Charles Lyell's Principles of Geology

Lyell, Darwin and the principle of reasoning in geology

An introduction by Gordon Chancellor

Charles Lyell's Principles of geology (1830-1833) has been described as "perhaps the most important scientific textbook ever written" (Gould 2000a) and was a major influence on Charles Darwin during the voyage of the Beagle. Darwin had been inspired to embark on the voyage by Alexander von Humboldt's Personal narrative (1819-1829). See an introduction to Humboldt's work and the complete text of the book on Darwin Online here. Once Darwin had read the Principles, Lyell¹ eclipsed Humboldt as his lifelong inspiration.

The first edition of Lyell's book appeared at a critical stage in Darwin's early career and played a major role in the emergence of his geological and biological theorising. This introduction is intended to summarise the key points Lyell was arguing and to explore how Darwin responded to them, especially in connection with the species question. The influence of the Principles on the young Darwin has been examined by many scholars and this introduction is indebted to them, as it is to Jim Secord's (1997) introduction to the Penguin Classics edition which explains the political and cultural context in which the book was published.

The foundation for all studies of the Principles remains Martin Rudwick's (1970) analysis of Lyell's strategy and how this determined the structure of his book. Rudwick's magnificent Worlds before Adam (2008) also discusses Lyell's work and geological milieu in great detail.

Captain Robert FitzRoy gave Darwin the first volume of the Principles at the start of the Beagle voyage in December 1831. Darwin had the second volume sent out to Montevideo for him to collect there in November 1832. He read the third volume in May 1834 at the mid-point of the voyage, after the Santa Cruz expedition. At that point, having seen the step-wise elevation of Patagonia for himself, Darwin eagerly read the third volume and declared himself a committed "Lyellian". In late May, in his Banda Oriental notebook(pp. 109-110), he instantly adopted Lyell's new terms for "Primary" rocks and he referred to the Principles many other times through the voyage. In his later career Darwin continued to cite Lyell in his notebooks, publications and correspondence and the second (1845) edition of his Journal of researches is dedicated to Lyell and the Principles.

The Principles is a massive book of approximately 1,300 pages and although elegantly written the first edition is rarely read in its entirety today. When it first appeared it was widely read – by those who could afford it - possibly because it stressed its own wider cultural implications, for example by such luminaries as George Eliot and Alfred Tennyson. In 1830 Lyell was already carving out a reputation as an experienced writer and knew exactly for whom his book was intended. He not only wanted to revolutionise geology; he wanted to be considered as the gentleman expert uniquely qualified by his heroic fieldwork to explain his science to the intelligentsia. Even today the book is rare among nineteenth-century geological treatises in that it remains in print and the Principles is still mentioned by contemporary writers such as A. S. Byatt when imagining discourse among the Victorian literati.

In the Principles Lyell sought to establish what he saw as the correct principle of reasoning in geology. He argued that the history of the Earth's surface – and by extension of the life which has inhabited it – should, as in his subtitle, he explained "by reference to causes now in operation". This in itself was nothing new, being a restatement of the methodology of actualism as espoused by James Hutton in his Theory of the earth (1795) and more lucidly by John Playfair in his Illustrations of the Huttonian Theory (1802) from whom Lyell sourced the quote at the start of the book. The mantra of actualism is "the present is the key to the past" and by 1830 this rather meaningless statement was the accepted starting point of geology, as it is today. In the Principles Lyell's strategy was to show the observed reality and adequacy of present day causes to explain the entire observed geological record, so that such causes would pass the vera causa ("true cause") test as spelled out in Newton's Principia of 1687. Darwin was to emulate Lyell's strategy in his Origin of species of 1859, the early chapters demonstrating the reality of natural selection and testing its ability to explain the origin of species, the later chapters showing its ability to explain the entire history of life (Hodge 2012).

Lyell was not the first to prefer an assumption of gradual change when interpreting Earth history. Neither was he the first to take issue with geologists who challenged that assumption when faced with geological phenomena which seemed inexplicable by observable causes. Humboldt, for example in vol. 3 of his Personal narrative (1818), clearly preceded Lyell in tackling this issue:

When we speak of the origin of caverns, we must choose between two systems of natural philosophy, one of which attributes everything to instantaneous and violent commotions; for instance, to the elastic force of vapours, and to the heavings occasioned by volcanoes; while the other has recourse to small powers, which produce their effect almost insensibly by progressive action. It would be in contradiction to the design of this work, which treats of the laws of nature, to discuss the origin of things, and abandon the small number of facts hitherto well observed, to wander amid vague conjectures. (Humboldt 1818, pp. 145-146)

What set Lyell in opposition to almost all other geologists of his time was his insistence that the "causes now in operation" had always been of the same magnitude in the past as they are today. This was an uncompromising statement of Hutton's belief that the Earth has had a steady state or "uniform" history with "no vestige of a beginning, no prospect of an end" - to paraphrase Hutton - as opposed to a developmental or progressionist history.

The reasons for Lyell's position are complex but we know from his private notebooks that they sprang from his deep-seated fears for an evolutionary history for Man (see Bartholomew 1973 for detailed discussion). At the time he was writing no-one was sure how far human history extended back before the ancient Egyptians and it was a central issue for geologists to find evidence of Man's first appearance. Lyell could see that the transmutation of species was theoretically possible and it was also easy to interpret the fossil record as showing progress from fish to mammals, in other words pointing towards Man. Lyell felt, therefore, that to protect Man from the indignity of a transmutationary origin he had to deny a progressionist interpretation of the history of life. As Secord (1997, p. xxxi) puts it: "no progress, no evolution". Much of the second volume of the Principles is a detailed refutation of transmutation so it is ironic that Lyell's discussion of species played such a significant role in exciting Darwin's interest in the subject during the Beagle voyage. As David Kohn (1980, p. 167, n118) suggests, Lyell's refutation strategy seems to have backfired!

Lyell's advocacy of uniformity may also have been partly a reaction to his Oxford professor William Buckland (1784-1856), who in 1823 had interpreted the "diluvial" deposits of the English Midlands as relics of the Biblical flood. By 1830 very few geologists accepted Buckland's interpretation and by the 1840s the "diluvium" would be reinterpreted as a glacial deposit, but in the Principles Lyell characterized "diluvialists" as "catastrophists" and rejected their explanations as non-actualistic and by implication "unscientific". By conflating actualism with anti-progressionism and anti-catastrophism Lyell was making a logical error and saddling himself with great difficulties when he had to explain geological phenomena which had no obvious present-day comparators.

Lyell's weakness was seized upon by Darwin's Cambridge geological teacher Adam Sedgwick (1785-1873) in his first Presidential Address to the Geological Society in 1831 as an unwarranted distortion of actualism. It is probable that Darwin attended this address as he refers in his Autobiography to hearing Sedgwick at the Society lauding the catastrophist theories of the Frenchman Jean Baptiste Élie de Beaumont (1798-1874; see discussion on vol. 3 below). Sedgwick's colleague William Whewell (1794-1866) in an otherwise favourable 1832 review of the Principles coined the word "uniformitarian" for the Huttonian non-directional view of Earth history. He implied that Lyell was almost the only geologist who held that view, as most geologists could plainly see that Hutton was contradicted by the directional fossil record.

The title page of volume 1 announces that there will be a second volume to the Principles, but while finishing the second volume in 1831 Lyell decided to split it into two and postpone publication of the second half as a third volume. This is why volume 2 has a frontispiece and map which had to wait for volume 3 for explanation. Lyell wanted the book to be read as one long argument, just as twenty-five years later Darwin's three-volume "big book" on species had also been intended. Darwin's book, which was never completed, would have exhausted most readers and would never have had the spectacular impact of his 1859 abridged version, The origin of species.²

There are many ways in which the Origin was a self-conscious successor to the Principles: the two books overlapped greatly in subject matter, had the same logical structure and were based on massive compilations of scientific facts. They were both expensive and aimed at the intellectual élite whose political and religious sensibilities needed to be carefully managed to gain a fair hearing, rather than at the more utilitarian book market. Of course, where they differed most profoundly was in their advocacy – or otherwise – for evolution. It is, however, not entirely co-incidence that both books were published by John Murray and that both authors were Oxbridge educated, Whig, gentlemen geologists sharing the same Christian name.

The first two volumes of the Principles deal seamlessly with Lyell's "causes now in operation". The first lays out the uniformitarian manifesto, before examining causes in the "inorganic" world; the second deals with causes in the "organic" world and interactions between the two worlds. The third examines the geological record in light of these causes and applies the "causes" to a reinterpretation of the Earth's history.

Volume One: The inorganic causes now in operation and Lyell's argument for uniformity

The first volume opens with a lengthy history of geological thought as the prelude to the exposition of Lyell's new science of geology. He traces this history from the earliest "doctrines" and "cosmogenies" to "the era of living authors." Lyell's "history" is in fact highly polemical in respect of his contemporaries and pulls off the trick of linking catastrophist geology with the outmoded "scriptural" geology. In so doing Lyell is implying that his own uniformist views are somehow more enlightened and scientific than those of his catastrophist colleagues.

Lyell commences by claiming that his own fieldwork at Etna and elsewhere proves that the geological record is far more complex than previously thought, with a timescale stretching ever further back. This allows him to argue that the usually small changes recorded in human history should be magnified millions of times in our imagination to explain gigantic features such as volcanoes and coral reefs. By arguing for gradual change over "deep time" Lyell is laying the ground for Darwin's later use of that time in the Origin to explain the power of natural selection:

He who can read Sir Charles Lyell's grand work on the Principles of Geology, which the future historian will recognise as having produced a revolution in natural science, yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume. (Darwin 1859, p. 282)

Lyell attacks the then prevalent view of a cooling Earth and argues that climate change, though real, is a local phenomenon and is merely a "fluctuation about a mean" with warmer periods being due to a high sea to land ratio and vice versa. He concedes that the European "Secondary" (from the Chalk down to the rocks beneath the Old Red Sandstone) seemed to have been warmer than today but he suggests that there may then have been "cooler-looking" strata deposited in parts of the world where there was more land and less sea than in Europe. Darwin refers in his Copiapò notebookin late June 1835 to Lyell's suggestion but says (p. 91) "…from his hypothesis this could not be". This confirms that, although Darwin was a committed "Lyellian" geologist from very early in the Beagle voyage, he was confident enough to challenge Lyell's views when they seemed too extreme.

Lyell accepts that fossil plants and animals were a good index of past climates, as for example the tree ferns of the Coal Measures which indicated a tropical environment, but he is correct to point out that this assumes that the ferns required the same conditions as they do today. In effect he is arguing here for "ecological uniformitarianism". This leads him to propose the existence of a "great year" of changes perhaps lasting millions of years so that at any given place falling temperatures and sea levels will eventually rise again to former levels (the term "great year" was misleading because it implied a regular periodicity which Lyell had not intended). The returning warmer conditions would be accompanied by the life forms which are adapted to those conditions, so that for example corals would grow again in the arctic circle (Lyell 1830, p. 123)³.

Lyell's view of geological processes as fluctuations about a mean is beautifully illustrated in his frontispiece to volume 1, an engraving of the "Temple of Serapis" (actually a Roman market) at Pozzuoli, near Naples, which he had examined first hand in 1828 and is discussed towards the end of volume 1. At the time of his visit the 12m-tall upright columns were smooth to about a 4m height, but above this they had been bored by marine molluscs for an additional height of about 3m. To Lyell this proved that since their erection in the early centuries A.D. the columns had been submerged under the sea, covered by 4m of volcanic debris then bored by the molluscs and later re-emerged as a result of elevation related to activity around Mount Vesuvius. If all this dramatic change can have happened in no more than sixteen centuries with the columns not toppling, it is no wonder that Lyell chose them as the perfect icons for his uniformitarian principles!

Lyell did not realise on his first visit that the "Temple" was actually sinking beneath the sea again but read of it in an 1838 report and referred to it in subsequent editions of the Principles as it strengthened his arguments even further. In fact, the bases of the columns have since re-emerged yet again and show evidence of the barnacles and oysters which grew above their bases. Lyell's image must have had a profound influence on Darwin at the Beagle's first landfall in January 1832 when, barely a month after leaving England, he discovered a seam of white limestone in the cliffs of St Jago in the Cape Verdes composed of marine species identical to those living in the sea nearby. From that moment on, we can trace in Darwin's notes how he began to see himself as "Lyellian", convinced that the Earth's crust is in constant vertical motion with on average a balance maintained between the land and the sea.

Lyell's next challenge was to counter a progressionist view of life history and this gets him into difficult water. He argued that the apparent increase in complexity in the fossil record is an illusion due to the imperfection of the record causing an under-representation of "higher" forms in older rocks. He makes the rash prediction that mammals, well known at that time only from Tertiary rocks, will turn up more abundantly in increasingly older rocks.⁴ Lyell is forced to admit the late arrival of Man but argues that this was a "moral" event and therefore a tolerable exception to his normal actualistic dogma. He is equally awkward when admitting that Man is capable of altering the natural environment, for example by transplanting animals and plants from their native countries, so that here we have a "cause" which has by definition no past comparator.

Lyell describes in detail the present day "aqueous" causes (river and sea action) and "igneous" causes (volcanoes and earthquakes). Many of his examples are taken from his fieldwork in Britain and Europe – as for example at Vesuvius and Etna - but he also makes use of the accounts of other geologists from around the world. The frontispiece to volume 2 of the Principles is an engraving of the Valle del Bove on the flanks of Etna and might more usefully have been included in volume 3 where the volcano is described in detail. He rejects the "crater of elevation" theory of Christian Leopold von Buch (1774-1853) as an unnecessary explanation for the caldera of Palma in the Canaries, but Darwin was not so convinced and carefully considered von Buch's theory in his own book on Volcanic islands (1844). Lyell has a fold-out map between pp. 16-17 of volume 1 showing the volcanic island arcs of the Molucca and Sunda Islands and also of the Greek Islands, neither of which he describes in any coherent way in the text. Both maps are copied from von Buch's Physikalische Beschreibung der Canarischen Inseln (1825).

Lyell explains that, given enough time, all the causes observable today are adequate to explain the entire geological record. He argues that the constructive causes, such as volcanism, are balanced by the destructive causes, such as marine erosion, thus maintaining the Earth's surface in equilibrium. He is able to use historical records of volcanic eruptions, such as Vesuvius in AD79, and accounts of earthquakes, such as Calabria in 1783, to demonstrate that such causes are adequate when extrapolated to explain the elevation and subsidence so clearly shown by geology. In a strange example of "reverse actualism", because there were no known earthquakes from the Baltic region, Lyell mocks the claims of the Swede Carl Linnaeus and others that the land there was emerging from the sea. Lyell had to eat his own words a few years later when he saw the evidence for himself.

In his account of igneous causes Lyell makes abundant use of examples from South America, mostly taken from Karl Ernst Adolf von Hoff's Geschichte der durch Ueberlieferung nachgeweisen natürlichen Veränderungen der Erdoberfläche [History of the natural changes at the Earth's surface] (1822-1824). He includes a summary of the volcanoes along the Andes and detailed historical accounts of the great earthquakes and resulting uplift recorded from Chile and Peru at locations which the Beagle was due to visit, the dates being 1822, 1750 and 1746. In a footnote (vol. 2, p. 265) Lyell expresses hope that the changes of level at the port of Callao in Peru "will soon be examined by our naval officers, and other intelligent persons" and will have a chance to report back on the impact of the 1746 earthquake. In July 1835 Darwin fulfilled Lyell's hope by examining the damage caused by the earthquake and made field notes on some scraps of paper which are now in DAR37.709-710 available here. Darwin summarized these in his Geological observations on South America in 1846.

One can imagine the young Darwin being deeply stirred by Lyell's exposition just as he was starting to explore for himself the geology of the Andes. In July 1836 in one of his last field notes (Despoblado notebook, p. 88b) Darwin refers to Lyell's account of the Andes, the southern limits of which had in 1830 yet to be determined. By the time of Darwin's note, four years after his first reading of Lyell, Darwin had an unrivalled knowledge of the Andes and was certain that they extended at least to Cape Horn.

Volume Two: The organic causes now in operation and Lyell's argument against transmutation

The second volume carries another epigraph from Playfair on its title page which says that change in the animal kingdom over time seems "to be part of the order of nature". This sets the tone for the volume, which carries a dedication to geologist William Broderip (1789-1859) and a preface explaining that contrary to Lyell's original plan there will now be a third volume which will "shortly be laid before the public". In the second volume Lyell discusses the present-day organic world. In effect it deals as much with what would today be called biology and ecology as with geology.

To avoid accepting that the apparent progress shown in the fossil record is real and implies some kind of evolutionary process, Lyell has first to deny that species are "created" by natural modification – the "transmutation" or evolution - of pre-existing species. He starts by examining the limits of variation within species as this is crucial to the question of whether species are "real" entities in nature, rather than imaginary human constructs. It was of course common knowledge that some species were highly variable under domestication, such as dogs and cabbages, and also that some species varied in the wild state, but he asserts the reality of species and argues that variation within a species is strictly limited (Coleman 1962). He also examines the question whether different but similar species can interbreed and concludes that hybrids are generally infertile so are not be able to pass on their traits for more than a few generations. In other words, "species" means something in a biological sense and is not just a convenient label for organisms which happen to look alike. He concludes that species are highly durable entities and there are no grounds for questioning their stability:

On what particular ingredient, or quality in the earth, these changes depend, has not yet been ascertained. But gardeners are well aware that particular plants, when placed under the influence of certain circumstances, are changed in various ways according to the species; and as often as the experiments are repeated similar results are obtained. The nature of these results, however, depends upon the species, and they are, therefore, part of the specific character; they exhibit the same phenomena again and again, and indicate certain fixed and invariable relations between the physiological peculiarities of the plant, and the influence of certain external agents. They afford no ground for questioning the instability of species, but rather the contrary; they present us with a class of phenomena which, when they are more thoroughly understood, may afford some of the best tests for identifying species, and proving that the attributes originally conferred, endure so long as any issue of the original stock remains upon the earth. (Lyell 1832, p. 35)

In the early years of the Beagle voyage there is no evidence that Darwin doubted the "reality" of species, but it seems likely that several things he encountered from very early on would have started him pondering on the issue. His encounters with mice and rats on various oceanic islands, where they were assumed to have been introduced centuries before, may have led him to wonder how much they might have varied and adapted to their new habitats. His encounters with "Indians" in Tierra del Fuego in December 1832 also had a profound effect on his assumptions about what it means to be human. His discovery at Port Desire in December 1833 that there was a second, smaller kind of Rhea "replacing" the better-known northern kind with no obvious change of habitat was a serious challenge to the Lyellian view of perfect adaptation of species to the environment. Hearing in the southern Spring of 1834 of the two sizes of fox on the two main Falkland islands and his realization over several years in South America of the relationships of the purported species of the bird Myothera can only have provoked further reflection on this "reality".

In 1832 Lyell had first, however, to examine and refute what was at the time the only serious attempt at an evolutionary theory, that of the French naturalist Jean Baptiste Lamarck in his Philosophie zoologiquepublished in 1809, the year of Darwin's birth,⁵ and first read by Lyell in 1827. We know from a letter Lyell sent Darwin in 1863 that Lamarck had at first "made a great impression" on him but that he had determined immediately to resist Lamarck's theory because of its implications for Man's history. It is worth noting that Lamarck advanced his theory even more clearly in the introduction to his Histoire naturelle des animaux sans vertebrès (1815-1822), the seven volumes of which Darwin used from the Beagle library and are available in Darwin Online (see Keynes 2000). Curiously Lyell does not seem to have been aware of the improvements made by Lamarck to his theory between the 1809 and 1815 versions and Herbert (2005, p. 297) in an otherwise excellent account mistakenly implies that the Histoire was not in the Beagle library.

Lyell argues that since no-one has ever witnessed the "birth" of a new species by modification of a pre-existing species, they must be highly stable entities. He cites the report by the Professors of the Paris Museum that mummified ibises and crocodiles found by Étienne Geoffroy St Hilaire (1772-1844) in Ancient Egypt are identical to those now living as support for this stability. Lamarck cited the lack of change in the mummified species as evidence that their environment had not changed, but Lyell asks why if this were true haven't the same species changed in countries where the environment is very different? Lyell turns Lamarck's view on its head by taking the mummies as evidence against transmutation. He accepts that species are perfectly adapted to their habitats and argues that in the present state of knowledge it is safer to assume that species are "created" where-ever and when-ever such habitats arise, as proposed many years before by Linnaeus.

Darwin must have been intensely interested to read Lyell's 1832 account of species origins. We know that Darwin studied Lamarck very closely after the Beagle voyage and he made extensive notes on Lamarck in his theoretical notebooks from the late 1830s (Barrett et al. 1987). In Notebook C, p. 119 he says that Lamarck "…was endowed with what may be called the prophetic spirit in science. The highest endowment of lofty genius". We may, therefore, assume that Darwin in fact learnt a great deal from Lyell's exegesis of Lamarck but once Darwin himself came up with a theory of evolution he strove mightily to differentiate his own theory from that of the Frenchman.

Lamarck's theory postulates two levels of processes or factors by which biological forms change over time. The first and higher-level factor is a tendency to progress and complexify through the various grades of organization from, say, jellyfish to molluscs to insects and so on. Lamarck believes that there is no real extinction, only a tendency to perfection as each form mounts the ladder of progress. The second and lower-level factor is adaptation to gradually changing environments, as for example, by stretching further upwards to reach higher leaves giraffes acquire longer necks. Lyell gives a very fair summary of Lamarck's two factors:

…this Nature is daily engaged in the formation of the elementary rudiments of animal and vegetable existence, which correspond to what the ancients termed spontaneous generations. She is always beginning anew, day by day, the work of creation, by forming monads, or 'rough draughts', which are the only living things she ever gives birth to directly.

There are distinct primary rudiments of plants and animals, and probably of each of the great divisions of the animal and vegetable kingdoms. These are gradually developed into the higher and more perfect classes by the slow, but unceasing agency of two influential principles: first, the tendency to progressive advancement in organization, accompanied by greater dignity in instinct, intelligence, &c.; secondly, the force of external circumstances, or of variations in the physical condition of the earth, or the mutual relations of plants and animals. For as species spread themselves gradually over the globe, they are exposed from time to time to variations in climate, and to changes in the quantity and quality of their food; they meet with new plants and animals which assist or retard their development, by supplying them with nutriment, or destroying their foes. The nature also of each locality is in itself fluctuating, so that even if the relation of other animals and plants were invariable, the habits and organization of species would be modified by the influence of local revolutions.

Now, if the first of these principles, the tendency to progressive development, were left to exert itself with perfect freedom, it would give rise, says Lamarck, in the course of ages, to a graduated scale of being, where the most insensible transition might be traced from the simplest to the most compound structure, from the humblest to the most exalted degree of intelligence. But in consequence of the perpetual interference of the external causes before mentioned, this regular order is greatly interfered with, and an approximation only to such a state of things is exhibited by the animate creation, the progress of some races being retarded by unfavourable, and that of others accelerated by favourable, combinations of circumstances. Hence, all kinds of anomalies interrupt the continuity of the plan, and chasms, into which whole genera or families might be inserted, are seen to separate the nearest existing portions of the series. (Lyell 1832, pp. 13-14).

Lamarck's second factor is about how organisms come to be adapted. Darwin's theory was meant to explain the same phenomena, but of course in a radically different manner from Lamarck's vague "influence of circumstances" with his own process of natural selection. Where these two great thinkers differ profoundly is that Darwin does not recognize Lamarck's first factor of a tendency to progress and argues that the entire panoply of life can be explained by extrapolating from the second, albeit supplemented by his principle of divergence. Darwin made extensive marginalia in response to Lyell's summary of Lamarck in the second volume of his copy of the fifth (1837) edition of the Principles. Against the passage quoted above Darwin wrote "very different from my view".

Whilst Lyell's refutation of Lamarck should be read in its entirety, his severest criticism strikes at Lamarck's most serious weakness, namely that his theory is not backed up by factual details:

We point out to the reader this important chasm in the chain of the evidence, because he might otherwise imagine that we had merely omitted the illustrations for the sake of brevity, but the plain truth is, that there were no examples to be found; and when Lamarck talks 'of the efforts of internal sentiment,' 'the influence of subtle fluids,' and the 'acts of organization,' as causes whereby animals and plants may acquire new organs, he gives us names for things, and with a disregard to the strict rules of induction, resorts to fictions, as ideal as the 'plastic virtue,' and other phantoms of the middle ages. (Lyell 1832, p. 8)

Darwin would take Lyell's criticism very much to heart while formulating his own theory over the next thirty years.

Having refuted an evolutionary explanation for the origin of species Lyell provided no clear alternative and left it to future science to decide the question, an obvious spur to Darwin (see for example vol. 2, p. 179). In a sense it was not essential for Lyell to have an explanation for the origin of species. For his geological methodology to stand up it was only necessary that species did come and go in a regular way that preserved the balance of nature. He hypothesized that species are probably created "evenly" in space and time, so he believed it unlikely that oceanic islands such as St Helena, being so small and far from land, would be "centres of creation". Darwin must have hoped to be able to test Lyell's hypothesis if the Beagle should be able, as in the event she was, to call there on her way back to England.

The remaining early chapters of volume 2 deal with the geographical distribution of animals and plants, largely based on James Prichard's (1786-1848) Physical history of mankind (1813). Prichard followed Linneaus's view that all the races of Man belonged to a single species: Homo sapiens – this was by no means a universally held view - and that like other species Man had a single origin at a single locality, possibly in a single pair (i.e., "monogenetic"). Lyell follows Prichard in stressing that many organisms, no matter where they are "created", have remarkable powers of dispersal which allow them to migrate and track changes in climate and sea level. With hindsight we cannot avoid reading his discussion of the many interactions between organisms, the "struggle for existence" (a phrase he uses on p. 56 of vol. 2) and so forth, as at times foreshadowing Darwin's subsequent arguments for natural selection. Lyell accepts Prichard's belief in the "single point" origin of species and builds on this by adopting Augustin Pyramus de Candolle's (1778-1841) ideas on how plants disperse from such points to create botanical provinces. Lyell had met de Candolle in Geneva in 1820 and could see how his ideas could be integrated with his own view of shifting climates and land areas.

Lyell was here inventing the new science of historical biogeography which laid the foundation for much of Darwin's thinking on the migrations and interactions of species. Lyell cites Georges Buffon's (1707-1788) late eighteenth-century observation that animals and plants, although clearly adapted to the climates they inhabit, are not the same species in Africa and South America, even though their environments are comparable. This point was not lost on Darwin, who instantly recognized in his Galapagos notebook the "American" appearance of the species on the Galapagos when he might have expected the species to be more similar to those of the closely comparable Cape Verdes which had "African" species. Darwin's earliest of at least nine mentions of the Principles in his Red Notebook ­– opened on the voyage and the first of his theoretical notebooks – is a reference to Lyell's biogeography on p. 52, datable to late July or early August 1836.

Lyell next examines the extinction of species which he argued must take place by some natural law to maintain the balance of nature. He cites the only proven total extinction of a species in historical times – the dodo in 1755 – and perhaps misses the irony that this was also the first proven extinction caused by Man. He examines the ideas of the Italian Giovanni Brocchi (1772-1826), whose studies of fossil molluscs from the Apennines (Conchiologia fossile subappenina) had suggested in 1814 the idea that species have fixed life spans. Brocchi was trying to explain why fossil species seem to go extinct in a regular fashion as one follows a rock sequence upwards and his idea was that a species would go extinct once its powers of reproduction failed. Lyell rejected this idea as not being a vera causa but he did leave the door open to further research. This did not stop Darwin from rethinking the whole question for himself when he was struggling to explain the extinction of the fossil mammals he had unearthed in Argentina and Uruguay.

Darwin was "much struck" (Origin p. 1) by the various species of large fossil mammals which were no longer found alive but seemed obviously related to living South American species such as the armadillo and the llama. He could not understand how these extinct animals were dug out of deposits which seemed to have been laid down gradually in the same way as dead cattle he saw being covered by mud in the Rio Plata. This convinced him that the extinctions could not easily be explained as due to a change of habitat, as Lyell would have suggested. As Darwin says in the Origin he was "astonished" at finding a horse's tooth in the same deposit with an extinct animal. Why would a native horse have died out when those introduced by the Spanish only three centuries ago had run riot and were obviously very well adapted to the Pampas?

Having pondered Lyell's discussion of extinction for over two years, in February 1835 Darwin wrote a note: 'The position of the bones of Mastodon (?) at Port St Julian is of interest' containing the following passage:

With respect then to the death the of species of Terrestrial Mammalia in the S. part of S. America. I am strongly inclined to reject the action of any sudden debacle.— Indeed the very numbers of the remains render it to me more probable that they are owing to a succession of deaths, after the ordinary course of nature.— as Mr Lyell supposed Species may perish as well as individuals; to the arguments he adduces. I hope the Cavia of B. Blanca will be one more small instance of at least a relation of certain genera with certain districts of the earth. This correlation to my mind renders the gradual birth & death of species more probable. DAR42.97-99

Darwin is here quoting "the gradual birth and death of species" from Lyell's vol. 3, p. 33, whereas Lyell has dropped the idea of species life spans in favour of piecemeal extinction due to changing environments. We can see that the student, with almost two years of the voyage still to go, is already challenging the master's opinions on the matter of species lifespans (for further discussion see the introduction to the Geological diary here).

The second half of volume 2 is taken up with discussion of the causes now in operation in the inter-relationships between the organic and the inorganic world. Lyell examines in great detail the processes of fossilization, demonstrating that aquatic organisms are far more likely to be preserved than terrestrial – so Darwin was all the more lucky to have found his mammal fossils. Lyell also argues that animals with calcareous shells, such as marine molluscs, have the best chance of preservation and will be of most use to geologists. This view is especially important for his thesis on geochronology in the third volume.

Lyell's final chapter is devoted to a discussion of coral reefs and the formation of limestone, matters of great interest at the time not only to geologists but also to hydrographers, to the extent that the Beagle's orders included the need to investigate coral atolls. It was known that reef organisms could only grow near the surface of the sea so it was puzzling that there were so many atolls in the Pacific and Indian Oceans which were mostly of unfathomable depth. Lyell subscribed to the majority view that atolls grew up on the bases of sinking volcanoes but it seemed to him extremely unlikely that all the volcanoes in an island chain would have risen to exactly the same height.

Darwin was eager for the chance to see atolls for himself to test Lyell's ideas. In the event his abundant observations of elevation across South America suggested to him that perhaps the Pacific seafloor was subsiding in compensation. This led to his theory that the Earth's crust was broadly divided into regions of uplift and regions of subsidence, probably driven by igneous forces. Darwin's theory was truly Lyellian and the older man was delighted when Darwin explained it to him on his return to England. Atolls were a clear marker of subsidence and the last stage in the sinking of volcanic islands which once were fringed by reefs. Since there were almost none in the Atlantic Ocean it was probably a region of elevation and regions of stability in the tropics might develop barrier reefs, as they do off the east coast of Australia. Darwin's insights led to his first major monograph, The structure and distribution of coral reefs (1842), a classic of scientific methodology and the prototype for his great work, The origin of species.

At the end of volume 2 there is a folded map of Europe, at the time the geologically best-known continent, based on a map prepared by the Austrian geologist Aime Boué (1794-1881). The map shows the extent of the areas which have at some point been submerged under water "since deposition of the older Tertiary strata". Lyell explains that it is obviously impossible to assess with any confidence which areas now under the sea were dry land during the same time interval. Lyell's main point is that the land and sea are in constant exchange due to uplift and subsidence caused by movements of the Earth's crust. These changes cause the climate to fluctuate and in turn these drive migrations of flora and fauna. A corollary to this is that since the climate seems to have been warmer in the Secondary (older) period sea level was correspondingly higher then with more marine deposits than in the Tertiary.

Around February 1835 Darwin wrote another important essay titled "Reflections on reading my geological notes" (DAR42.93-148, transcribed in Darwin Online here). The essay, which provides a narrative framework for the history of life in South America, has been discussed by Hodge (1983) and Herbert (1995) and was first drafted in mid-1834. It includes a note to himself to compare Lyell's map "for extent and size" with the later Tertiary elevations he has mapped in Patagonia.

Volume Three: Lyell's attempt to explain the former changes in the Earth's history by reference to present-day causes

The first and second volumes of the Principles sit together naturally as discussions of the "causes now in operation" on the Earth's surface, in other words they set out Lyell's "principles" of geology. The third volume is more comparable to a modern geology textbook in that it deals with the history of the Earth, albeit heavily biased towards the Tertiary or more geologically recent periods. Lyell justified this bias on the grounds that in the previous volumes he was more concerned with methodology so could now only afford to deal slightly with the older Secondary, Transition and Primary periods. Volume three also "feels" different from the previous two in that it is far more descriptive and is illustrated not just by full-page engraved plates but by no less than 93 smaller woodcut maps and diagrams. The volume segues from the previous two by reiterating the need for a full grasp of present-day processes, then embarks on a journey through the geological record, working backwards in time from the known to the unknown in decreasing detail, applying the "principles" established earlier.

The volume opens with a dedication to Roderick Murchison (1792-1871), the Scottish ex-soldier and geologist who had accompanied Lyell on parts of his geological tour of France and Italy in 1828, followed by an engaging twelve-page autobiographical preface. This preface explains how Lyell's findings on the geological tour caused him to refine his planned four-fold division of the Tertiary, with type localities in descending age order – the first and youngest at Ischia/Val del Noto, the second in the Subappenines, the third at Superga in northern Italy and the fourth and oldest in the London-Paris basin. He describes his collaboration with the French and Italian conchologists and explains that the delays in publication of volume 1 provided the opportunity in the summer of 1829 to examine the Crag of East Anglia and to revisit Paris in 1830. In 1831 he decided to extend the Principles to three volumes and in 1832 he went to Germany and Switzerland and delivered his first course of lectures at King's College.

Probably the most important aspect of Lyell's European fieldwork was his detailed examination of Mt Etna, in Sicily, the largest active volcano on mainland Europe. Etna was beautifully illustrated in the frontispiece to volume 2 and Lyell's fieldwork combined with his studies of the historical record of its eruptions convinced him of the vast timescale of geology. It was also critical to shaping his views concerning the so-called "Primary" rocks which at the time were still generally thought to be older than strata of Secondary age. There were, however, instances of granite and other such "Primary" rocks intruded into and therefore younger than Secondary deposits. In the Val di Noto south of Etna he found enormous beds of limestone elevated above the sea and full of fossils of recent species of molluscs. These limestones were in close contact with "Primary" volcanic rocks, proving once and for all that the term "Primary" should be dropped altogether, as discussed in his penultimate chapter.

The first five chapters explain the principles of dating rocks and of reconstructing geological history. Lyell discusses the principle of superposition, by which geologists can usually assume that the strata at the top of a sequence are the youngest, since they were laid down most recently on top of the older beds. He shows that superposition at best will only show relative ages of rocks and he also argues that similar rock types (lithologies) in different places indicate similar ages but cannot be relied on for dating. He argues that marine invertebrate fossils are the best indicators for dating rock strata because they are well preserved and are found over wide areas. Fossil mammals in terrestrial deposits are far less reliable because they are rare and fragmentary and the species come and go too quickly. He also argues that this may be due to them having a more rapid physiology and less resistance to environmental change.

In a very important section linking straight back to the second volume, Lyell explains why we should not expect the fossil record always to be continuous but should be very cautious about interpreting the discontinuities as catastrophes. To illustrate his point he uses the imaginary archaeological metaphor of finding two buried cities at the base of Vesuvius, the lower one being ancient Greek, the higher one being Italian, judging from inscriptions. Lyell warns against any hasty interpretation that the Greeks abruptly became Italians. He says that later excavations might reveal a Roman city at an intermediate depth between a Greek and an Italian city, proving that Vesuvius does not erupt at regular intervals and that the transition from Greek to Roman had been gradual. The passage is worth quoting in full:

Suppose we had discovered two buried cities at the foot of Vesuvius, immediately superimposed upon each other, with a great mass of tuff and lava intervening, just as Portici and Resina, if now covered with ashes, would overlie Herculaneum. An antiquary might possibly be entitled to infer, from the inscriptions on public edifices, that the inhabitants of the inferior and older town were Greeks, and those of the modern, Italians. But he would reason very hastily, if he also concluded from these data, that there had been a sudden change from the Greek to the Italian language in Campania. Suppose he afterwards found three buried cities, one above the other, the intermediate one being Roman, while, as in the former example, the lowest was Greek, and the uppermost Italian, he would then perceive the fallacy of his former opinion, and would begin to suspect that the catastrophes, whereby the cities were inhumed, might have no relation whatever to the fluctuations in the language of the inhabitants; and that, as the Roman tongue had evidently intervened between the Greek and Italian, so many other dialects may have been spoken in succession, and the passage from the Greek to the Italian may have been very gradual, some terms growing obsolete, while others were introduced from time to time. If this antiquary could have shown that the volcanic paroxysms of Vesuvius were so governed as that cities should be buried one above the other, just as often as any variation occurred in the language of the inhabitants, then, indeed, the abrupt passage from a Greek to a Roman, and from a Roman to an Italian city, would afford proof of fluctuations no less sudden in the language of the people. So in Geology, if we could assume that it is part of the plan of nature to preserve, in every region of the globe, an unbroken series of monuments to commemorate the vicissitudes of the organic creation, we might infer the sudden extirpation of species, and the simultaneous introduction of others, as often as two formations in contact include dissimilar organic fossils. But we must shut our eyes to the whole economy of the existing causes, aqueous, igneous, and organic, if we fail to perceive that such is not the plan of Nature. (Lyell 1833, pp. 33-35)

Darwin extends Lyell's metaphor in his chapter on the imperfection of the fossil record in the Origin:

For my part, following out Lyell's metaphor, I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated formations. On this view, the difficulties above discussed are greatly diminished, or even disappear. (Darwin 1859, pp. 310-311)

Lyell's reason for discussing dating methods in detail is to clear the ground for his proposal for quantitative dating of rocks from around the world. He explains that as we work downwards from the most recent strata the suites of fossil species gradually become less similar to the suites of species living today. If we assume a uniform average rate of change of species, we can estimate theage and duration of the strata by calculating the percentage of still-living species in each bed. Here we see why Lyell in the first two volumes has gone to such lengths to argue for uniformity through geological history: only if species come and go uniformly can his method be relied on to provide a fossil "chronometer". This may also partly explain his aversion to transmutation as he needs species to be stable throughout their term of life.

In order to develop his proposed dating methodology Lyell collected thousands of Tertiary bivalves and gastropods across Europe and had them identified by the French palaeontologist Gérard Paul Deshayes (1795-1875) with whom he had worked closely in Paris in 1830 (most of Lyell's collections are now in the Oxford Museum of Natural History). Examples of these molluscs are illustrated in the four plates which appear at the start of volume 3. The fifth plate shows microfossils which Lyell calls cephalopods which are in fact unicellular foraminifera.

What is perhaps revolutionary about Lyell's dating system is its use of whole suites of fossil species, rather than the standard method used by geologists then (and now) of relying on one or a few "index" species for correlating rocks in different places. In effect he is using what might today be termed a "palaeoecological" approach to dating rocks. Obviously, the Tertiary is the easiest time period over which to develop Lyell's new dating system as it allows living species to be used to "calibrate" his fossil chronometer. Likewise, Lyell describes Etna in so much detail partly because so many of its eruptions and lava flows are recorded in human history, providing a unique opportunity to calibrate the rocks of the most recent past. In theory Lyell's method can be extended backwards - provided there are fossils available - by "recalibration" at various earlier points.

Lyell is aware of the massive discontinuity between the fossils of the Tertiary and the Secondary, at which point he would struggle to extend his dating system as there would be no present-day species for comparison. This is surely why the bulk of the third volume is given over to reconstructing the history of the Tertiary with such scant attention paid to the Secondary, although Lyell claims (vol. 3, p. 324) that this is because he wants to avoid writing a fourth volume. He seems always ready to turn a "catastrophic necessity" into a "uniformitarian virtue", however, and suggests that – since species he assumes change at a uniform rate - the Tertiary/Secondary boundary instead of showing a dramatic change of fauna and flora at that time, actually proves that a whole vast chunk of time must be missing from the record. Since he knows that it has taken the entire Tertiary for present-day species to dwindle to zero, the time gap must be at least as long as the Tertiary!⁷

Lyell then proposes his new arbitrary division of the Tertiary based on his method, as follows: The Newer Pliocene being the most recent with c.90-95% of present-day species, the Older Pliocene with c.35-50%, the Miocene with c.18% and the Eocene with c.3%. In other words, by the time we "get back" to the Eocene, c.97% of today's species have disappeared.

Lyell then commences a detailed account of the European Tertiary using his new divisions.⁸ Chapters six to eleven are a detailed analysis of the Newer Pliocene. Lyell shows that this period was one of continuous, gradual change and he dismisses claims that the "diluvium" is evidence of any global catastrophe such as the Biblical flood. His first-hand researches at Etna show a great mountain built on top of limestone containing mostly recent species of shells. What clearer demonstration could one ask for of how present-day causes, as witnessed in the historical record, can gradually build mighty edifices provided we grant the almost unimaginable vastness of geological time. The fact that the limestones are thousands of feet above sea level also proves the power of gradual elevation - due presumably to the swelling of lava beneath the volcano - and negates any requirement for catastrophic "crater of elevation" explanations. He also pays special attention to the cliffs and valleys eating into the volcano, exemplifying the continuous interplay of constructive igneous causes with destructive aqueous causes.

Lyell next discusses Vesuvius and some of the other Italian volcanoes, then examines the evidence for elevation in the Andes, some of which Darwin would soon have the opportunity to examine. Of particular interest were the "parallel roads" or terraces of Coquimbo in Chile. Captain Basil Hall (1788-1844) initially attributed these in his Extracts from a journal (1824, v.2, p. 9) to the step-wise draining of a lake which had been dammed at the valley entrance. This was the identical explanation then current for the parallel roads of Glen Roy in Scotland, except that Glen Roy did not face directly into the sea.

Lyell was confident that the roads at Coquimbo had once been sea beaches, proving the elevation of the coast, and when the Beagle arrived there in May 1835 Darwin seized the opportunity to examine them carefully. In his Coquimbo notebook he drew several maps and diagrams of the roads (e.g., p.129) and in his Geological diary (DAR 37.662-670) agreed totally with Lyell's marine explanation. On  his return to England Darwin declared his support for Lyell's explanation in his Journal of researches (1839) and made it a priority to demonstrate the same explanation for Glen Roy.⁹

This leads Lyell to consider the Newer Pliocene terrestrial and freshwater deposits, such as cave breccias and alluviums. He has to explain the mysterious giant "erratic" boulders encountered around the Alps which are far too large to have been transported by any river. Lyell postulates that they were dropped from icebergs as they are known to do today in the Arctic, but it had been argued by Élie de Beaumont that a "paroxysmal" elevation of the Andes could have created a great "diluvial wave" sufficient to deposit the erratics. Lyell explains that while marine icebergs cannot have floated over to the Alps, valleys downstream of the glaciers there, if dammed for any reason, could be big enough to have icebergs. If such dams burst, the flood waters could flow far enough to explain the erratics, thus doing away with the need for any catastrophe. Lyell is of course writing a few years ahead of Louis Agassiz's (1807-1873) glacial theory which proved that these erratics were dropped by glaciers during an Ice Age which post-dates the Newer Pliocene. Darwin had already seen icebergs when he read Lyell's account and would have been eager to assess their power to move such large boulders.

Moving further back in time Lyell next considers the Older Pliocene strata of the Subappenines, as studied by Brocchi, and elsewhere around the Mediterranean, before reporting his findings from the Crag of Norfolk and Suffolk. As before, he then considers the volcanic rocks thought to be of this period and gives special attention to the well-preserved extinct cones around Olot in Catalonia, as depicted in the frontispiece to the volume, which he had examined in the summer of 1830. He also describes those of Eifel as reported by Leonard Horner (1785-1864), his son-in-law and later one of Darwin's closest friends.¹⁰ These volcanic districts were very important for Lyell as he was seeking evidence against the views of some influential geologists that there was in Europe a clear distinction between those of "antediluvian" as against "postdiluvian" date.

The preceding period is the Miocene, the marine deposits and volcanics of which age are treated very briefly before Lyell's account of the Eocene in chapters 17-20. He begins with the freshwater deposits of central France, then moves up to consider those of the Paris Basin, the geology of which had been made famous by Cuvier and Brongniart's classic "catastrophist" memoir of 1811. Cuvier believed that the freshwater deposits alternated with marine deposits and he hypothesized that there had been a regular cycle of "révolutions" and marine inundations. Constant Prévost (1787-1856) had more recently re-interpreted all the deposits as those one might expect to accumulate in a gulf or estuary where the depth and salinity would fluctuate, so removing Cuvier's need for cycles of elevation and subsidence. Cuvier had argued that the extinction all the fossil mammal and bird species in the deposits required a catastrophic explanation, but Lyell countered that since only c.3% of the fossil molluscs in the deposits are present-day species, the extinction of the other c.97% can easily be explained by the passage of time with no need for catastrophes.

Lyell's next chapter deals with the volcanics of central France, as studied by his contemporary the political economist George Poulett Scrope (1797-1876). Scrope had seen Vesuvius erupt in 1817/1818 and this had inspired him to reinterpret the extinct volcanoes of the Auvergne (known as "puys") with a strict actualistic methodology. One of his objectives had been to decide whether the older lavas were separate from the newer lavas and to assess claims that the older lavas showed evidence of a diluvial episode. Lyell was deeply impressed by Scrope's Memoir of the geology of Central France (1827) which showed no evidence of any such dramatic erosive event.

With Murchison, Lyell carefully checked Scrope's fieldwork and agreed with his gradualistic interpretation of the erosion of the river valleys. In the Principles he said there was no proof that any of the Auvergne volcanoes were younger than Miocene, although some looked to him identical to the much younger ones on the flanks of Etna (in fact the youngest French volcanoes are now known to be only a few thousand years old). Lyell described the preservation of some of the puys as just as fresh as those in Catalonia proving that appearance was not a reliable guide to age. More importantly, however, none of these volcanoes showed any evidence of the erosional damage needed by supporters of Élie de Beaumont's neodiluvial catastrophism. Lyell supplements his discussion with a short essay on the Biblical flood aimed at banishing appeals to scripture in geology. He stresses the fresh appearance of the French and Spanish volcanoes as sufficient to demolish the "diluvial wave" theory.

Rudwick (1974a) has concluded that Lyell owed Scrope a considerable debt for promoting an actualistic methodology and for supporting his belief in an enormous age for the Earth. Rudwick argues that some of Scrope's highly original geology had been inspired by his anti-Malthusian, anti-Corn Law political economics. He suggests that Scrope needed to extend the geological timescale to explain the complex history of eruption and erosion in the Auvergne. He did this by "making drafts on the bank of time", an approach which came straight from the economic views he later expressed in his Principles of political economy (1833).

Rudwick argues that Scrope's economics were basically Ricardian in that he believed credit should be freed up by the banks to relieve rural poverty, so the analogy of time in geology and credit in economics seems clear. Scrope had applied a probabilistic approach to his work on human populations then re-applied it to the "populations" of Auvergne volcanoes. Lyell then applied this approach to his "populations" of Tertiary mollusc species. There are parallels here to Darwin's later application to natural history of the ideas of political economists such as Adam Smith and Thomas Malthus which led to his discovery of natural selection in 1838 (Radick 2009).

Lyell concludes his Eocene section with an account of the deposits from elsewhere in Europe, including those at Monte Bolca in the Italian Alps with their exquisite fish and insect fossils, and from the London and Hampshire basins.

Lyell then refers in chapters 21 and 22 to the geological map of southeast England which appears at the start of the volume. The point of this map is to show the structure of the Weald of Kent and Sussex, which is an anticline (upwards fold) of Chalk and older Cretaceous rocks, with Tertiary deposits on its flanks. Lyell's point is that all the Cretaceous rocks must originally have been lain horizontally on the sea floor and since – that is during the Tertiary – gradually uplifted and folded perhaps by forces linked to the raising of the Alps in southern Europe. The Weald therefore provides a natural laboratory for estimating the time it has taken for the Chalk and older strata running east/west along the crest of the upfold to have been gradually eroded to their present condition. Lyell has a particular interest in demonstrating that there has been ample time available for this uplift and erosion: he wants to refute the suggestions of catastrophist geologists like Buckland who claim the Weald as evidence of "paroxysmal elevation" and greater forces at work in the past.

In theory, by estimating the rates of present-day erosion of the rock types present in the Weald, it should be possible to extrapolate backwards to estimate the time since the youngest Chalk deposits were laid down, in other words an estimate of their ages. At this point it is important to note that Lyell assumes that the Weald has been mainly eroded by the sea, basing this (erroneous) assumption partly on his view that the Chalk escarpments are "the exact likeness" of sea cliffs. Lyell makes no attempt to quantify the time available, but presumably this is because it was the principle of gradual and uniform change that he wanted to prove rather than an estimate of actual years. Darwin, who came to live on the North Downs within a few years of the end of his voyage, was well placed to provide such an estimate in the Origin. Lyell must have been delighted with Darwin's wildly optimistic first estimate of over 300 million years, more than enough to explain the Tertiary fossil record!¹¹

Lyell then moves on to his somewhat cursory treatment of the Secondary, or the stratified rocks older than the Tertiary. The oldest group he terms the "Carboniferous" but this includes virtually all known older stratified rocks which within a few years would be split off as separate by his colleagues Sedgwick, Murchison and others. He contrasts the Tertiary and the Secondary rocks, the former he says showing, at least in Europe, a more terrestrial aspect, the latter being more marine, a difference he attributes to changing geography. Not surprisingly the Secondary rocks, being older, also display more tectonic complexity. Chapter 24 is a discussion of the relative antiquity of mountain chains which is actually a lengthy polemic against the paroxysmal theory of Élie de Beaumont. Lyell says de Beaumont's violent theory is simply unnecessary if one accepts the vast amounts of time available to build even the largest mountains by gradual causes.

The final two chapters of the Principles deal with the so-called Primary rocks. As previously mentioned Lyell points out that Primary rocks are still being formed today and are not always ancient. He rejects the name "Primary" in favour of "Hypogene" (i.e. nether-formed) and he divides these rocks into "plutonic" which are unstratified and formed from molten rock (such as granite) and "metamorphic" which are stratified and formed by modification of previously stratified sediments (such as gneiss). Since metamorphic rocks are formed from older rocks they are still likely to be relatively old, but not always older than Secondary. Darwin had every opportunity to examine rocks of all these types in the Andes and he readily adopted Lyell's new terms.

Lyell concludes with a final reference to Hutton and says it is unfair to label either of them as eternalistic. They never claimed, he says, that time had no beginning and will have no end, only that specifying the limits of time is beyond the power of science. He signs off by reassuring his more religious readers that they need not fear that geology will take them down a faithless path:

… in whatever direction we pursue our researches, whether in time or space, we discover everywhere the clear proofs of a Creative Intelligence, and of His foresight, wisdom, and power (Lyell 1833, p. 384).

Darwin signed off the Origin (1859, p. 490) with his famous passage about "…life with its several powers, having been originally breathed into a few forms or into one…". Within weeks he hastily inserted into the second edition, after "…breathed…", the phrase "…by the Creator…". We do not know exactly why he made this insertion, but perhaps he had been reminded of Lyell's reassuring last sentence.¹²

The rest of volume 3 of the Principles is given over to tables, the first showing the relations of all the classes of rocks in all geological periods, with a diagram, the second showing the order of superposition of European sedimentary rocks. These are followed by a massive 52-page appendix listing the fossil shells collected by Deshayes and Lyell's quantitative analysis of the Tertiary, then a shorter appendix listing the fossil shells collected by Lyell and identified by Deshayes. Finally, there is an excellent 23-page glossary of geological terms intended for the "general reader".

Gordon Chancellor, 2021

RN5

NOTES

1 Sir Charles Lyell (1797-1875) was a Scottish barrister by profession who switched to writing and geology from 1827. Darwin regarded him as the person to whom the science of geology was most indebted and the two men became lifelong friends. After graduating from Oxford he took advantage of the post-Napoleonic peace to travel in Europe between 1818 and 1830, in which year the first volume of his Principles was published. Secord (1997) discusses the political tensions in Lyell's early life and shows how his opposition to "révolutions" in geology reflects the need for stability in Britain during the Age of Reform.

From 1831 to 1833 Lyell was professor of geology at King's College, London and was president of the Geological Society in 1836 and 1850. In 1838 he published Elements of geology which was essentially vol. 3 of the Principles revised for a wider audience. Darwin hated the size of the sixth edition of this book and famously broke it in two to make it easier to hold! Lyell visited America in 1845 and 1849 and wrote two books based on his experiences there and in 1863 he published Geological evidences of the antiquity of Man. This book was a great disappointment to Darwin because it failed to give unconditional support for the theory of evolution.

The Principles went through 11 editions up to 1872 of which Darwin owned the first, fifth to seventh and ninth to eleventh. It was only in the tenth edition that Lyell eventually accepted evolution in full. The first edition has been reprinted by Wheldon and Wesley in 1970 and by Chicago University Press in 1990-1991, in both cases with introductions by Martin Rudwick. See Wilson (1972) and van Helvert and van Wyhe,  Darwin: A Companion, p. 193 for further details. In 2020 access to Lyell's notebooks and a great deal of his other material was ensured by a campaign organized by the University of Edinburgh, where they are now held.

2 Darwin's "big book", written in the second half of the 1850s, was never published in its entirety. The surprise arrival of Alfred Russel Wallace's letter in June 1858, independently outlining a theory of natural selection, led Darwin to publish a condensed "abstract". Darwin's "big book" as described to Asa Gray would have had three volumes, of which the first was eventually published as the two-volume Variation under domestication in 1868. The unfinished second and third volumes were published as Natural Selection by Robert Stauffer in 1975. Darwin's "abstract" which appeared in 1859 was of course The origin of species.

3 In 1830 the geologist Henry De la Beche ridiculed Lyell's prediction in his cartoon "Awful Changes". The cartoon shows a future "Professor Ichthyosaur", having returned to a now warmer England where his ancestors last swam in Jurassic times. The professor is lecturing by the sea to an audience of other marine reptiles and pterosaurs, against a background of exotic plants (there are no dinosaurs as these would not be "invented" until 1842, by Richard Owen). He is demonstrating that a fossil human skull has very feeble teeth and that humans must therefore have had great difficulty procuring food!

4 In 1818 Georges Cuvier (1769-1832) had identified an "opposum" - like jawbone from the Secondary (Jurassic) Stonesfield Slate of Oxfordshire, but by 1830 it was obvious that such mammals were very rare compared to reptiles in these earlier rocks. Cuvier was a French zoologist and anatomist who made major contributions to comparative anatomy, geology and vertebrate palaeontology. He became famous for his ability to pronounce the lifestyle of any fossil vertebrate on the basis on a single tooth. He proved that Lamarck was mistaken in believing that extinction was only an illusion and on the evidence of mummified cats and ibises from Ancient Egypt being identical to those of the present day he became implacably opposed to the idea of evolution. His major publications include Recherches sur les ossemens fossils des quadrupèdes (1812), La règne animal (1817) and Discours sur les revolutions de la surface du globe (1826). The "Discours préliminaire" to the "Recherches…" (1812) was translated as "Theory of the Earth" by Robert Jameson (see note 5).

5 The theory of evolution of Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (1744-1829) was laid out in its greatest detail in his Philosophy zoologique (1809). Although sometimes both logically inconsistent and inadequately explained at many points, it is undoubtedly the first complete theory of evolution and the only academically respectable one before Darwin's. Lyell's excellent analysis – and refutation - of Lamarck's theory in the Principles was by far by the best available and was therefore the route by which almost all nineteenth-century Anglophone scientists encountered Lamarck. The major features of Lamarck's theory can be summarized as follows:

At the higher level there is a continuous tendency for animal life to progress upwards. There is continuous spontaneous generation of the simplest forms from raw chemicals by the direct action of fire, water and so on (Lamarck rejected Lavoisier's "new" chemistry). As the organism is subjected to fluid action, channels are carved which modify and complexify the organism and its offspring to commence an upward "climb" to perfection. Once the animal develops an exoskeleton (at around the sea urchin stage) the fluid action becomes internal. All steps on the ladder are occupied at all times so that it becomes an escalator, with Man at the summit. Lamarck explicitly states in his Histoire naturelle des animaux sans vertebrès (1815) that the upward tendency to complexify is the most important process of organic change.

During preparation of the Philosophie zoologique Lamarck made some very significant changes. In the first volume he presented the linear series described above, but he added an appendix to the second volume which changed that linear series into possibly the first branching evolutionary diagram ever published (Gould 2000b). In the appendix version Lamarck shows two branches, the first off the main series for the insects, spiders and crustacea, the second near the top with the reptiles splitting into birds and mammals.

At a lower level there is adaptation to environment which disrupts the higher-level tendency and causes offshoots from the ladder. Lamarck was an extreme uniformitarian who believed, as did Hutton, that the land and the sea are in a never-changing cycle of creation and destruction so that at any given locality the environment will gradually change. This causes pressures on animals to change their habits and adapt, so that form follows function, as the giraffe was supposed to acquire a longer neck by stretching higher. By the inheritance of these acquired characters (the "blacksmith's sons" inherit his strengthened arms as in the folk wisdom of Lamarck's day) new forms gradually evolve. This part of Lamarck's theory is basically the same as Darwin's, although for Darwin adaptation was caused by selection pressure with "soft inheritance" being of relatively minor effectiveness. They were both gradualists and functionalists, believing that form follows function rather than the other way round, without denying the constraints of form on adaptation.

Although we know Darwin held Lamarck in great esteem, he always distanced himself from the Frenchman, much to Lyell's irritation as he also had great admiration of Lamarck as a scientist (see CCD 13, p. 411). Darwin claimed that he learned nothing from Lamarck and only grudgingly acknowledged him as a predecessor in the historical introduction to the Origin (third edition). We know, however that he was exposed to Lamarck's theories early in his career when he came under the sway of Robert Grant (1793-1874),⁶ perhaps the best-known Lamarckian in Britain, as a seventeen-year-old at Edinburgh University. Secord (1991) has shown that Robert Jameson (1774-1854), Darwin's much-maligned geology professor in Edinburgh, was also sympathetic to Lamarck, as he made clear in the preface to the fifth edition of his translation of Cuvier's Essay on the theory of the earth (1827). We also know that Darwin made notes on Lamarck's Systeme des animaux sans vertebrès (1801) while in Edinburgh (these are in DAR5, f.28 and transcribed in Darwin Online here). Corsi (2021) has shown the crucial importance of information exchange between Edinburgh and the Continent in the 1820s in giving British naturalists access to the latest thinking on the history of life.

Gould (2000b) has shown that on the evidence of Lamarck's manuscript notes in his own copy of the Systeme he may have planned a second edition which was superseded by his later publications. The notes show that Lamarck, having heard a lecture on worms by his colleague Cuvier, was increasingly unhappy about Linnaeus's grouping of the "vermes" ("worms") and was starting to experiment with a new classification. By the time he published his Philosophie zoologique in 1809 Lamarck had not only split the "annélides" off from the "vermes" but converted his "ladder of progress" into the first published branching tree of life (see above). In the Histoire naturelle he took this a stage further, with two separate branching trees – one for "inarticulate" animals, the other for "articulate" animals. His revised view was available to Darwin on the Beagle in the Histoire naturelle (1815, vol. 1, p. 457, available in Darwin Online here).

As Gould (2000b) first pointed out, Lamarck in his last book completely reversed the priority that he had always insisted for his primary factor of upward complexification over his secondary factor of environmental influence. In his rare Système analytique des connaissances positives de l’homme of 1820 – published around the time he went blind - he stated clearly that he now realized that there was only one branching tree of life, not the two he proposed in the 1815 first volume of the Histoire Naturelle. Lamarck now stated that all animals derived from the monad, giving rise to infusorians, then to polyps which then branched into all the other ‘classes’. This of course is essentially the same as Darwin’s later ‘descent with modification’. In what may be the greatest irony in the history of science, Lamarck’s new model seems to have been completely missed by everyone, including Lyell, and it seems from Darwin’s summary of Lamarck’s theory in the third edition of the Origin that he never heard of it either.

Although Lamarck never saw the power of natural selection, his last words on evolution show that he now saw the influence of circumstances as the primary cause driving change, as of course did Darwin. So, in words comparable to the last words in the Origin, Lamarck says we must attribute all ‘l’extrême diversité des productions de la nature’ to the power of ‘circonstances’ (1820, p.142). How different might things have turned out if Lyell had been aware of Lamarck’s change of view.

6 Grant seems to have been a brilliant zoologist and Darwin was extremely fortunate to have been taken under his wing at such an early stage in his scientific education. Grant knew most of the great naturalists of Europe and had studied in Paris from 1815 to 1820. He could speak several languages, but he seems to have been a difficult person and once Darwin's own reputation was established the two men had little contact. Grant was an authority on marine invertebrates (a word coined by Lamarck) which he believed held the secrets of evolution. He explained his Lamarckian views in a series of papers on sponges published in the Edinburgh Philosophical Journal, one of which Darwin cites in his historical introduction to the Origin. Darwin was inspired by Grant to focus on marine invertebrates during the Beagle voyage and on his return asked Grant to describe some of his specimens. Grant left Edinburgh in late 1827 to become the first professor of comparative anatomy and zoology at University College London where he remained for the rest of his career (Desmond 1989).

7 Ironically the boundary between the Cretaceous and the Tertiary (the "K/T boundary") is today almost universally regarded as a classic "catastrophic" event due to a massive bolide impact 66 million years ago. Whilst many groups of organisms seem to have survived the impact, almost all dinosaurs famously did not. So far as we know the only dinosaurs which survived were a handful of species of ground-living birds (Benton 2020).

8 In common with many of his contemporaries Lyell is inconsistent in the terms he uses for the various divisions of geological time (Epoch, Period, Era etc) which are today applied with rigid consistency.

9 Darwin's marine explanation for the parallel roads of Glen Roy (Darwin 1839) was soon disproved and he regarded his theory as a "great failure" (Rudwick 1974b).

10 Darwin wrote to Horner in 1844 saying "The great merit of the Principles was that it altered the whole tone of one's mind, & therefore that, when seeing a thing never seen by Lyell, one yet saw it partially through his eyes." (CCD, vol. 3, p. 55).

11 As discussed in our introductions to the various editions of the Origin, Darwin was roundly criticized for his initial estimate, but within weeks he came up with the far more reasonable estimate of 150 or more million years in the second edition. This figure though still much too great is closer to today's estimates of the duration of the Tertiary (see note 7).

12 Darwin's insertion, together with several others in the Origin'ssecond edition, may have been triggered by his receipt of a letter dated 18 November 1859 from Rev. Charles Kingsley (1819-1875) quoted obliquely on p. 481 (See CCD, vol. 7, p. xxiii).

 

REFERENCES NOT ALREADY IN DARWIN ONLINE

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Benton, Michael. 2020. The dinosaurs rediscovered: How a scientific revolution is rewriting history. Thames and Hudson. London.

CCD: F. Burkhardt, Sydney Smith et al eds. 1985-. The Correspondence of Charles Darwin. Cambridge University Press.

Coleman, William. 1962. Lyell and the 'reality' of species. Isis 53, pp. 325-338.

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Desmond, Adrian. 1989. The politics of evolution: morphology, medicine, and reform in radical London. University of Chicago Press. Chicago

Gould, Stephen Jay. 2000a. Lyell's pillars of wisdom. In The lying stones of Marrakech. pp. 147-168. Jonathan Cape. London.

Gould, Stephen Jay. 2000b. A tree grows in Paris. In The lying stones of Marrakech. pp. 115-143. Jonatan Cape. London.

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Rudwick, Martin. 1974b. Darwin and Glen Roy: a 'great failure' in scientific method. Stud. Hist. Philos. Sci, 5, pp.  97-185.

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