations among individuals — natural variations that make a difference for survival and reproduction.

By systematically studying all of the barnacle species and comparing their bodily structures, he learned a great deal about the extent of variation in nature. As he said in a letter to a friend: “I have been struck . . . with the variability of every part in some slight degree of every species: when the same organ is rigorously compared in many individuals I always find some slight variability.”4

Another piece of supporting evidence came from a set of surprising discoveries he made about barnacle reproduction. Barnacles are usually hermaphrodites (possessing both male and female reproductive organs). But in the course of his study, Darwin found a number of curious exceptions to this rule.

A number of barnacle species turned out to have two separate sexes, but the adult males were tiny, rudimentary organisms that live as parasites within the shell of the female. In some cases, Darwin found that each female had a collection of, as he put it, “little husbands”5 that consisted of “mere bags of spermatozoa”6 without even a mouth or stomach.

Other species Darwin found were even more curious. They were hermaphrodites, but also possessed a collection of tiny, parasitic males. The male organs of each hermaphrodite individual, though functional, were smaller than usual and were supplemented by the extra, “complemental” males.

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What this proved is that, within a species, in addition to the fact that sexual identity can be mixed, the degrees of maleness and femaleness can vary. It is not true that a species is either fully hermaphroditic or composed of fully separate male and female sexes. As Darwin had long suspected, an initially hermaphroditic species can evolve by a continuous series of slight modifications into a species with separate sexes.

Indeed, in the end Darwin actually found a whole series of barnacle species displaying a graduated range of varying degrees of sexual differentiation. Darwin’s notebook musings on the origin of male nipples had been validated in nature beyond his wildest imaginings. In great excitement, he wrote about all this to a friend: “I never shd. have made this out, had not my species theory convinced me that an hermaphrodite species must pass into a bisexual species by insensibly small stages; & here we have it, for the male organs in the hermaphrodite are beginning to fail, & independent males already formed.”7

The Origin of The Origin

Having completed his barnacle work at long last, Darwin emerged from the project with strong new evidence for his theory of species, and he was able finally — eighteen years after his return from the Beagle voyage — to turn his attention to evolution full-time.

Although he had been collecting evidence and reflecting on the arguments for and against his theory for years, he still had myriad details to consider and unresolved questions to explore.

For example, he had gathered large amounts of evidence about the distribution of plants and animals around the world, such as the facts concerning the Galapagos species; but in order to explain that distribution, he had to be able to explain how organisms could get from one place to another. How, for instance, could mainland plant species get to distant islands?

It occurred to him that seeds might simply float across the ocean. But when he asked his botanist friends about this, he was told that seeds are killed by salt water. On the other hand, it turned out that nobody he talked to had ever put them in salt water to see if that was true! So Darwin decided to try it. He launched into a series of experiments, floating seeds of all types in salt water. He kept them there for weeks at a time and then checked whether they would germinate.

To his surprise and delight, he found that the assumption that they would quickly die was simply false. He was able to grow plants from seeds that had been immersed in brine for twenty-eight days, and a colleague was able to do so after an even longer immersion. Using a nautical atlas, he calculated from the speed of various currents how far a seed could travel in the time available and found that it could go a distance of up to nine hundred miles, certainly far enough to account for the data he had collected. In addition to floating seeds, he tested a number of other methods of transport, including by birds. To get a sense of the lengths he went to in these inquiries, consider the following quotation from The Origin of Species, where he describes some of this work:

I forced many kinds of seeds into the stomachs of dead fish, and then gave their bodies to fishing-eagles, storks and pelicans; these birds after an interval of many hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds retained their power of germination.8

Other lines of evidence also required further development. Consider Darwin’s argument from domestic breeding. To explore the similarities between natural selection and man’s “artificial selection,” he decided to study a particular domestic species in depth: domestic pigeons.

Recognizing the degree to which his immersion in barnacles enabled him to acquire expertise at taxonomical classification, he threw himself into the strange subculture of the fancy pigeon breeder to explore the power of domestic selection. He built an aviary at his house and began to acquire exotic breeds — at one point keeping almost ninety birds. He read treatises on pigeon breeding, hunting down references going back to ancient Rome and Egypt. He had correspondents send him pigeon skins from India and Persia. He joined pigeon clubs and talked pigeon lore with an assortment of pigeon fanciers.

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This effort produced strong evidence in support of his theory. In the few centuries that people had been keeping pigeons, they had produced a variety of breeds that were so different that zoologists finding them in the wild would have classified them as different species — and some as even belonging to different genera. Yet, a number of lines of evidence established that they were all descended from a single pigeon species. This showed that, starting with small, naturally occurring variations among individuals in a single species, the careful selection of breeding pairs could magnify those variations into substantial differences.

If man could do this over a mere few hundred years, could there be any doubt of the power of natural selection to bring about, over the immense time scale of geological history, the diversity of forms that exist in nature? Whales and horses differ so greatly that it seems impossible that they could be descendants of a common ancestor. But if man, who can select breeding pairs based only on visually observable differences, can produce a wide variety of pigeon breeds in the blink of a geological eye, then surely over tens of millions of years natural selection, which acts on all characteristics — seen and unseen — that have bearing on survival and reproduction, could transform an ancient mammal species into both whale and horse.

Contemplating the “thousand intermediate forms”9 that must have existed between such creatures as the otter and its land ancestor, Darwin wrote: “Opponents will say, show me them. I will answer yes, if you show me every step between bull Dog & Greyhound.”10

The image of Darwin as an experimental scientist is as atypical as the image of Darwin as the Beagle’s resident Indiana Jones. Yet he conducted any experiment he thought was necessary and relevant to his theory. At one point, he even tickled aphids to see if he could get them to excrete nectar for him as they do for ants. There is no room in science (or in any area of thought, for that matter) for unfounded assertions. If something needs to be put to the experimental test, then a true scientist will engage accordingly, even if it means tickling insects or collecting fishy pelican poop.

Darwin spent a number of years conducting these sorts of experiments and filling in details on his theory. At long last, he was ready to pull together all the data and threads of evidence he had accumulated, and to arrange and organize his vast collection of facts.

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He began writing a book, to be called Natural Selection, that would present the full, scientific case for his theory. He planned it to be a massive, multi-volume tome, like Lyell’s Principles of Geology — systematic and thorough, and replete with the full apparatus of scholarly discourse, including voluminous notes and references.

But almost two years into the writing, with ten chapters finished, Darwin’s plans were completely overturned. On June 18, 1858, he received a letter from a young naturalist named Alfred Russel Wallace. Wallace had discovered independently the essential elements of the theory of evolution by natural selection, and he wrote to see what Darwin thought of it!

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Darwin was shocked and distraught. Wallace had asked him to circulate the letter to other interested scientists, if he thought it worthy. Being intimately aware of the worthiness of the theory, Darwin felt obliged to send Wallace’s letter to a scientific journal for immediate publication. And being the gentleman that he was, Darwin would not dream of doing otherwise, even though it would mean that Wallace would be forever credited with the major scientific discovery that had comprised Darwin’s life’s work.

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Fortunately, the crisis was resolved amicably and to the satisfaction of all parties (a rare occasion in the history of scientific priority disputes). Friends arranged for the publication of Wallace’s letter alongside excerpts from Darwin’s unpublished writings, including his 1844 sketch. These were read before a meeting of the Linnaean Society on July 1, 1858, and published in its journal.

The incident, however, was a wake-up call to Darwin, who realized that he had better put out a book quickly presenting the details of the theory, rather than continue to labor on his large, technical treatise. In less than a year, he dashed off one of the immortal works in the history of science: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.


It is important to keep in mind that evolution by natural selection is a theory that pertains to all life on earth. To prove a fundamental truth concerning all life on earth requires a range of facts and evidence drawn from every part of the organic world. The scope of the evidence must be commensurate with the scope of the principle it supports — and evolution is one of the fundamental integrating principles of all biology.

To prove his theory, Darwin assembled and organized “long catalogues of facts.”11 He gathered evidence from all corners of the plant and animal kingdoms, and from all corners of the globe. He had correspondents all over the world who sent him specimens and answered questions on local flora and fauna. He drew on the most obvious and the most obscure facts, spanning all branches of life’s family tree — from the smallest, microscopic organisms to the tallest trees and the largest whales, and from as far back in time as the fossil record went in his day.

The Origin of Species offers an impressive survey of the enormous scope and depth of knowledge that Darwin brought to bear on his theory. To get a sense of that scope and depth, consider the following brief laundry list of some of the kinds of facts he refers to in the book. At various points he bases arguments on, or draws conclusions from, facts concerning: the relative weights of the wing and leg bones in domestic versus wild ducks;12 the fertility via self-fertilization of numerous species of hermaphrodite flower;13 the differences in the method that different species of honeybee use in constructing their hives;14 the sexual relations of snails and of earthworms;15 the number of feathers in the tails of various breeds of domesticated pigeons, followed immediately in the same paragraph by observations on the root stems of different varieties of turnip and rutabaga;16 the slave-making instincts of different species of ant17; the differences in the marine faunas of the eastern and western shores of South and Central America, which have “hardly a fish, shell, or crab in common”;18 the hooks on the ends of the branches of a species of bamboo in the Malay Archipelago;19 the similarities in the embryos of a whole range of vertebrate animals, including birds, reptiles, and mammals;20 the absence on remote oceanic islands of all mammals except bats, which can fly;21 the rules governing the intercrossing of different varieties of cabbage, radish, and onion22; the reappearance of ancestral stripes on various breeds of horse, ass, and zebra.23

One could keep going with this list — and going and going. But the point should be clear. By the time Darwin published the Origin, he had assembled, organized, reflected on, and digested a vast body of factual evidence, and he had integrated that body of evidence into a fundamental principle of nature.

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Darwin published The Origin of Species in 1859 and spent the rest of his life continuing to explore the foundations and the implications of his theory. The list of books that he wrote after the Origin reveals that although he wrote on a broad range of biological topics, all of his works have some bearing on evolution. For example, his 1862 book on orchids (On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects) grew directly out of his attempt to understand complex, interrelated adaptations such as the shape of the flower and the behavior of the insects that fertilize it — and why these would have arisen together by natural selection. His 1868 book on domesticated plants and animals (The Variation of Animals and Plants under Domestication) was the fruit of his labors in the pigeon clubs and barnyards. His 1871 book, The Descent of Man, and selection in Relation to Sex, was his application of his theory to human evolution. And so on.

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Even his last book in 1881, which explains the work of earthworms in reconstituting the soil, is relevant to evolution insofar as it addresses the power of small effects, accumulated over time, to bring about important changes.

Having explored Darwin’s life and work, and the stages by which he discovered and painstakingly validated the theory of evolution, one can readily see how unjust is the accusation that Darwin was an armchair theorist indulging in baseless speculation.

It is notable that Darwin himself anticipated that accusation and answered it in his introduction to The Origin of Species. With his characteristic understatement and modesty, Darwin described his years of painstaking work developing his theory and then expressed the hope that his readers would not think he was too quick to accept his conclusions.

When on board H.M.S. “Beagle” as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species — that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years’ work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I have given them to show that I have not been hasty in coming to a decision.24

Indeed, he was not hasty.

And as careful as he was about assessing the status of his work, when he finally became convinced that his theory was true he was confident in expressing that conviction. By the time Darwin published his theory he was certain of its validity; he was, as he put it, “thoroughly convinced” by the vast body of evidence he had accumulated “that species have changed, and are still slowly changing by the preservation and accumulation of successive slight favorable variations.”25

Today, 160 years after the publication of Origin of Species, the truth of Darwin’s conclusions is rationally indisputable. Darwin’s discoveries, as well as discoveries that were unimaginable in Darwin’s day (such as those concerning the genetic mechanism of heredity and the structure and properties of DNA), have all been integrated into evolutionary theory and further attest to its validity.

Despite the fervent wishes of creationists, the facts leave no room for believing that evolution is “just a theory” or that Darwin was “just a theorist.” On the contrary, the evidence demonstrates that Darwin was the quintessential hands-on scientist — and that his theory is true.26 

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A version of this article was previously published in The Objective Standard (Spring 2008).

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  1. Charles Darwin to J. D. Hooker, 2 October 1846, The correspondence of Charles Darwin vol. 3: 1844–1846.
  2. White and Gribbin, Darwin, 178.
  3. Desmond and Moore, Darwin, 407.
  4. Browne, Charles Darwin: Voyaging, 514.
  5. Desmond and Moore, Darwin, 355.
  6. Desmond and Moore, Darwin, 357.
  7. Desmond and Moore, Darwin, 356.
  8. Darwin, The Origin of Species, 289.
  9. Desmond and Moore, Darwin, 248.
  10. Desmond and Moore, Darwin, 248.
  11. Darwin, The Origin of Species, 14.
  12. Darwin, The Origin of Species, 20.
  13. Darwin, The Origin of Species, 86–90.
  14. Darwin, The Origin of Species, 186–94.
  15. Darwin, The Origin of Species, 89.
  16. Darwin, The Origin of Species, 136.
  17. Darwin, The Origin of Species, 182–86.
  18. Darwin, The Origin of Species, 278.
  19. Darwin, The Origin of Species, 164–65.
  20. Darwin, The Origin of Species, 347–55.
  21. Darwin, The Origin of Species, 313–14.
  22. Darwin, The Origin of Species, 88–89.
  23. Darwin, The Origin of Species, 139–42.
  24. Darwin, The Origin of Species, 11.
  25. Darwin, The Origin of Species, 378.
  26. Readers interested in more rigorous and scholarly accounts of Darwin’s life and work—especially in discussions of Darwin’s inductive methodology—would benefit from the following works by James Lennox, emeritus professor of history and philosophy of science at the University of Pittsburgh:

    James Lennox, ‘Darwin’s Methodological Evolution’, Journal of the History of Biology, 38/1 (March 2005), 85-99.

    James Lennox, ‘From Darwin to Neo-Darwinism’, chapter 5, in Sahotra Sarkar and Anya Plutynksi, eds. A Companion to Philosophy of Biology, Malden, MA: Blackwell Publishing Ltd., 2008.  

    James Lennox, ‘The Evolution of Darwinian Thought Experiments’ and ‘Thought Experiments in Evolutionary Biology Today’, chapters 2 and 4 in W. J. Gonzalez (ed.), Evolutionism: Present Approaches, La Coruña: Netbiblio, 2009, 63-76, 109-20.

    James Lennox, ‘Darwinian Thought Experiments: A Function for Just So Stories’, in Tamara Horowitz, Gerald Massey, eds., Thought Experiments in Science and Philosophy, Savage, MD: Rowman and Littlefield, 1991, 173-96.

    James Lennox, “Darwinism”, The Stanford Encyclopedia of Philosophy (Fall 2019 Edition), Edward N. Zalta (ed.),

    A 2013 talk by Professor Lennox offers a fascinating overview of his work on these topics:

    “The Inductive Basis of Darwin’s Origin,” Oslo Objectivist Conference, 2013.

    I’m grateful to Professor Lennox for his assistance in selecting these references.

    Readers interested in the relationship between evolutionary biology and the phenomenon of teleology (or goal-directedness) in nature may consult the following works:

    Harry Binswanger, The Biological Basis of Teleological Concepts, Los Angeles: ARI Press, 1990.

    James Lennox, ‘Darwin and Teleology’, Chapter 17 in Michael Ruse (ed.) The Cambridge Encyclopedia of Darwin and Evolutionary Thought. Cambridge: Cambridge University Press, 152-57.

Keith Lockitch

Keith Lockitch, Ph.D. in physics, is a senior fellow and vice president of content at the Ayn Rand Institute. He focuses primarily on the intersection of science with current events and policy issues. He is a senior editor of New Ideal.

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