Invasion on So Grand a Scale: Darwin, Lyell, and Invasive Species

Abstract

The importance of naturalization—the establishment of species introduced into foreign places—to the early development of Darwin’s theory of evolution deserves historical attention. Introduced and invasive European species presented Darwin with interpretive challenges during his service as naturalist on the HMS Beagle. Species naturalization and invasive species strained the geologist Charles Lyell’s creationist view of the organic world, a view which Darwin adopted during the voyage of the Beagle but came to question afterward. I suggest that these phenomena primed Darwin to question the “stability of species.” I then examine the role of introduced and invasive species in Darwin’s early theorizing and negotiation with Lyell’s ideas, recorded in his post-voyage “transmutation notebooks.” Therein, the subject was an inflection point in his contention with Lyell’s views and moreover, his theorizing on invasive species occasioned some of his earliest inklings of natural selection. Finally, I examine how naturalization was crucial to Lyell’s own eventual conversion to evolutionism. I conclude with brief reflections on the implications of this narrative for our understanding of Darwin’s reasoning, his intellectual relationship to Lyell, and the historical context that shaped his theory.

Introduction: The Naturalization Conundrum

In On the Origin of Species, Charles Darwin appeals to evidence from naturalization, or the survival and proliferation of species introduced into foreign places, to support his theory of evolution. “It might have been expected,” writes Darwin, that the species “which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country” (Darwin 1859, p. 115). In other words, if species are created with traits pre-adapted to the conditions of their native environments, then we would expect introduced species which are closely “allied” (taxonomically related) to the native species—and thus which bear traits similar to theirs—to be the most successful at naturalizing. But Darwin argues that the existing data supports the opposite conclusion: naturalized species tend not to be close taxonomic relatives of the indigenous species. Darwin interprets this surprising fact as indirect evidence for his theory that competition drives species to adaptively diverge. Introduced species which are not closely related to the native residents—and thus are adapted to occupy different “stations” in the “economy of nature”—will tend to prosper because they face less competition in their prospective new homes. Organic competition, rather than the pressures of the physical environment, is the chief determinant of naturalization—and also, Darwin goes on to argue, of adaptation and divergence (speciation) by natural selection.

The theory that successful invaders tend not to be close relatives of resident species is known by ecologists as Darwin’s “naturalization hypothesis” (e.g., Daehler 2001). His hypothesis is relevant to efforts to predict whether introduced species will become ecologically problematic invasives. Efforts to test Darwin’s hypothesis have had mixed results: some studies have supported it, others have found counterevidence, and still others have been inconclusive. This state of confusion has in turn been dubbed Darwin’s “naturalization conundrum.”¹ Predicting which species will become invasive is confoundingly difficult—but this would come as no surprise to Darwin. On his theory, whether a given species will succeed in a new environment entirely depends on the details of the species and the environment—both its physical and climatic features and, more importantly in Darwin’s view, the other organisms present with which the introduced species will interact and compete. Darwin’s hypothesis that successfully naturalizing species will tend not to be closely related to the natives does not support predictions in the opposite direction; one cannot predict that, if an introduced species is not a close relative to the natives, it is therefore likely to naturalize. In Darwin’s view, naturalization is an inherently contingent—and hence, in particular cases, unpredictable—process.

While Darwin’s contributions to the beginnings of invasion biology are well recognized, the converse relationship remains unexamined.² The importance of invasion phenomena to the beginnings of Darwin’s theory of evolution deserves historical attention. The contingency of naturalization was not a minor detail or corollary of Darwin’s mature theory; it was his solution to problems that incited his departure from creationism in the first place. In this paper, I will in no way dispute the longstanding historiography of Darwin’s conversion: in short, that he only questioned creationism in the final months of the Beagle voyage, in a sudden passage recorded in his ornithological notes; and that only after the identification of his Galápagos birds and his South American mammal fossils by experts back in England did he begin to pursue a theory of the transmutation (evolution) of species in his private research (Sulloway 1982b; see also Gruber 1985, part VI). Rather, I submit what I believe has been an overlooked aspect of the context of his conversion and of his subsequent theorizing. The HMS Beagle sailed into the midst of what the historian Alfred Crosby called “the greatest biological revolution in the Americas since the end of the Pleistocene era” (Crosby 1972, p. 66). The “portmanteau biota” of introduced European crops, weeds, livestock, and pests (and of course, people) were, as Crosby was the first to show, more essential to the radical global success of European colonialism than any technological or military advantage (Crosby 1986). The astonishing organic ramifications of this “ecological imperialism” confronted Darwin at every stage of his voyage and proved to be crucial evidence in the subsequent development of his theory of the mechanics of life. Naturalization phenomena revealed the intended and accidental effects on nature of human agency. Darwin’s use of this anthropogenic evidence in his early theorizing would be echoed in his later use of the effects—again, intended and accidental—of selective breeding in his analogical argument for the potency of natural selection. The effort to understand invasive species is not just an application of evolutionary theory. It was central to its origin.

In Part I of this paper, I detail Darwin’s observations during the voyage of the Beagle of the changes wrought and undergone by introduced and invasive species. These phenomena posed practical challenges to Darwin as a field naturalist, but more importantly, they strained the geologist Charles Lyell’s creationist theory of organic nature, the framework under which Darwin was working. I contend that puzzles surrounding introduced, feral, and invasive species primed Darwin to open his inquiry on transmutation. In Part II, I show how the subject of naturalization remained important in Darwin’s post-voyage effort to develop a transmutationist theory of life, documented in his private research notebooks of that era. The subject was an inflection point in his negotiations with Lyell’s ideas, and his theorizing on invasive species apparently occasioned some of his earliest inklings of natural selection. In Part III, I explore Lyell’s re-litigation of the species question in private journals of his own and in correspondence with Darwin in the late 1850s. Naturalization, I show, was also a central point of contention in this final period of debate between Darwin and Lyell before the publication of the Origin. In the end, the success of European invasive species in colonized regions, reportedly more than any other fact, converted Lyell to evolutionism. I conclude with brief reflections on the implications of this narrative for our understanding of Darwin’s reasoning, his intellectual relationship with Lyell, and the historical context that shaped his theory.

The Invasive Species Question

In mid-September 1833, Charles Darwin traveled overland through northern Argentina. In a passage from his published narrative of the voyage, the Journal of Researches (Darwin 1839b, pp. 137–139), Darwin reflected on the dramatic changes that three centuries of European influence had wrought on the landscape of this region.³ After crossing the river Salado, he noticed that the typical “coarse herbage” of the pampas suddenly became a “carpet of fine green verdure”—a change he at first supposed was a result of a difference in the soil on either side of the river. But locals informed Darwin that it was wholly attributable to “the grazing and manuring of the cattle.” Whether by introducing foreign species or by affecting the growth or proportions of native species, cattle ranching had transformed a rough scrubland into a verdant pasture.

Meanwhile, European cardoon—Cynara cardunculus, or artichoke thistle—had spread for hundreds of square miles in the surrounding countryside, replacing what had been a diverse hierarchy of native plants with an unbroken prickly mass “impenetrable by man or beast.” It seemed that nothing else could live in the vast tracts where the cardoon had taken over. Darwin doubted “whether any case is on record, of an invasion on so grand a scale of one plant over the aborigines.” The whole region was completely transformed:

According to the principles so well laid down by Mr. Lyell, few countries have undergone more remarkable changes, since the year 1535, when the first colonist of La Plata landed with seventy-two horses. The countless herds of horses, cattle, and sheep, not only have altered the whole aspect of the vegetation, but they have almost banished the guanaco, deer, and ostrich. Numberless other changes must likewise have taken place; the wild pig in some parts probably replaces the peccari; packs of wild dogs may be heard howling on the wooded banks of the less frequented streams; and the common cat, altered into a large and fierce animal, inhabits rocky hills. (Darwin 1839b, pp. 138–39)

The changes that Darwin attributed to the flora and fauna, native and foreign, of this territory of South America scarcely hinted at the transmutationist theory he was privately developing during the same post-voyage period in which he drafted his Journal of Researches.⁴ Still, this passage indicates that Darwin viewed European colonization and invasive European species as evidence of the mutability of the organic world. Darwin had received the newly published, second volume of Charles Lyell’s Principles of Geology (1832) while at port in Montevideo in October 1832 (Desmond and Moore 1991, p. 131; Browne 1995, p. 272),⁵ and as indicated in the passage above, Lyell’s ideas (which I will discuss shortly) informed Darwin’s thinking, during and after the voyage, about change in the organic world—including his thinking about the impacts of introduced species.⁶ But, as I will show, Darwin’s observations of introduced species during the voyage would strain Lyell’s already conflicted account of the principles of organic nature—and prime Darwin to question the stability of species.

Most of Darwin’s biological work during the voyage was purely descriptive, in contrast to his vigorous (and thoroughly Lyellian) geological theorizing on volcanoes, coral atolls, and the South American continent. In R. D. Keynes’ introduction to his transcription of Darwin’s Zoology Notes from the voyage (Keynes 2000), Keynes acknowledges this general lack of biological theorizing but flags several entries that display Darwin’s early aptitude for (what would eventually be called) ecology. Several of these entries notably concentrate on interactions between American and European species.

In Brazil in May 1832, Darwin wrote that he “could not help noticing how exactly the animals & plants in each region are adapted to each other.” When transported to the Americas, European lettuce and cabbage were not attacked by the native insects and snails like they were in England. Referring to the vegetables and pests of Europe, Darwin mused that “Nature, when she formed these animals & these plants, knew they must reside together” (Keynes 2000, pp. xviii, 58–59). A year later, exploring the ranchlands of Uruguay, Darwin made an analogous observation: huge piles of horse and cattle manure were untouched by native dung beetles. To Darwin, it showed “a connection in the creating between animals as widely apart as Mammalia & Insects” (Keynes 2000, pp. xxi, 175). The absence of American beetles utilizing European cattle manure, even after 80 years of ranching in the region, spoke to the stability of what Charles Lyell called “centers of creation:” geographical foci in which assemblages of species were created to coexist. The fact that British dung beetles do feed on horse and cattle manure in England was proof, Darwin wrote, in accordance with his Lyellian logic, that those mammals were created to inhabit Britain. The same logic led Darwin to predict that he would also observe an absence of native dung beetle activity in the colonial cattle ranches of Australia.

But there was an exception. Darwin observed a species of South American Aphodius scarab burrowing in the horse manure in Uruguay. He pondered “what animal [it] belonged to before the introduction of horses” (Keynes 2000, p. 175), for the native mammals—deer, guanaco, capybara—produced pellet-shaped dung totally unlike horse manure. He saw the same thing a year later on the island of Chiloe, Chile, where there are no native large mammals: a Geotrupes scarab had taken to the horse manure and even to the sheep dung, proliferating in a “curious instance of increase in number & change in habit” (Keynes 2000, p. 234). When Darwin finally reached Australia, he counted no fewer than 6 different species of native beetle making use of the cattle and horse manure—dung totally unlike the native kangaroo pellets—in the ranches of Hobart, Tasmania. Adding these observations to his Chiloe notes, Darwin remarked that “the subject is a curious one” (Keynes 2000, p. 234).⁷

The dung beetles puzzled Darwin because they challenged his Lyellian view of natural history. But what, more precisely, was the Lyellian view? In particular, how were the impacts of naturalized foreign species—a long familiar phenomenon, after hundreds of years of European imperialism—understood in Lyell’s creationist theory? Lyell held that species are providentially fitted to their native environments. Hence, one might expect Lyell to believe that the permanent establishment of a species in a foreign place would violate the divinely legislated centers of creation, and hence could only happen through the free exercise of human agency. On such a view, without human maintenance, any introduced species will inevitably go extinct in a foreign territory, restoring the temporarily disturbed natural order. Later, we will see that Lyell entertained this view in the 1850s; but in 1832 his position was more complicated, more radical—and, I suggest, less internally consistent.

Lyell began his book with a severe critique of the French naturalist J. B. Lamarck’s controversial theory of the transmutation of species, protesting a “chasm in the chain of evidence” between Lamarck’s data and his conclusions (Lyell 1832, p. 8). Lyell, a proponent of the Newtonian vera causa ideal, held that scientific explanations should be founded on verified causes.⁸ He argued that the development of a new organ or faculty had never been observed and that all evidence shows variation to be strictly limited within specific bounds. Lyell contemptuously lampooned Lamarck’s theory with a sarcastic narrative of the evolution of man from an orangutan, Lamarck’s supposed final stage of evolutionary progress up the scale of nature (Lyell 1832, pp. 14–17).

Lyell’s creationist view held that new species are “introduced” (by unspecified means) as single individuals (or sexually reproductive pairs) preadapted to the organic and inorganic conditions of the environments in which they are created. The inorganic conditions (climate, soil, etc.) depend on features of the surface of the earth (mountains, rivers, coastlines, etc.) which are gradually but perpetually transforming (as detailed in his previous volume). Haphazard geological “vicissitudes”—the accumulating effects, over eons, of earthquakes, volcanoes, erosion, etc.—will eventually drive species, created to inhabit vanished worlds, extinct. And since species are also fitted to the organic conditions of their native regions, that is, preadapted to cohabitate with the other species present, the extinction of one will beget the extinction of others. This chain of contingencies means that all species will eventually die out, some quickly, some after a long time. Thus, life on earth will disappear unless new species are perpetually being created, fitted to whatever geological and organic conditions happen currently to prevail in each district of the earth. Lyell believed this continuous series of creations was evidenced by, and explained, the fossil record.

In Lyell’s account, nature does not require human intervention to spread species into foreign territories. He detailed at length plants’ natural means of establishing “new colonies” (Lyell 1832, pp. 73–81), and he invoked the “accidental” rafting of animals across bodies of water to explain disjunctive species ranges (Lyell 1832, pp. 97–99).⁹ But he did not discount the impact of modern human activity. For plants, man is “one of the most important of all” instruments of naturalization, spreading seeds to new regions both on purpose, as with crops, and “by accident,” as with countless weeds (Lyell 1832, p. 82). The same is true of animals, as with domestic quadrupeds, ship-borne rats, cockroaches, etc. Given the chain of dependencies among species in the scheme of nature, Lyell emphasized that the impacts of just one colonizing species could “ramify through all classes of living creation, and be almost endless” (Lyell 1832, p. 144). The dizzying interactions of so many causes and dependencies would make it effectively impossible “for us to declare, what ultimate alteration the presence of any new comer […] might occasion” in the economy of nature of a colonized region (Lyell 1832, p. 146).

Yet, amid all this unpredictable, chaotic, permanent change, Lyell constantly invoked forces of balancing, equilibration, and uniformity. Every plant has its “proper insect” to limit its growth (Lyell 1832, p. 132); in turn, parasites (such as the ichneumon wasp) curb the spread of plant-eating insects, “a good illustration of the checks and counterchecks which nature has appointed to preserve the balance of power amongst species” (Lyell 1832, p. 133).¹⁰ Nor do the extreme impacts of human activity upset the higher balance of nature, on Lyell’s account. The functions, in the economy of nature, of animals driven extinct by humans “devolve upon the human race,” such that we naturally assume their appointed roles of consuming plants and animals, dispersing seeds—even of unintentionally spreading weeds and insects to distant places (Lyell 1832, p. 84). At the same time, Lyell conceded the extraordinary organic disruptions wrought by European imperialism: the global multitudes of naturalized domestic species, the native floras and faunas replaced by vast tracts of agricultural monotony. But he contended that “in thus obtaining possession of the earth by conquest […] we exercise no exclusive prerogative” (Lyell 1832, p. 156). For every species increases at the expense of others; every being, from the locust to the lion, slaughters thousands as it disseminates itself across the world. The human impact, however unprecedented in scope, does not violate “the uniformity of the system, unless, indeed, we are precluded from speaking of uniformity, when we characterize a principle of endless variation” (Lyell 1832, p. 157).

Rudwick (1998, p. 13) remarks that, in retrospect, Lyell’s Principles seem “paradoxical.” He is referring to the tension between Lyell’s vera causa argument for perpetual change and his evidence-free insistence that earth’s history is not directional but cyclical, which in his day was a controversial aspect of his commitment to “uniformity.” According to Lyell’s environmentally deterministic theory of creation, in the course of ages, when inorganic conditions similar to those of the past recur, extinct organic forms will consequently reappear (see Lyell 1830, p. 123). I suggest Lyell’s account of organic nature exhibits a parallel internal conflict. Nature has “appointed” checks and counterchecks to maintain balance within her economy, i.e., species are created to interact with other species such that a stable, harmonious equilibrium among them is sustained. Yet at the same time, it is equally natural that species will actively venture, or be haphazardly carried, outside their native lands to colonize foreign places, where such checks and balances are not provisioned for them, resulting in cascades of chaotically ramifying effects, not least of which are distinctly inharmonious slaughters and extinctions; and that is in addition to the eternally disruptive geological vicissitudes. Nature appoints regulatory forces of harmonious balancing and at the same time, admits forces which subvert them. Nature’s careful design and forethought is everywhere confounded by agency, accident, and unpredictable change.

Lyell’s conflicted ideas would provide scant help to a field naturalist endeavoring, as Darwin was, to identify species’ natural ranges and distinguish natives from nonnatives. In Lyell’s scheme, there is supposed to be an essential difference between a species that is at home in its district of birth and one that is not. Indigenous status should be inferable by whether a species is a functional part of the system of checks and counterchecks appointed to keep the organic economy of a region balanced. But dispersal and naturalization, in addition to the steady march of geological transformation, will inevitably confound such inferences. Consider Darwin’s remark in his voyage ornithological notes on the distribution of the bird genera Certhia and Troglodytes:

In central Chile both are found, but extremely in few numbers. — In that country (& in a like manner in a like case in other countries) one is apt to feel surprise that a species should have been created, which appears doomed to play so very insignificant a part in the great scheme of nature. One forgets, that these same beings may be the most common in some other region, or might have been so in some anterior period, when circumstances were different. (Barlow 1963, p. 259)¹¹

Since landscapes are changing and species are dispersing to new territories all the time, as part of the regular goings-on of nature, species appearing to play insignificant parts in the scheme of nature will be commonplace. Their insignificance could result from several distinct historical causes.

Darwin’s dung beetle observations were troubling because they suggested that indigeneity inferences could be stymied from the other direction. The American and Australian beetles utilizing horse and cattle manure were developing new associations, actively integrating known interlopers into the local organic system. If organisms have such powers, then even the observation that a species is well integrated is not a sure sign that it is in its district of birth. Natural associations and harmonies were supposed to be the creationist natural historian’s scientific bedrock, yet inferences on their basis were hopelessly bedeviled.

Darwin did not critically scrutinize Lyell’s ideas until after the voyage. But he did struggle to distinguish native from non-native animals. There was an endless parade of island rats and mice, each of which Darwin tried to interpret either as endemic, as having swum from the mainland or crossed on former land or ice bridges, or as accidental immigrants introduced over the years by passing ships. By its “habits,” he judged a mouse on Gorriti Island, Uruguay, to be aboriginal. It eventually proved to be European. He made a similar error in the Falkland Islands (Keynes 2000, pp. xx, 171, 209; see also p. 134).

Darwin continually observed the changes wrought, and undergone, by introduced species.¹² Feral domestic animals were particularly interesting. Based on the uniform coloration and head shape of the feral goats on Ynche Island, in the Chonos Archipelago, Darwin believed they were “retrograding into their original figure & kind” (Keynes 2000, p. 281).¹³ The feral horses of the Falkland Islands were bizarrely “fond of catching cattle,” a strange “aberration of instinct” (Chancellor and van Wyhe 2006, p. 12a). The Falklands were home to black rabbits which a French naturalist had previously identified as an endemic species; but Darwin believed the local gauchos, who insisted they were a feral variant of the imported gray European rabbit. The effects of colonization on native faunas were interesting, too, and troubling. Darwin predicted the tame, endemic Falkland “fox” (“very curious, thus having a quadruped peculiar to so small a tract of country”) would soon go extinct because of human culling: “how little evidence will then remain of what appears to me to be a centre of creation” (Keynes 2000, pp. 209–210).¹⁴ Incidentally, the coloration of the fox also caught his attention: locals insisted there were two variants, a dark one on East Falkland Island and a rusty one on West.¹⁵ It was odd because, environmentally, the islands were little different.

When Darwin reached the Galápagos Islands in September 1835, they were not the pristine, virgin world of popular imagination. After only a few years of permanent human habitation, feral pigs and goats were already rampant in the woods of Charles Island (Keynes 1988, p. 356).¹⁶ Expanses of the same woods had been cleared to make way for tracts of imported sweet potatoes and plantains, which grew “with luxuriance” (Keynes 1988, p. 355). Darwin was aware that the tortoise population had fallen precipitously; the governor of Charles Island, Nicholas Lawson, predicted that the tortoises would hold out only another twenty years (Keynes 1988, p. 356).¹⁷

Darwin paid little mind to the introduced species in the Galápagos. His chief interest from the start was determining “to what district or ‘centre of creation’ the organized beings of this archipelago must be attached” (Keynes 1988, p. 356). Darwin believed the islands were of volcanic origin and had arisen from the sea, devoid of life, in the geologically recent past. The species of the archipelago must have either been created there or arrived there from elsewhere. If a unique assemblage of species had been created on the islands, then in a narrow sense, that of Lyell (1832, p. 126), the Galápagos archipelago was itself a “center of creation.” But this was only part of Darwin’s question; even if the archipelago’s species were endemic, the question of their “alliance” to other floras and faunas remained: were the Galápagos plants and animals similar to those of Polynesia, of South America, or of somewhere farther away? Local centers of creation appeared to be nested within global ones, and Darwin wanted to know to which global center of creation the Galápagos Islands belonged.

The “manifestly S. American” ornithology famously answered Darwin’s question. The mockingbirds, which appeared to exist “as varieties or distinct species in the different Is^ds,” appeared remarkably “closely allied to the Thenca of Chili,” with identical habits but, Darwin believed, a different “note or cry” (Keynes 2000, pp. 297–298). Several months after leaving the Galápagos, Darwin reflected on the mockingbirds in his ornithological notes:

When I see these Islands in sight of each other, & possessed of but a scanty stock of animals, tenanted by these birds, but slightly differing in structure & 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 & West Falkland Islds. If there is the slightest foundation for these remarks the zoology of Archipelagoes will be well worth examining; for such facts [would inserted] undermine the stability of Species. (Barlow 1963, p. 262; brackets original)¹⁸

Scholars have not connected this celebrated note, which marks Darwin’s first recorded transmutationist suspicions, to his attention throughout the voyage to the changes undergone by introduced foreign species.¹⁹ For that was exactly what Darwin suspected the Galápagos mockingbirds to be: descendants of a foreign (Chilean) mockingbird introduced to the islands—not by human intervention but by an accident of nature—and subsequently altered, island-by-island. On this view, they were in principle no different than the horses, rabbits, goats, beetles, weeds, etc. that Darwin had, with so much curiosity, observed changing, if only incrementally, under new conditions of life.

The Transmutation Notebooks

After the voyage, Darwin developed his evolutionary ideas, and critically examined Lyell’s creationism, in his private “transmutation notebooks.”²⁰ There is a rich body of historical work on these notebooks and this period in Darwin’s intellectual development.²¹ Few of the passages and episodes I will discuss in this section are not addressed in greater depth by previous authors. My intention here is not to improve on the existing notebook historiography but merely to trace the subject of naturalization—a theme to which previous authors have not directed their focus—through the notebooks, and to situate its importance in the context of Darwin’s larger effort. Introduced species and their implications appear repeatedly in notebooks B and C, written roughly between July 1837 and June 1838. While the topic was less relevant in Darwin’s development of selection theory, which he worked out in subsequent notebooks D and E (from July 1838 to July 1839), naturalization was nevertheless crucial evidence of the looseness of the connection between adaptation and native environment, and of the reality of adaptive change—and it even prompted Darwin to consider how differently chance varieties might fare in the “wars of organic beings.”²²

As his transmutationist ideas developed, Darwin began to envision formulating a theory of the “laws of life” that would be for biology what Newton’s laws were for physics: a small set of simple principles with wide-ranging and unifying explanatory power (Barrett et al. 1987, pp. 195, 219, 228 [Notebook B, pp. 101, 196, 229]). He would pursue a theory free of the creationist crutch of perpetual divine interventions and free of Lamarck’s appeal to a teleological ascent up an eternal scale of nature.²³ He would seek to explain organic phenomena as the inevitable consequences of the physical causes of geology, reproduction, and the dispersal of species:

Let animal be created, then by the fixed laws of generation, such will be their successors.—let the powers of transportal be such & so will be the form of one country to another.— let geological changes go at such a rate, so will be the numbers & distribution of the species!! (p. 195 [Notebook B, pp. 101–102])

In this, he would ally himself with Lyell, who always sought explanations based on verified causes (with the glaring exception of those mysterious, perpetual creations).²⁴ But the early stages of Darwin’s theorizing occasioned severe doubts about Lyell’s account of the organic world.²⁵

From the beginning, Darwin was critical of the idea of a nested hierarchy of centers of creation. On Lyell’s theory, it made no sense that the creation of island species would be influenced by the “halo” of neighboring continents, “as if any creation taking place over [a] certain area must have [a] particular character” (p. 61 [Red Notebook, pp. 127–128]; see also p. 271 [Notebook C, p. 106]). If species are preadapted to the conditions of their birthplaces, then isolated island species should have no reason to resemble those on an environmentally dissimilar mainland, just because it is nearby. Darwin now began to entertain doubts about the entire relationship between local environment, adaptation, and extinction.²⁶ One line of evidence was the startling ease with which a single domestic species could naturalize in diverse foreign territories, implying that adaptation and environmental circumstances are not so closely linked:

Dogs. Cats. Horses. Cattle. Goat. Asses. have all run wild & bred. no doubt with perfect success.—showing non Creation does not bear upon solely adaptation of animals.—extinction in same manner may not depend.—There is no more wonder in extinction of species than of individual. (p. 63 [Red Notebook, p. 133]; see also p. 202 [Notebook B, p. 130])

If adaptation was not environmentally determined, maybe extinction was not either. Darwin now considered whether lineages, like individuals, might have natural “lifespans.”²⁷ This idea was distinctly non-Lyellian, relying on a more Lamarckian, internal cause to explain life’s trajectories.²⁸ Toying with this idea (p. 176 [Notebook B, pp. 22–23]), Darwin quickly noticed that, if species are connected by branching ancestry (as he now believed), then a fixed lineal lifespan would mean the simultaneous extinction of whole groups of related species (pp. 177, 179–80 [Notebook B, pp. 29, 35–36]). While extinction seemed sporadic as a rule, the idea called to mind the mysterious mass extinction of the giant Quaternary mammals of America and Eurasia, an event that seemed like “some great system acting over the whole world” (p. 183 [Notebook B, p. 53]):

It is a wonderful fact Horse, Elephant & Mastodon dying out about same time in such different quarters.—Will Mr Lyell say that some circumstance killed it over a tract from Spain to S. America.—Never. (p. 187 [Notebook B, pp. 62–63])

Darwin flirted with a hypothetical “law:” changing environmental conditions superinduce changes in species, extending their lifespans, whereas species go extinct in the absence of environmental change (p. 186 [Notebook B, p. 61]).

This hypothesis would not last. Recognizing that physically uncaused extinctions were the “weakest part” of his theory (p. 203 [Notebook B, p. 135]), Darwin reverted to his early, Lyellian view that extinction was the consequence of “unfavorable conditions.” With hereditary changes in structure being “excessively slow,” rapid, haphazard changes to the physical environment could indeed turn fatal: “the animal cannot change quick enough & perishes;” Lyell, who had argued that geological vicissitudes will be “unequally rapid, with respect to their effects,” was right after all (p. 285 [Notebook C, p. 153]).²⁹

The ever-present problem was, rank historical contingency is distinctly un-lawlike. Lyell had sought order in the chaos by his appeal to nature’s balancing, harmonizing provisions. Having abandoned the foresight of a creator, Darwin had to doubt the reality of “appointed” checks and counterchecks: it was an error to believe that “every animal [is] born to consume this or that thing” (p. 204 [Notebook B, p. 141]). And Darwin would break with Lyell over another source of self-conflict in his theory: “man’s place in nature.” Lyell did not believe the fossil record evidenced Lamarck’s anthropocentric story of progress: if species’ traits are determined by the conditions prevailing in a given place, which themselves depend on a haphazard series of geological vicissitudes, then there should be no Lamarckian pattern of ascent up the scale of nature toward man.³⁰ Moreover, for Lyell, humans effect no net change in the underlying systemic uniformity: they are just another mountain rising where elsewhere, a seabed drops. But Lyell simultaneously believed man to be the culmination of nature’s creative power, as evidenced by his moral intellect and unprecedented capacity to influence the economy of nature. Lamarck’s contention that man and orangutan were kin disgusted Lyell. Lyell rejected a history of organic progress, but still insisted that man, the latest creation, sat at the top of the scale.³¹

Darwin agreed with Lyell that the direction of organic change depends on prior conditions, not on an eternal template (see p. 224 [Notebook B, pp. 214–215]). Any apparent progress in the fossil record was the trivial effect of life having necessarily begun in a simple state: “the simplest cannot help.—becoming more complicated,; & if we look to first origin there must be progress” (p. 175 [Notebook B, p. 18]). But unlike Lyell, Darwin was willing to bite the bullet: man’s alleged superiority is only a projection of arrogance. A bee would rank bees at the top of the scale (p. 189 [Notebook B, p. 74]).

Hence, in Darwin’s developing view, Lyell had not erred in his belief in environmental contingency, nor in his rejection of Lamarckian teleology.³² He only erred in mitigating and muddling these insights with baseless notions of divine foresight and human superiority. But his biggest error was his repudiation of species’ capacity to change. The capacity to change was, of course, where Darwin directed his search for the explanatory “laws of life.”

Darwin came to focus on two distinct, if often combined, sources of change: geographic isolation, and altered conditions of life. Evidence from naturalized species was telling in both cases. He remained certain that the black rabbits in the Falkland Islands were merely a different color morph of domestic European rabbits that had spread and been mistaken for an endemic species (pp. 178, 184 [Notebook B, pp. 31, 54], pp. 244, 247 [Notebook C, pp. 23, 29]). This known case proved that the relocation and isolation of a small number of individuals could result in a permanently altered population. It would have struck Darwin not as evidence of adaptive change, but of apparently accidental change—presumably analogous to the two varieties of Falkland foxes with slightly different coats. But he knew where to look for evidence of adaptive change. He made a note to ask entomologists “whether they know, of any case of introduced plant, which any insects hav[e] become attached to” (p. 225 [Notebook B, p. 218]).³³ Such a case would be exactly parallel to the dung beetles: evidence not only of adaptive change but, he now believed, indicative of incipient speciation. He predicted that the “S. American dung beetles will each become the fathers of many species” (p. 300 [Notebook C, p. 197]).³⁴

Finally, Darwin returned to the subject of introduced species flourishing to excess, as in feral domestic animals and invasive weeds. At first, he had viewed this phenomenon as a challenge to the connection between environment and adaptation. Now, he reinterpreted it in light of his developing theory:

Study the wars of organic being.— the fact of guavas having overrun— Tahiti,^[35] thistle. Pampas. show how nicely things adapted— These “aberrant” varieties will be formed in any kingdom of nature, where scheme not filled up […] & in round of chances every family will have some aberrant groups. (p. 262 [Notebook C: 73])

In this passage, Darwin interprets the European cardoon he had seen overrunning the Argentine pampas as an “aberrant” variety.³⁶ He attributes the existence of aberrant varieties to the inevitable “round of chances,” and this variant evidently happened to be well-suited to dominate in the “war”among plants in the region.³⁷ While it is still a far cry from the theory of individual competition and consequent natural selection that he would formulate in the following autumn and winter, this entry, probably written in April 1838, appears to be one of Darwin’s first articulations of the propagation of chance variation through competition.³⁸ It hardly needs adding that adaptation by natural selection would be Darwin’s solution to the contingency problem, the “law” imposing explanatory order among Lyell’s chaos of geological vicissitudes and endless variation.

Lyell’s Conversion

Lyell’s eventual conversion to evolutionism was drawn-out and fraught. Historians have tended to focus on the gradual erosion of his conviction in the uniformity of earth’s history and in the uniqueness of man.³⁹ However, Dixon and Radick (2009, p. 32, note 15) and Radick (2018, p. 171) have suggested that a series of exchanges with Darwin in 1859 concerning naturalization shattered Lyell’s confidence in the creationist rationale. In this section, I situate this episode in Lyell’s larger efforts to re-examine the “species question.”

In the years after the voyage of the Beagle, Lyell and Darwin became friends, intellectual allies, and frequent correspondents. However, while Lyell was aware that Darwin harbored transmutationist sympathies, he would not be let in on the details of Darwin’s still-unpublished theory until a visit to Darwin’s residence, Down House, on April 16, 1856. By then, Lyell had been re-litigating transmutation in his own private journals for several months, apparently provoked by Alfred Russel Wallace’s recent “Sarawak” paper (Wallace 1855). Lyell’s journals evidence his remarkable even-handedness in reassessing the evidence that life had evolved, despite his longstanding partisan stance on the issue.⁴⁰

Based on fossil and biogeographical evidence, Wallace had proposed a “law:” “Every species has come into existence coincident both in space and time with a pre-existing closely allied species” (Wallace 1855, p. 196). Lyell is commonly thought to have perceived evolutionary implications in Wallace’s law, but this interpretation has been vigorously disputed (van Wyhe 2016). Nevertheless, Lyell sought a creationist explanation for Wallace’s conclusion. Lyell began by considering what a “not omniscient” creator would need to predict—that is, infer on the basis of present conditions—in order to create a well-adapted species (Wilson 1970, p. 3). This perhaps surprising framing probably reflects Lyell’s natural-theological commitments: nature was supposed to reveal the Creator’s workings, not be explained away by His all-knowingness. As he always had done, Lyell supposed that the physical conditions and the traits of the pre-existing species in a region would unambiguously determine the traits of any newly created species. So, assuming organic and inorganic conditions are coincident in space and time, then, to be fitted to its environment, a new species would inevitably resemble the existing ones: “[T]he new species will partake of the characters which others of the same genus have had in reference to past cond.^s of the animal, vegetable & inorganic worlds” (Wilson 1970, p. 4). This would explain Wallace’s law. On this view, apparent continental “halos”—nested centers of creation—are simply the result of similar physical conditions prevailing over wide areas. Thus, Lyell reasoned that the Galápagos fauna resemble those of South America simply because the islands and the continent share a similar climate—at least, that was his assumption (Wilson 1970, p. 4).

Yet Lyell immediately recognized the same problem Darwin had years earlier: if there is such a close mapping between birthplace and adaptation, then species should be ill-fitted to survive in foreign environments—but the success of domestic animals and plants introduced by humans to colonized regions contradicts that expectation. To explain this difficulty, Lyell reasoned that, were they not maintained by humans, introduced species would die after all. He contrasted the human-directed naturalization of domestic species with natural migrations, which “must have always occurred & have caused mere temporary disturbance” (Wilson 1970, p. 4). With this line of reasoning, Lyell appears to have radically attenuated the ecological contingency that characterized his earlier account of natural history. Physical conditions are now the principal determinant of form. Migrations, however common, no longer chaotically disturb the scheme of nature. Centers of creation are now stable, and human agency is required to bring about prolonged violations of their boundaries. (Note that this also restores to humanity a unique position in the natural world.)

Lyell may, to some extent, have entertained this position for the sake of argument, articulating a simplistic but plausible creationist stance against which to test transmutationist reasoning. In any case, he continued to weigh both sides, an effort that his April 1856 meeting with Darwin at Down House injected with new energy.⁴¹ The theoretical advantages of natural selection could not be dismissed. While he felt that the choice between evolution and creation was still underdetermined by the evidence, Lyell recognized, provisionally, that Darwin’s appeal to verified causes broke the tie (Wilson 1970, p. 106). Whereas creationism required perpetual miraculous interventions (Wilson 1970, pp. 55–56), natural selection was a vera causa solution to the species question: “we may accept it for the present” (Wilson 1970, pp. 288, 294). But the case was not settled in Lyell’s mind.

Lyell’s final effort to contest transmutation came in a series of letters with Darwin in October of 1859, as the final edits to On the Origin of Species were being made. In a now-lost letter, Lyell apparently tried the argument from his journal, contending that European species in Australia, such as cattle, only survive by human maintenance, without which they would inevitably die—which they nearly do anyway in the extreme Australian droughts. Darwin replied that feral cattle in Australia survive without the aid of humans; and moreover, even the indigenous mammals in, for example, Argentina and India die by the thousands during droughts, sometimes taking as long as a decade to regain their former numbers.⁴² Thus, not only do non-native species often flourish in foreign climates, but native species frequently suffer. Back in their April 1856 discussion, Darwin had emphasized the far greater importance, in the struggle for life, of organic competition than the physical environment (Wilson 1970, p. 54).⁴³ Now he doubled down: “[A]gain I feel inclined to swear at climate!”⁴⁴

Lyell had read the latest draft of the Origin, and by now he knew what he somehow had not known before: there is little environmental similarity between the Galápagos Islands and the South American coastland (see Darwin 1859, pp. 398–399). So, he confronted Darwin with a new creationist rationale for the American character of the Galápagos fauna, one that relied on the importance of organic competition. Lyell suggested that, to prepare the Galápagos species to compete with the inevitable South American migrants, a forethinking creator would create them on the American type.⁴⁵ Darwin replied that, in fact, American species tend to be dominated by foreign genera—as evidenced by the transformed Argentinian landscape: “Look at the unbroken & untilled ground in La Plata, covered with European products, which have no near affinity to the indigenous products: they are not American types which conquer the aborigines” (Darwin’s emphasis).⁴⁶ Darwin then reiterated his naturalization hypothesis: because they face less competition, successful invaders tend to be species of foreign genera rather than indigenous genera.

Previous commentators have missed the fact that Darwin’s reply to Lyell does not quite make sense on its own terms. True, foreign species have an outsize advantage in America; but according to Darwin’s naturalization hypothesis, American species will have an advantage in foreign environs. This is evidenced by, for example, the invasive guava in Tahiti, which Darwin was aware of.⁴⁷ Hence, a creator wishing to protect an isolated center of creation from domination by migrants ought to create its species on the same type as those of the nearest mainland, wherefrom future migrants are most likely to come. This would predict, rather than contradict, the American character of the Galápagos fauna. Lyell’s interim letter is lost, but he seems to have tried to counter Darwin by reiterating this very line of reasoning, based on Darwin’s own principle of competition. More precisely, he appears to have taken it to its logical conclusion, suggesting that within a local center of creation, such as America, species are created on the same type, over time, to prevent newly created species from competitively dominating the existing species. Reluctantly conceding Lyell’s logic, Darwin nevertheless dismissed the notion that species are created, in a sense, handicapped: it seemed to him “somehow a monstrous doctrine.”⁴⁸

Whatever he may have thought of this particular response from Darwin, Lyell’s confidence in a match between species and their environments coherent enough to evidence divine foresight was ultimately shaken beyond repair. Darwin’s naturalization hypothesis was only a tendency, not a fast law: a given species’ chances of naturalizing in a given territory were ultimately dependent on the finest details. Lyell may have felt that the necessary calculations of a “not omniscient” creator had become too hopelessly contorted. They could scarcely rely at all on the physical environment. The creator must now contend with every contingency, predicting the most likely future migrants and weighing their competitive advantages and disadvantages. Accidental migrations from far away would tend to dramatically disrupt the harmony and stability of nature. And there is little reason for a creator to make a new species in a territory resemble the existing ones, because any number of forms could do quite well in the same environment. The possibility of a creationist rationale for Wallace’s “law” had crumbled. Adaptation to the physical environment and the seeming harmony of nature could no longer be held as evidence of creation. Lyell was forced back to the radically contingent view expressed in the first edition of the Principles of Geology, except now he knew he could not temper it with divinely appointed balancing mechanisms.⁴⁹

Years later, in January 1867, Lyell’s friend and extended family member Sir Charles Bunbury recorded in his diary a conversation they had while out for a walk:

Lyell said, that nothing contributed more to shake his belief in the old doctrine (which he formerly held) of the independent creation of species, than the facts of which so many have lately been recorded, relating to the rapid naturalization of certain plants in countries newly colonized by Europeans. He remarked that these introduced plants, many of which have spread to an enormous extent and with surprising rapidity in the Australian colonies, New Zealand and parts of S. America, belong in many cases to families entirely wanting in the indigenous floras of the countries in which they have thus settled themselves, and hardly ever to families prevailing in or characteristic of those indigenous floras. When one sees, he said, a particular genus or order of plants abounding very much in a particular country and exhibiting there a great variety of specific forms, one is naturally inclined to suppose (on the “independent creation” hypothesis) that there are particular local conditions in that country, which render it peculiarly suitable and favourable to that family of plants. But when we see an introduced stranger which has no affinity to that prevalent family, intruding itself in its place, overpowering and superseding it, this explanation becomes less satisfactory, and one is led to search rather for some law of descent with variation, to explain the multiplicity of nearly allied forms in a particular region. (Lyell 1906, pp. 216–217)⁵⁰

Naturalization finally convinced Lyell that species are not providentially fitted to their places of birth. He accepted that the biogeographical patterns we observe can only be coherently explained by historically contingent processes of migration and descent with modification.⁵¹ Lyell never fully embraced the materialism of natural selection, nor did he accept Darwin’s account of human evolution.⁵² But he accepted that life had gradually evolved. Evidence from invasive European species was crucial to Lyell’s conversion, just as it had been to Darwin’s.

Conclusion: Naturalization in Context

Historians have long recognized that Darwin’s study of domestic breeding in his development of, and argument for, selection theory was unusual.⁵³ It notably marked a stark contrast with Wallace, whose research and arguments were almost always centered on nature in its “natural” state.⁵⁴ It has not been appreciated that Darwin’s unique approach of investigating nature through the lens of human interference went all the way back to the Beagle era, and was equally foundational to his conviction that species evolve as it was to his theory of how they evolve.

Darwin never drew an explicit analogy between the effects of human-introduced species and the effects of natural migrations the way he did between artificial and natural selection. But his exploration of these relationships seems to reflect a unified pattern of reasoning. In both cases, he drew insights from the outcomes of human interventions which were both intentional and accidental. In the Origin, Darwin emphasized the permanent changes caused by “unconscious” artificial selection: changes wrought not by breeders’ deliberate efforts to alter their stock, but merely through the more mundane effort to maintain—not improve—the quality of herds or crops (Darwin 1859, pp. 34–37). Arguably, this type of selection creates a kind of bridge between the more familiar “methodical” artificial selection and wholly human-independent natural selection, demonstrating that even within the sphere of human influence, our intentions are inessential: unequal reproduction inevitably results in permanent change.⁵⁵ In his early theorizing, Darwin recognized the same implications for invasive weeds and feral domestic animals: humans may have been responsible for their arrival in foreign countries, but our agency was only incidental. Once arrived, they flourished indefinitely on their own, demonstrating that an organism’s success and its native environment are not so closely linked, and that the scheme of nature is capable of permanent change irrespective of human interference.

This story also lends new texture to Darwin’s relationship with, and debt to, Lyell. That Lyell’s uniformitarian geology and vera causa reasoning were the bases of Darwin’s science has rarely been doubted. It has not been appreciated, however, that puzzles over naturalization, perhaps more than any other phenomenon, led Darwin to question Lyell’s assumptions and the latent contradictions in his account of natural history. Naturalization forced Darwin to re-assert contingency where Lyell had tried to mitigate it—and to affirm mutability where Lyell had denied it. And naturalization eventually brought Lyell over to Darwin’s side.

Finally, this narrative adds a neglected dimension to Darwin’s context of discovery. Much attention has rightly been paid to the social and economic contexts—capitalist competition and division of labor, Malthusianism—that shaped Darwin’s theory.⁵⁶ But few have recognized that the extraordinary ecological changes wrought by European imperialism were essential to the development of Darwin’s thinking, reflected not only in the content but the very language—“colonies,” “invasions,” “wars,” etc.—of his and his scientific forebears’ theories of natural history and naturalization.⁵⁷ In formulating an historical science, Darwin had to infer the changes of the past from the observable present. But it was not a static present which confronted him. Organic nature was everywhere in flux, not in peaceful stasis but chaotic transformation, transformation triggered by a history of empire but at every opportunity taking on a life of its own.