Introduction to South America

This introduction to Darwin's Geological observations on South America (1846) is intended to complement the bibliographical introduction by Richard Freeman. It should perhaps also be read alongside the introduction to Darwin’s Volcanic islands (1844). Herbert 2005 provides a general survey of Darwin’s geological work and Rudwick 2008 is highly recommended for placing Darwin’s geology in the context of geological theory in the 1830s and 1840s.

South America is a tremendous book. It makes considerable demands on the reader but repays in proportion. It may be heavy going for the non-geologist, but is more readable than many other technical books of its vintage. Darwin’s style is far more personal than would be found in almost any scientific book today. For example, Darwin rarely writes a page without using the word ‘I’! The book is well-illustrated by the standards of its day and Darwin comes across as a skilful user of his carefully prepared, coloured sketch sections (plate 1). It must be said, however, that the modern reader will regret that Darwin did not listen to his friend Joseph Dalton Hooker’s suggestion in February 1846 that South America include geological maps of the continent (Correspondence vol. 3, p. 283). The Darwin Archive in Cambridge University Library contains several coloured geological maps which, if they had been included in the book, would have made it far easier to follow.

The history of composition of South America is usefully summarised in Correspondence, vol. 3, pp. xv, 396 and that volume includes many letters bearing on the book. The first draft was finished on 24 April 1845, then revised in October. The Falklands were split off as a separate paper in March 1846. Today those islands have been shown to be a geological continuation of South America. Darwin must have been relieved when he put the proofs aside on 1 October 1846, ten years after the return of the Beagle, as the very same day he started work on his barnacles.1 First editions of South America are rare today; the original price was 12/- (60p) whereas a copy was advertised in December 2008 for over £10,000. We are aware of only one contemporary review in the British Quarterly Review in 1847.

South America consists of eight chapters, plus appendices on Darwin’s Tertiary fossils by George Sowerby and on his Secondary (now known as Mesozoic) fossils by Edward Forbes respectively. The chapters fall into two halves, the first half taking the geological history of South America back to mid Tertiary times, the second half taking it back to Secondary times. In this way Darwin breaks from a simple ‘regional’ approach and instead organises his material along chronological and thematic lines. This follows the approach used by Charles Lyell in his Principles of geology (1830-3) of starting with the strata laid down comparatively recently, which might be expected to have been formed in conditions closest to those of the present, then working backwards into steadily less familiar environments. The book is a goldmine of fascinating details, covering for example such esoteric concerns as loxodromism, the fractionation of lavas and advice on keeping cheese. It also contains some of Darwin’s most eloquent prose, such as his description of mist in the Andes appearing as he believed the sea must once have done when filling the same space in the ancient past, and some of his finest statements on deep geological time. At one point (p. 126) he summarises the entire geological history of South America on one page.

The first two chapters of South America deal with elevation of the east and west coasts respectively. The third deals with the plains and valleys of Chile with a section on salt deposits, while the fourth deals with the Pampas. In the second half of the book we have the ‘Older Tertiary’ of Patagonia and Chile in chapter five, then a general chapter on cleavage and foliation (metamorphic rocks). The last two chapters concern Chile only, which takes the story back to Secondary times, the seventh being a detailed description of Central Chile and the structure of the Cordillera, the eighth being Northern Chile and a general conclusion. In terms of the length of the chapters, the average is about thirty pages, but the third is shortest and the eighth is longest.

In terms of the way Darwin laid out his arguments, South America bears very strong relationships both to Lyell’s Principles and to Darwin’s most important book, Origin of species first published in 1859. The early chapters of South America interpret the more recent geological periods in great detail and focus on devising a methodology for proving that southern South America has been and continues to be elevated by a series of step-wise uplifts. Later chapters use this approach going back (Darwin would have said ‘ascending’) into ‘deeper’ time to decipher earlier periods, building the case for the emergence of the entire continent from its origins under the sea. This is Lyell’s method of using the ‘present as the key to the past’, a method Darwin applied in the Origin, in which the first two chapters examine domestication of animals and plants at the present day as an analogy to the process of selection in nature. Having made the case for natural selection acting today, Darwin worked backwards in the later chapters of the Origin to show how natural selection can explain the history of life in former periods.

As when dealing with anticipated objections to natural selection in Origin, in South America Darwin dismisses potential objections to his elevation theory, as for example by showing that ‘difficulties’ with the sea transport of pebbles (p. 23) are more ‘apparent’ than real. The connection between Origin and South America is in fact even closer as there is much more biology in South America than is usually assumed. Darwin devoted considerable space in South America to palaeontological issues especially in relation to the extinction of Pleistocene mammals, but also to some general discussion of the history of life, as for example in the discussion of ‘change of organic forms’ on p. 105. It is true, however, that nowhere in South America does Darwin mention the issue of the obvious relationships between the extinct mammals and their present day descendants, preferring to direct the reader to his more overtly ‘evolutionary’ Journal of researches, the second edition of which had been published in 1845.

At the risk of straining the comparison, it is possible to find a strong similarity between the famous last paragraph on the evolution of life by natural selection in Origin and the following passage concerning the elevation of Patagonia in chapter 1 of South America:

Hence, on this view of a slow and gradual rising of the land, interrupted by periods of rest and denudation, we can understand the pebbles being of about the same size over the entire width of the step-like plains,—the occasional thin covering of sandy earth,—and the presence of broken, unrolled fragments of those shells, which now live exclusively near the coast. (p. 18)

In reading South America it helps to understand Darwin’s mature view of the geological history of South America. He came to view the history of the whole continent as dominated by the Andes, which is a reasonable point of view given that they are the longest mountain range in the world. Today we see the Andes as continuous with the Cordillera of North America, with both ranges resulting from the westward movement of their continents overriding and subducting the eastern Pacific oceanic plate. Darwin perceived that the Andes generally taper southwards both in width and altitude. He visualised them as an elongated volcanic archipelago rising from the sea since Secondary times to become a very long, thin, mountainous country, with a granitic margin down the western coast. This long, thin country has continued to elevate Patagonia and the central part of the continent to join up with the ancient mountains of Brazil to form the continent we know today. Darwin knew that the story was more complex, with periods of submergence, as proved by the fossil forest which he discovered high in the mountains, but his overall narrative is suffused with a belief that the continent is still rising due to pressure from beneath. This pressure, Darwin believed, is periodically relieved by earthquakes and eruptions.

Darwin sometimes over-extended his commitment to elevation from the sea, as for example when extrapolating from his explanation of the Coquimbo terraces to embrace the history of some of the Scottish lochs on p. 24 and in his explanation of the parallel roads of Glen Roy p. 65. In both Scottish cases Darwin ignored the effects on sea level of the Ice Age. He was also over-committed – almost to the point of obsession – concerning the power of icebergs to transport boulders over what he imagined were seabeds since risen above the waves. Today we would interpret most if not all these boulders as glacial erratics.

Several other geologists are cited in South America, in particular Charles Lyell, Christian Gottfried Ehrenberg, William Hopkins and Adam Sedgwick, the latter in connection with cleavage and foliation. Darwin also argues against Léonce Élie de Beaumont in connection with the Frenchman’s belief that the Andes were the most recent example of a mountain range formed ‘at one blow’. Darwin’s strongest criticism was, however, reserved for that other great South American traveller Alcide d’Orbigny, whose name occurs dozens of times, partly because he identified many of Darwin’s shells but also because the two men differed fundamentally over the origin of the Pampean Formation.

Darwin interpreted the Pampean Formation as the result of deposition in a proto-Plata estuary over millions of years, whereas d’Orbigny preferred a catastrophic interpretation based on the idea of a gigantic flood or debacle. Of course Darwin had a vested interest in a ‘gradualist’ interpretation as by 1846 he knew that natural selection would be swamped as a creative force in evolution if geological history was dominated by catastrophies. Darwin wrote that he ‘should not have noticed the theory of a debacle, had it not been adduced by a naturalist so eminent as M. d’Orbigny’ (p. 98). Both geologists could cite a section at St Fé which supported their own view, and the matter was not resolved until a century later when it was shown that the two men were talking about two different formations which actually looked very similar. Some of Darwin’s arguments against d’Orbigny’s view that the Formation was all laid down at once seem rather close to special pleading, for example on p. 101.

Darwin’s attitude to d’Orbigny is at best ungrateful, in view of all the help he received from the French palaeontologist. Darwin criticised d’Orbigny’s catastrophist opinion that the deposits containing fossil mammals at Punta Alta had all been washed together:

Will anyone be so bold as to affirm that it is possible, that a piece of the thin tessellated armour of a Dasypoid quadruped, at least three feet long and two in width, and now so tender that I am unable with the utmost care to extract a fragment more than two or three inches square, could have been washed out of one bed, and re-embedded in another, together with some of the small bones of the feet, without having been dashed to atoms? We must then wholly reject M. d’Orbigny’s supposition… (p. 86).

Several other quotes could be made here to show Darwin’s rather spiteful attitude to d’Orbigny but only one more is required to make the point. Darwin, after itemising all his evidence for gradual elevation at Punta Alta, made this remark in connection with d’Orbigny’s view of the locality, saying that if he too had seen this evidence: "I cannot believe he would have thought that the elevation of this great district had been sudden." (p. 17). Regardless of this unnecessary – and it has to be said, rare - example of Darwin’s posturing, South America is forever assured a classic place in the annals of geology. It is far more than some scientific scrapping mixed with a dry description of the rocks and fossils of a continent. South America is a masterful application of theory to a vast mass of observation, leading by carefully crafted logic to a unified history spanning unimaginable lengths of time.

In view of Darwin’s subsequent influence on our view of nature, South America deserves to be read by everyone interested in understanding his complete oeuvre and the development of his great theory of evolution. It was the evidence Darwin saw in South America during the voyage of the Beagle which convinced him, firstly, that extinction of species was a natural process and, secondly, that species were not created supernaturally to preserve the balance of numbers. Having imbibed deeply of Lyell’s gradualistic interpretation of Earth history Darwin applied this approach in a wholly novel way to the geological history of a vast ‘new’ continent. Before he left its shores, he had not only rejected Lyell’s theory of coral reef formation but also his belief in perfect adaptation of species to their environments. This prepared him to see the significance of the South American nature of the species which he saw in the Galapagos and thus to open the challenge to Lyell’s view of species production.

Gordon Chancellor

February 2009

Darwin, C. R. 1846. Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder and Co. Text Image PDF F273

1 A second edition, containing a few additions and combined with Volcanic islands, appeared in 1876 under the title Geological observations. A third edition, with a critical introduction by John Judd and combined with Coral reefs and Volcanic islands, appeared in 1890.

RN1

 

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