Origins of Darwin's Evolution

Solving the Species Puzzle Through Time and Place

J. David Archibald

• Publisher: Columbia University Press
• Year: 2017
• ISBN:
  9780231545297
• Topic: History
• Subtopic: Science History
• Index: History

Historical biogeography—the study of the history of species through both time and place—first convinced Charles Darwin of evolution. This field was so important to Darwin's initial theories and line of thinking that he said as much in the very first paragraph of On the Origin of Species (1859) and later in his autobiography. His methods included collecting mammalian fossils in South America clearly related to living forms, tracing the geographical distributions of living species across South America, and sampling peculiar fauna of the geologically young Galápagos Archipelago that showed evident affinities to South American forms. Over the years, Darwin collected other evidence in support of evolution, but his historical biogeographical arguments remained paramount, so much so that he devotes three full chapters to this topic in On the Origin of Species.

Discussions of Darwin's landmark book too often give scant attention to this wealth of evidence, and we still do not fully appreciate its significance in Darwin's thinking. In Origins of Darwin's Evolution, J. David Archibald explores this lapse, showing how Darwin first came to the conclusion that, instead of various centers of creation, species had evolved in different regions throughout the world. He also shows that Darwin's other early passion—geology—proved a more elusive corroboration of evolution. On the Origin of Species has only one chapter dedicated to the rock and fossil record, as it then appeared too incomplete for Darwin's evidentiary standards. Carefully retracing Darwin's gathering of evidence and the evolution of his thinking, Origins of Darwin's Evolution achieves a new understanding of how Darwin crafted his transformative theory.

Chapter One

ESTABLISHING THE FACT OF EVOLUTION

During the voyage of the Beagle I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely allied animals replace one another in proceeding southwards over the Continent; and thirdly, by the South American character of most of the productions of the Galapagos archipelago, and more especially by the manner in which they differ slightly on each island of the group; none of these islands appearing to be very ancient in a geological sense.

It was evident that such facts as these, as well as many others, could be explained on the supposition that species gradually become modified; and the subject haunted me.

—CHARLES ROBERT DARWIN, 1887

Without a doubt, many shards of evidence came together to form Charles Darwin’s theory of descent with modification by means of natural selection, or what we simply call evolution. Explanations for when and how he developed his theory fill many of pages of fact and conjecture. Some of the most telling evidence comes from Darwin himself; the above epigraph originates from a private account Darwin penned for his family in 1876, later published by his son Francis in 1887 five years after Charles’s death. Importantly, nothing of the mechanism—natural selection—appears in this passage. Here he merely acknowledges his acceptance of the fact of gradual modification of species, that is, evolution. He writes of the facts and observations that brought him to his conclusion: fossils in South America linking specifically to living forms on that continent, which in turn dispersed to and radiated in the Galápagos Islands, as proof that evolution occurred. He realized that the concept of “special creation” could not account for this pattern.

Darwin mentions fossils in the above epigraph, but the reference to Pampean fossils with armor similar to that of modern armadillos does not represent an argument for an ancestor–descendant relationship, but rather proof of the overall continuity of ancient with modern relatives in South America, on a much grander scale. Although citing a specific example, his intent was to emphasize the importance of faunal succession in the context of biogeographical history in demonstrating the change of species over time. The quite incomplete rock and fossil record alone could not provide the sort of evidence he sought. Darwin’s attempt to muster more direct evidence from fossils and rocks, as we will see in later chapters, led to a rather futile attempt in his aptly titled Chapter IX, “On the Imperfection of the Geological Record.” As indicated by this title and the fact that he penned the chapter only months before the publication of On the Origin of Species in 1859, Darwin already placed his confidence in another form of evidence for evolutionary history: faunal succession within the framework of biogeographical history. This becomes even clearer in the three chapters following that on the imperfection of the geological record. He titles Chapter X “On the Geological Succession of the Geological Record” and the next two Chapters XI and XII “Geographical Distribution.” Darwin regarded geographical succession and geographical distribution as the greatest evidence of species change through time. Thus, Darwin attempted to use these four chapters to document that evolution had occurred, whereas his other ten chapters deal with aspects of living creatures and his other proposed mechanism—natural selection.

INTEGRATING THE NEW SCIENCES OF GEOLOGY, PALEONTOLOGY, AND BIOGEOGRAPHY

We read about new fossils bridging a gap within or between groups in scientific journals and newspapers almost daily. Molecular results are playing an increasingly important role in unraveling complex relationships, a methodology to which Darwin and his contemporaries had no recourse. During Darwin’s life, the science of geology gradually matured as field studies tested theories of Earth history. The sciences of paleontology and biogeography, although newer, also developed because of field work and global exploration. Although the idea of species evolution began bubbling under the scientific surface by the early nineteenth century, work on the fossil record remained firmly entrenched in the special creation paradigm.

Any number of stratigraphic columns detailing fossil ranges appeared in print, tracking the turnover of whole faunas. Almost without exception, none of these studies implicated evolution as the cause for the observed changes in fossils over geological time. No wonder then that Darwin could not use such records to support his evolutionary ideas, when their authors did not describe or place them in such an evolutionary context and seldom related them to living species. The irony of this is that the paradigm of a creationist analysis of the rock and fossil record shifted within a few years of Darwin’s publication of On the Origin of Species in 1859. Darwin’s findings established a new evolutionary paradigm in which to study Earth history.

Darwin’s catalytic effect proved most keen among scientists under the age of thirty and those involved in work on recent as well as fossil species. In papers in 1865 and 1867, the English biologist St. George Jackson Mivart (1827–1900) outlined evolutionary trees dealing with relationships of primates. The French paleontologist Jean Albert Gaudry (1827–1908), in his 1866 monograph on Greek fossil mammals, created a diagram showing the evolutionary relationships of mammals from around the world. In 1867 German paleontologist Franz Hilgendorf (1839–1904) produced a detailed evolutionary history of fossil snails from one small region of Germany. In 1873 Russian paleontologist Vladimir Onufrievich Kovalevskii (1842–1883) began presenting his analysis of the evolutionary relationships of ungulates (or hoofed mammals) and their relatives. Finally, German zoologist Ernst Haeckel (1834–1919), perhaps the most prolific and influential of all these young scientists, began in 1866 what became a lifetime’s pursuit of documenting the evolutionary history of no less than the entire history of life on this planet. The majority of these breakthroughs appeared fewer than ten years after the 1859 publication of On the Origin of Species, without a doubt testifying to Darwin’s far-reaching effect (see Archibald 2014 for references for the authors mentioned here). The work of Darwin and the scientists who followed him provided a reason why fossils aligned as they do throughout stratigraphic sections, leading eventually to our modern biota. Darwin’s influence in this realm cannot be overstated.

No such evolutionary framework allowed Darwin to pursue his own studies into Earth history. He confronted an almost exclusively creationist-tinged interpretation of the fossil record. Nevertheless, Darwin relied heavily on fossils as starting points in his goal to determine that biogeography explained why different parts of the world experienced quite different paleontological histories. In the mid-nineteenth century, biogeographical history could still embrace a creationist framework as the foundation for special and separate creations, but when Darwin demonstrated that close oceanic islands (such as the Galápagos) shared their biota with nearby continents (like South America), special creation could no longer stand. This insight proved the major importance of the Galápagos themselves for Darwin’s theorizing, rather than any specific discussion on finches, mockingbirds, tortoises, or iguanas.

Darwin wrote of the species he encountered on the Galápagos in what we commonly call the Voyage of the Beagle, first published in 1839. Thorough species descriptions and illustrations are provided in Zoology of the Beagle, which was published from 1838 to 1843 and edited by Darwin. Somewhat surprisingly, however, the only Galápagos species Darwin mentions in On the Origin of Species are his beloved mocking-thrushes (mockingbirds); he notes the occurrence of three species. The four or perhaps five Galápagos species recognized today reside only on these islands, a fact made clear to Darwin after his voyage.

The importance of the Galápagos in his biogeographical stratagem is clear from the fact that of the seventeen times he mentions the islands in his 1859 text, all but two occur in the second of his two chapters on biogeographical distribution. Ancestral species arrived from South America and speciated on the various islands. Thus the Galápagos served as the linchpin for his theory: evolution, not multiple centers of special creation, explained the origin of differing biotas found worldwide. And in this argument, the grand view of the Galápagos, not the specific species themselves, were what truly mattered to Darwin. In this way, Darwin found a means to show a profound biological history of Earth based on evolution by using faunal succession in concert with biogeography, as opposed to relying on poorly understood and poor-quality geological and fossil records, thus solving the species puzzle through time and place.

THE FACT OF EVOLUTION BEFORE A MECHANISM

The first part of Darwin’s phrase “descent with modification through natural selection” states the fact of evolution, whereas the second part refers to a mechanism—Darwin’s own contribution. All too often Darwin is believed by the public to be the discoverer of evolution, which is an incorrect perception. Nevertheless, Darwin’s lasting and still relevant contribution remains the mechanism: natural selection. Biologists and historians of science know well that the fortunes of this mechanism waxed and waned through the end of the nineteenth century, becoming broadly accepted only in the first third of the twentieth century. Not so with the acceptance of evolution, which Darwin firmly planted within both the scientific and public realms. Thus, although he did not discover evolution, he nonetheless firmly established it as scientific fact. Whereas acceptance of natural selection experienced rough going early on, acceptance of evolution never again wavered within the scientific community.

All evidence points to Darwin developing his ideas of natural selection as the mechanism of evolution relatively soon after returning from his voyage in 1836. He began his first notebook dealing with evolution in July 1837. As noted in his 1887 autobiography, soon after his return Darwin began asking breeders and gardeners about their methods of selecting traits they desired to preserve. Not much more than a year later, in October 1838, his reading of An Essay on the Principle of Population (1798) by Thomas Malthus (1766–1834), a treatise that theorized on the unsustainability of unchecked human population growth, led to the following conclusion:

[B]eing well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here then I had at last got a theory by which to work…. (Darwin 1887, 83)

Determining when Darwin first came to the realization that evolution occurs, however, remains a more contentious topic. Nowhere does Darwin outright indicate that he began developing his ideas on evolution while on the HMS Beagle, but some authors believe that a number of his statements suggest that he tested evolution while on the voyage, whereas others find these same statements more equivocal.

Perhaps the most cited and contested proof of his acceptance of evolution while still on the Beagle first came to public attention in a 1935 letter to Nature magazine from Darwin’s granddaughter Nora Barlow (1885–1989). She also included the letter in the more comprehensive Darwin’s Ornithological Notes (1963). This collection contains transcribed and edited versions of the notes Darwin made concerning bird observations and collections he made while on board the Beagle. (The original notes can be viewed online at the Cambridge University Library through the Darwin Manuscripts Project.) Darwin’s entry reads:

When I recollect the fact, that from the form of the body, shape of scales and general size, the Spaniards can at once pronounce from which Isd. any tortoise may have been brought:- when I see these Islands in sight of each other and possessed of but a scanty stock of animals, tenanted by these birds but slightly differing in structure and filling the same place in Nature, I must suspect they are only varieties. The only fact of a similar kind of which I am aware is the constant asserted difference between the wolf-like Fox of East and West Falkland Isds. If there is the slightest foundation for these remarks, the Zoology of Archipelagoes will be well worth examining; for such facts would undermine the stability of species. (Barlow 1935, 391)

Darwin inserted the word “would” in the last sentence probably very soon after writing it.

When exactly Darwin penned the sentence remains uncertain. He clearly wrote this while still on board the Beagle, sometime after leaving the Galápagos on October 20, 1835, and before reaching England on October 2, 1836. Without offering specifics, Chancellor and van Wyhe (2009) suggest that Darwin likely penned this about the time he visited the South Atlantic island of St. Helena in July 1836. If St. Helena sounds familiar, it entered historical lore as the last place of exile for Napoleon Bonaparte. Much has been made of this quote. In her letter, Barlow points out how this note seems to indicate the earliest upheaval in his thinking on evolutionary lines. Along the same lines, Briggs (2009) suggests it identifies the first indication of Darwin pondering the problem of the modification of species. Eldredge (2015, 141) presents a more emphatic, but by no means unique, argument that this quote shows Darwin had “all but formally declared adoption of transmutation.” On the other end of the spectrum, Sulloway (1982b) finds the meaning of the passage obscure, particularly on whether Darwin endorsed or rejected evolution.

Focus tends to rest on whether or not this passage reveals Darwin clearly voicing an acceptance of evolution in his reference to “undermining the stability of species.” However, the cause for this undermining of species stability becomes all but lost in most discussions of the above quotation. The cause, as Darwin plainly writes, is island biogeography—that is what is responsible for the differences of the tortoises and birds in general between islands of the Galápagos, as well as between the now extinct Falkland Island foxes (actually not true foxes) and other canids on the South American mainland. Separate from whether this represents a complete change of view by Darwin regarding the origin of species—and I have my doubts—it does emphasize the importance of the role that biogeographical history later plays in the formulation of Darwin’s evolutionary ideas.

Various authors also cite another passage from an 1835 essay Darwin wrote pertaining to South American geology to argue he was thinking about evolution. In part of this essay, Darwin takes issue with the view of Charles Lyell (1797–1875) regarding the wholesale extinction of species, particularly terrestrial mammals of South America. Instead Darwin favored the idea of the “gradual birth and death of species.” Eldredge found this a convincing argument for Darwin’s shift to evolutionary thinking. Another author, Herbert (1995), did not read this as an evolutionary argument but rather as a case of sequential change, a view with which I agree. It must be remembered that at this time a majority of paleontologists, biologists, and geologists viewed the sequences of species in the fossil record as a series of replacements under the guidance of a Creator, not as proof of evolution. Whether Darwin differed in this belief by the time he returned from his voyage in 1836 remains unclear. With few exceptions, the geologically based bifurcating diagrams drawn of species succession in most of the nineteenth century, which to us look like phylogenetic trees, definitively identify a Creator as the cause. Brinkman (2009) posits that because Darwin argued in On the Origin of Species that vertebrate succession represented a key line of evidence making him question the fixity of species, it follows that Darwin seriously contemplated evolution during the voyage. Though I tend to agree with Brinkman, I feel the evidence remains elusive. I do believe the case can be made that Darwin showed in these passages the beginning of his belief in gradual change in both the rock and fossil records, but whether to attribute it to a Creator or to evolution remains uncertain. Whichever the case, this observation became exceedingly important later as he argued that the theory of evolution required very slow change over time. Echoing Brinkman (2009), Eldredge (2015, 76) feels “that Darwin sailed home already thoroughly convinced that life had evolved through natural causes.”

I find that none of these passages explicitly demonstrates Darwin’s commitment to the origins of species via the process of evolution, let alone anything about natural selection, while on board the Beagle. Although less convinced than authors such as Eldredge and Brinkman about Darwin’s becoming an evolutionist by the time he reached home in 1836, I do agree that at the very least the subject began to haunt Darwin while still on the voyage.

We do possess the text of a letter Darwin wrote to German zoologist Otto Zacharias (1846–1916) in 1877 supporting this view (Darwin 1903, 367):

When I was on board the Beagle I believed in the permanenc...

Table of contents

• Cover 
• Series Page
• Title Page
• Copyright
• Dedication
• Contents 
• Preface
• Acknowledgments
• 1. Establishing the Fact of Evolution
• 2. Darwin’s Geological Education
• 3. The Gravest Objection
• 4. Marking Time
• 5. The Immutablists
• 6. Discovering the Long Dead
• 7. Relating the Long Dead and Collecting the Recently Living
• 8. Describing the Long Dead and the Recently Living
• 9. Private Musings then Shared Sketches
• 10. Darwin’s Historical Biogeography
• Epilogue: What Many Reviewers Missed
• References
• Index