mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== /Based on a Professorial Lecture given at Nottingham Trent University 25 April 1996/ * Summary <#summary> * Introduction <#introduction> * Uniformitarians and Catastrophists <#uniformitarians> * The Rise and Apparent Triumph of Uniformitarianism <#rise> * The Disregarded Cosmic Dimension <#cosmic> * Catastrophism Re-Born <#reborn> * Epilogue <#epilogue> * References <#references> /SUMMARY/ /Until/ recently, the Modern Synthesis of neo-Darwinism, together with the contemporary geological paradigm, has seemed secure in representing the triumph of a gradualistic- uniformitarian view of Earth history over the catastrophist alternative. Uniformitarianism was formulated by Charles Lyell in a geological context, but many of its principles and attitudes were adopted by Charles Darwin for his theory of evolution by natural selection, subsequently developed into the Modern Synthesis. There was a general belief that catastrophism and evolution were mutually exclusive explanations for the fossil record and, as fossil evidence accumulated during the second half of the nineteenth century, it became increasingly clear that the features could not be explained by the Earth-centred model of catastrophism then in vogue, which linked extinctions of species to crustal upheavals on a global scale. /So/,gradualism-uniformitarianism became dominant. Then, as a result of distortions propagated by Lyell and others, subsequent generations were led to believe that the views of the catastrophists owed more to pre-conceived ideas than to observation, whereas the theories of the uniformitarians were all derived by logical deduction from observed data. At best this was over-simplistic, at worse a reversal of the truth, yet the myth became widely accepted. It also became received opinion, quite falsely, that the catastrophists relied on supernatural explanations for the cause of the major catastrophic events which had supposedly taken place. /I/gnorance about threats to the Earth from asteroids and comets contributed to the demise of catastrophism in the nineteenth century. From the time of Aristotle onwards, there had always been considerable resistance to suggestions that the earth might be subject to interference by external factors. Nevertheless, the extraterrestrial origin of meteorites was well-established by the start of the twentieth century, but not until the 1960s did it become generally accepted that impacts had caused many of the large craters found on the Earth's surface. The arguments of a few individuals that such impacts could have influenced the evolution of life continued to fall on deaf ears, even though episodes of mass extinction were evident from the fossil record. /T/here have been dramatic changes in attitude towards catastrophism since 1980, stimulated by the hypothesis of Luis Alvarez and colleagues that high iridium concentrations found at the Cretaceous-Tertiary boundary throughout the world could be taken as evidence that the mass extinction episode at the end of the Cretaceous Period had been caused by the impact of a large asteroid. Alternatively, the iridium abundance anomaly might have been the result of extensive vulcanism, which is known to have occurred at this time, but this would also have to be regarded as a catastrophist mechanism, and could even be linked to an impact. /S/uch events may have had a considerable bearing on the course of evolution. When species diversity is high, it is likely that natural selection acts mainly as a stabilising factor, rather than driving evolution forward. However, when species diversity is low and many ecological niches are vacant, as in the aftermath of a mass extinction, there are much greater opportunities for new variants to become established, and to give rise to further variants. The characteristic pattern of evolution is of extinctions followed after a pause by the rapid radiation of new species into vacant ecological space. This picture is very different from that envisaged by the founders of the Modern Synthesis. /INTRODUCTION/ /P/erhaps the most memorable trans-Atlantic cable of all time was sent by the American writer, Mark Twain, to point out that the reports circulating about his death were somewhat exaggerated. That incident has surely served as a reminder to countless obituary writers to check that their subject really is dead, before rushing into print. Someone claiming a resurrection has to be even more careful: here it is necessary to establish not only death. but also subsequent life. /I/n the case of catastrophism, as applied to geology (the study of the Earth) or palaeontology (the study of fossils), there can be little doubt that, in the eyes of the scientific establishment for a century or more, it has seemed as defunct as any theory could be. Now, however, catastrophism is making a very real contribution to geology and evolutionary theory. A resurrection would seem to have taken place. /W/hat precisely is catastrophism? In the McGraw-Hill Dictionary of Scientific and Technical Terms (1974 edition), we read: / Catastrophism [Geol.]. The theory that most features in the earth were produced by sudden, short-lived, worldwide events./ /H/owever, the scientific issues have been clouded by a supposed association between catastrophism and religion. Rightly or wrongly, it has generally been thought that the catastrophists of the nineteenth century and earlier believed that God was directly involved in determining the history of the Earth. So, for example, American palaeontologist, Steven Stanley, claimed in 1987 that catastrophism was: /the outmoded belief that sudden, violent and widespread events caused by supernatural forces formed most of the rocks visible at the earth's surface./ (Stanley, 1989) /C/oncerning the succession of fossil forms, the Oxford University zoologist, Richard Dawkins, wrote in 1986: / Catastrophism was an eighteenth - and nineteenth - century attempt to reconcile some form of creationism with the uncomfortable facts of the fossil record./ (Dawkins, 1986) /S/imilarly, the science historian, Michael Ruse, wrote in 1982 about the catastrophists: /They argued flatly that new species of organism, including God's final creation, man, were produced miraculously by God. God wants no nonsense about unbroken laws coming between them and His handiwork. He interevenes personally./ (Ruse, 1982) /I/t should go without saying that twentieth century catastrophism, often called neocatastrophism, is founded entirely in science, relying solely on natural forces for its explanations, but was eighteenth and nineteenth century catastrophism completely different? Was it so dominated by supernatural elements that any scientific content it may have claimed was without value? That was certainly the prevailing view for most of the present century. Catastrophists have been condemned for putting dogma before observational science, whereas their rivals, the gradualists (also called uniformitarians) have been praised for taking the opposite stance. /A/ major influence in establishing this picture was the 1897 book, Founders of Geology, by Sir Archibold Geikie, who was then Director General of the British Geological Survey. Geikie condemned the "monstrous doctrines" of the early catastrophists, and contrasted their approach most unfavourably with what he claimed was that of the gradualist James Hutton, regarded as the founder of uniformitarianism: / In the whole of Hutton's doctrine, he vigorously guarded himself against the admission of any principle which could not be founded on observation. He made no assumptions. Every step in his deductions was based upon actual fact, and the facts were so arranged as to yield naturally and inevitably the conclusions which he drew from them./ (Geikie, 1897) /T/hat is the story which has been presented to generations of geology students. In the 1965 edition of their textbook, Physical Geology, L Don Leet and Sheldon Judson claimed: / Modern geology was born in 1795 when James Hutton ... formulated the principle that the same physical processes operating in the present also operated in the past./ (Leet and Judson, 1965) /T/he same view was prevalent in the 1970s. The CRM textbook, Geology Today, stated: / The first to break formally with religion-shrouded tradition was James Hutton. (Geology Today, 1973)/ /S/imilarly, in 1982, the British palaeontologist, Beverley Halstead, wrote: /The first detailed attempt to understand the meaning of rocks was made by the Scotsman James Hutton (1726-97), after 30 years of observation and study of the rocks in Scotland and elsewhere./ (Halstead, 1982) /E/ven closer to the present day, we find the science journalist, Roger Lewin, writing in 1993: /At the beginning of the nineteenth century, the great French geologist and naturalist Baron Georges Cuvier proposed what came to be known as the Catastrophe theory or Catastrophism. According to the theory, the abrupt faunal changes geologists saw in rock strata were the result of periodic devastations that wiped out all or most extant species, each successive period being repopulated with new kinds of animals and plants, by God's hand. [Charles] Lyell rejected so nonscientific a hypothesis (as did James Hutton before him), and replaced it with the notion that geological processes proceeded gradually - all geological processes./ (Lewin, 1993) /H/owever, as I shall demonstrate, this view of scientific uniformitarianism and dogmatic catastrophism was, at best, over-simplistic, failing to take into account the range of beliefs and attitudes of individual uniformitarians and catastrophists in the late 18th and early 19th centuries. For many centuries, the Church had exercised almost complete control over academic communications in the western world, and everything was viewed within a spiritual context. For most of that time, it would have been heretical to deny the testimony of the Bible, as accepted by the Church, that the Earth was only a few thousand years old, and that there had been a major cataclysm, the Flood in the time of Noah. Today, we ridicule Archbishop Ussher, who in the middle of the seventeenth century calculated, from information given in the Bible, that the Earth was created in 4004 BC. For whatever reason, we ignore the fact that Ussher's chronology was supported, in very positive terms, by no less a scientist than Sir Isaac Newton. Similarly, when rightfully praising Newton for formulating the mathematical laws of gravity, we turn a blind eye to the fact that he thought the gravitational forces themselves required a supernatural rather than a physical explanation. /A/s with Newton, the uniformitarians had some views which today we find very strange, and there is absolutely no intellectual justification for making allowances for these, because of the times in which they lived, whilst refusing to do the same for the catastrophists. If we take an objective look, we find that uniformitarians and catastrophists alike were products of their times, overlapping to a considerable extent in their religious beliefs and approaches to science. Throughout Europe in the eighteenth century, and in Britain well into the nineteenth, science was dominated by the philosophy of Natural Theology, so that observations of the material world were expected to show evidence of the power, wisdom and goodness of God. This applied to uniformitarians as well as catastrophists. /UNIFORMITARIANS AND CATASTROPHISTS / /J/ ames Hutton, the founder of uniformitarianism (although he himself never used the term), formulated his theory of a self-renewing Earth late in the eighteenth century. According to this theory, there were essentially three stages in a cycle: erosion of rocks produced sediment; sediment built up until eventually its weight generated enough heat to liquefy the bottom layers; and then this molten rock forced its way upwards. The process then started all over again, continuously regenerating the Earth's surface over countless ages, maintaining an environment capable of supporting living animals and plants. Hutton specifically wrote that "the Earth functions as a machine whose purpose is to sustain life". The theological basis of it all is apparent from the following quotation from a paper written by Hutton in 1788: / If no such reproductive power, or reforming operation, after due enquiry, is to be found in the constitution of this world, we should have reason to conclude, that the system of this earth has either been intentionally made imperfect, or has not been the work of infinite power and wisdom./ (Hutton, 1788) /H/utton produced evidence in support of his theory, such as the rock formations at Siccar Point and in the Tweed Basin, Scotland. However, contrary to the myth propagated by Geikie, it is clear that he looked for the evidence after, not before, he formulated the theory. He had never seen the Siccar Point and Tweed Basin formations when he first presented his ideas. The self-renewing Earth model was derived in the first instance from Hutton's religious views, not from the geological evidence (Gould, 1988; Bowler, 1989; Hallam, 1989). /I/n France, particularly after the Revolution of 1789, science was more independent of religion. In 1812, Georges Cuvier published the results of many years painstaking investigation of the geology of the Paris Basin, concluding that there had been several sudden advances and retreats of the sea. These were associated with major catastrophes for, on each occasion, almost all the animals and plants then living were anhililated, a new set emerging in the aftermath, as judged by the fossils found in the rocks. As evidence of the speed of the process, at least concerning the most recent catastrophe, Cuvier drew attention to the discovery of unputrefied carcasses of large, extinct mammals in the northern ice. /T/he establishment of species extinction was a significant step in the development of our understanding of the Earth's history. By any standards, Cuvier was one of the great scientists of his day, being particularly renowned as an anatomist. Nevertheless, it would be easy to jump to the conclusion, in view of the supposed association between catastrophism and religion, that he saw the fossil record as providing evidence of divine intervention. In fact, Cuvier's approach to science was to avoid any speculations which could not be supported by facts. Characteristically, he wrote: / But what then was this primitive earth where all the beings differed from those that have succeeded them? What nature was this that was not subject to man's dominion? And what revolution was capable of destroying it to the point of leaving as trace of it only some half-decomposed bones? But it is not our duty to engage ourselves in the vast field that these questions present. Let hardier philosophers undertake it. Modest anatomy, limited to detailed examination, to the meticulous comparison of objects submitted to its eyes and to its scalpel, will content itself with the honour of having opened this new route to the genius who will dare travel along it. / (Cuvier, 1796) /C/uvier was more than prepared to accept the scientific evidence that there had been more than one major catastrophe, and that some animal species had become extinct, both of which were against Christian tradition. He opposed the possibility of evolution, not because of any adherence to Christian dogma, but because his anatomical studies showed that each species possessed a high degree of structural organisation, making a transition, particularly a gradual transition, from one to another extremely unlikely. He insisted, though, that fresh creations were not necessary to explain repopulation of an area after a catastrophe, because the `new' animals could have migrated from another part of the world. In all his published writings, he took care to keep science and religion separate (Corsi, 1988; Albritton, 1989; Bowler, 1989). /S/uch an attitude would have been most unusual in Britain during the same period, because of the continuing influence of natural theology. Many of the leading British scientists were also clergymen, because that was still a requirement for being a fellow at either Oxford or Cambridge. The Rev. William Buckland, a geologist at Oxford, eventually became Dean of Westminster. He was an amiable eccentric who conducted field trips in full academic dress and fed guests exotic animals sent to him from around the world. Another eminent geologist was the Rev. Adam Sedgwick, of Cambridge. The Rev. William Whewell, a Cambridge mineralogist and philosopher, wrote extensively on the nature of science, and coined the terms "scientist" and "physicist". All were catastrophists, being impressed by sudden breaks between rock formations, which seemed to indicate equally sudden changes in the conditions under which the rocks were laid down, and by the changes in fossil populations in successive formations. The dual role of these men, as scientists and clergymen, has confused many into thinking that their science must have been subservient to their religion, but this was not so. Buckland and Sedgwick once thought they had found traces of Noah's Universal Flood, but changed their minds when more evidence became available. Nevertheless, the myth persisted (Gould, 1988; Hallam, 1989). /T/he British science historian, Robert Young, writing in 1985, claimed that: / William Whewell was the most articulate and sophisticated interpreter of the providential view which was associated with the belief that God intervened in the course of nature with catastrophic alterations of its geology and its complement of species./ (Young, 1985) /H/owever, did Whewell really believe in wholesale and direct interventions by the Creator? In a quotation prominently displayed on the frontispiece of Charles Darwin's On the Origin of Species, Whewell actually wrote: / But with regard to the material world, we can at least go as far as this - we can perceive that events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws./ (Whewell, 1833) /B/y around 1830, the British catastrophists had generally adopted a model originally put forward by Cuvier's pupil, Léonce Élie de Beaumont. This held that if, as seemed likely, the Earth was gradually cooling from an incandescent beginning, the shrinkage would give rise to episodic large-scale disruptions of the crust, these decreasing with intensity as time passed. During each of these intermittent upheavals, mountain building would take place because of "wrinkling" of the crust, volcanoes would erupt, some former continental regions would be flooded by sea-water, and many species would become extinct. This was a completely natural process. The intellectual power of Élie de Beaumont's synthesis undoubtably made a big impression on his contemporaries, and catastrophism ruled supreme (Bowler, 1989; Hallam, 1989). /THE RISE AND APPARENT TRIUMPH OF UNIFORMITARIANISM / /T/his situation did not prevail for long. The key event in the establishment of gradualism as the ruling paradigm was the publication of Charles Lyell's Principles of Geology in three volumes between the years 1830 and 1833. Lyell, son of a wealthy Scottish landowner, was destined to be a lawyer until he attended Buckland's lectures at Oxford, which aroused a passionate interest in geology. His career as a geologist was largely self-financed, although he was Professor of Geology at King's College, London, from 1831 to 1833. His legal training was not wasted, however, for he used all the techniques of a lawyer to promote his views (Gould, 1988; Albritton, 1989; Hallam, 1989). /F/or a start, he gave the same term - uniformity - two quite different meanings. One was a methodological principle which was widely acclaimed by scientists, the other was a gradualistic world view which originally had far less support. Somehow, he managed to convince people that if they accepted the principle, they had to accept the world view as well, even though the two were not obviously linked. The Harvard geologist and evolutionary biologist, Stephen Jay Gould, has commented, with the aid of a mythological analogy: Like wily Odysseus clinging to the sheep's underside, the dubious substantive meaning of uniformity sneaked into geological orthodoxy - past an undiscerning Cyclops, blinded by Lyell's rhetoric, by holding fast to the methodological principles that all scientists accepted. (Gould, 1988) /L/yell also advanced his case by denigrating his catastrophist opponents, attacking them for their supposed reliance on speculation and mysterious forces. He dismissed the perfectly rational model of ?lie de Beaumont as being "mysterious in the extreme, and not founded on any induction from facts". /T/he irony was that the catastrophists, not Lyell, were the ones who interpreted the evidence of the rocks in straightforward fashion. Catastrophists and uniformitarians alike contributed to the division of the geological record into Periods, on the basis of fossil populations, and these Periods were then grouped into Eras, the divisions between Eras coming where there were particularly sharp breaks in the fossil succession. One of these came at the end of the Permian Period, another at the end of the Cretaceous Period. The catastrophists took these sharp breaks between formations as indicating that sudden changes had taken place at the Earth's surface at these times. Lyell, however, with great skill, managed to turn the situation around, and present the catastrophists as blinkered and over-simplistic in their arguments. Had they been more knowledgeable and open-minded, they should have been able to appreciate that where there was no evidence for a gradual transition, this was simply because the evidence was missing. As a result of erosion and other effects over countless millenia, the geological record was far from perfect (Gould, 1988; Hallam, 1989). /L/yell, moreover, was not just an arch-gradualist, he had an a priori belief in a steady-state Earth, for which there was no evidence whatsoever. Whereas the catastrophists took a directional view of the Earth's history, interpreting the geological record as revealing that a series of irreversible changes had taken place, Lyell thought that conditions simply fluctuated about a mean. Like the catastrophists, he believed that when the environment changed, different forms of life would come into being, more suited to the new conditions. Unlike them, however, he thought that extinct forms of life could eventually come back into existence, when there was a reversion to former environmental conditions. In the Principles of Geology, Lyell wrote: /Then might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree ferns./ (Lyell, 1830) /W/hewell attacked Lyell's geological ideas, using purely rational arguments, in his History of the Inductive Sciences from the Earliest to the Present Time of 1837. He pointed out the inconsistencies in Lyell's approach, writing: /The effects must themselves teach us the nature and intensity of the causes which have operated; and we are in danger of error, if we seek for slow and shun violent agencies further than the facts naturally direct us, no less than if we were parsimonious of time and prodigal of violence./ (Whewell, 1837) /H/owever, as to the question of how new forms of life came into being, whether as a consequence of sudden or gradual changes, Whewell wrote: / We conceive it undeniable (and Mr Lyell would probably agree with us) that we see in the transition from an earth peopled by one set of animals, to the same earth swarming with entirely new forms of organic life, a distinct manifestation of creative power, transcending the known laws of nature: and, it appears to us, that geology has thus lighted a new lamp along the path of natural theology./ (Whewell, 1837) /A/mazingly enough, in view of the myth that uniformitarianism was divorced from religion, Lyell did agree with Whewell. Like the catastrophists, Lyell was a staunch creationist, totally opposed to evolution, and especially to any evolutionary origin for humankind. They may all have been rational scientists when considering geology, but the question of the origin of new species was a very different matter. By this time, as evidence had accumulated, it became impossible to sustain Cuvier's argument that migration could provide a sufficient explanation for the replacement of extinct species in a particular location. The issue had to be faced: creation or evolution? Very few people were prepared to consider the possibility of the latter. The details of precisely what was meant by creation were left vague, but the process had to involve the `First Cause', i.e. God, either acting directly or through the operation of `intermediate (or secondary) causes' (Bowler, 1989). /L/yell tried to make out he was being more scientific than the catastrophists, by advocating the operation of intermediate causes, giving rise to "laws of creation". However, the laws only operated intermittently, when required to advance a plan set in motion by God, and they could not be tested. As the eminent evolutionary biologist, Ernst Mayr, has pointed out, when Lyell was asked to explain what he meant by `intermediate causes', he always included in his answer some reference to the arrival of a new species being `pre-ordained, pre-determined' or `appointed', so his views could only be described as `rank special creationism' (Mayr, 1982). /T/he man who was to challenge those views, Charles Darwin, became a member of Lyell's circle of friends in 1836, after his round-the-world voyage in H.M.S.Beagle. Darwin's observations on the voyage had made him a supporter of the gradualist school of geology, but they had also started him thinking about the possibility of evolution, which would have been just as abhorrent to Lyell as to the catastrophist clergymen. So Darwin worked on his theory in private, keeping copious notes (which still exist), but not disclosing them to anyone. Eventually in 1844, he summarized his views on evolution, with natural selection as a mechanism, in a 200 page essay. He was still afraid of the reaction, so he disclosed the contents in the first instance only to the botanist, Joseph Hooker, a kindred spirit who like Darwin had travelled the world as ship's naturalist (Bowler, 1989; Mayr, 1991; Desmond and Moore, 1992). /L/yell eventually came to know of Darwin's evolutionary beliefs and, although shaken, remained friendly. His own views, however, remained unchanged. In 1851, Lyell addressed the Geological Society in London and re-affirmed that humankind had not arisen by a process of evolution. As to how new species originated, Lyell told the members that he was not prepared to speculate, but nevertheless made it clear that God must be involved in some way: / Whether such commencements be brought about by the direct intervention of the First Cause, or by some unknown Second Cause or Law appointed by the Author of Nature, is a point upon which I shall not venture to offer a suggestion./ (Lyell, 1851) /E/ven after the publication of Darwin's On the Origin of Species in 1859, Lyell continued to resist arguments for evolution as a natural process. Evolution might be acceptable with God giving it an appropriate steer, but the "law of higgledy-piggledy", as the astronomer, Sir John Herschel called natural selection, hardly appeared of value to someone like Lyell, looking for the working out of a divine plan. Eventually, at the age of seventy, in the tenth edition of the Principles of Geology, Lyell felt able to admit publicly that some form of biological evolution must have taken place, but he still resisted the notion that humankind was part of the scheme. Darwin and Lyell had little further contact with each other before Lyell's death in 1875. As Adrian Desmond and James Moore wrote in their biography of Darwin: / After Lyell's failure to back the Origin, their cameraderie had cooled. It was a grim end to a once famous friendship./ (Desmond and Moore, 1992) /A/ccording to the myth, Darwin's theory of evolution was simply an extension of Lyell's uniformitarian ideas. So, for example, Roger Lewin wrote in 1993: /Gradualism superceded Catastrophism. Darwin's and Lyell's worldviews were therefore perfectly complementary./ (Lewin, 1993) /I/n fact, there was a three way relationship. Lyell and the catastrophists were as one in rejecting Darwin's view that humankind could have arisen by a natural, evolutionary process; Darwin and the catastrophists were as one in preferring a directional history of the Earth, in contrast to Lyell's steady-state view; and Darwin and Lyell differed from the catastrophists in maintaining that everything could be attributed to gradualistic processes (Bowler, 1989; Mayr, 1991). /O/f Darwin's commitment to gradualism, there can be no doubt. In the final chapter of the Origin, he wrote: / As natural selection acts solely by accumulating slight, successive favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps./ (Darwin, 1859) /T/he whole process operated by competition between variant forms. And, if there were winners, there also had to be losers: /The extinction of species and of whole groups of species, which has played so conspicuous a part in the history of the organic world, almost inevitably follows on the principle of natural selection; for old forms will be supplanted by new and improved forms./ (Darwin, 1859) /T/he problem was that there was little or no fossil evidence for gradual transitions. Towards the end of chapter 10 of the Origin, after summarising (perfectly legitimately) reasons why the formation and preservation of fossils might be unrepresentative of the history of life, Darwin wrote: / All these causes taken conjointly, must have tended to make the geological record extremely imperfect, and will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps. He who rejects these views on the nature of the geological record, will rightly reject my whole theory./ (Darwin, 1859) /S/imilarly, when the Modern Synthesis of neo-Darwinism, incorporating advances in population genetics, was put together in the mid-twentieth century, the emphasis was on gradualism, allied to the imperfections of the fossil record. So Ernst Mayr, one of the? founding fathers, wrote in 1942 in his Systematics and the Origin of Species: / The species of each period are the descendants of the species of the previous period and the ancestors of those of the next period. The change is so slight and gradual and should, at least theoretically, not permit the delimitation of definite species. In practice, the fossil record is fragmentary, and the gaps in our knowledge make convenient gaps between the `species'./ (Mayr, 1942) /A/t the time of the Darwinian revolution, however, the details were relatively unimportant. To champions of Darwin such as Thomas Huxley, Darwinism represented an attempt to explain the world by natural processes. Huxley himself was no gradualist, but by contributing to the triumph of Darwinism, he helped to impose a gradualistic paradigm (Bowler, 1989; Mayr, 1991). /I/n geology, at around the same time, accumulating evidence was providing no support for the model of ?lie de Beaumont, much favoured by catastrophists, that a cooling Earth would give rise to intermittent global convulsions of the crust, accompanied by wholesale extinctions. The demise of this model left no coherent alternative to the growing uniformitarian consensus. However, contrary to general perception, this consensus incorporated elements of catastrophism as well as gradualism, and the steady-state aspect of Lyell's views quietly dropped out of sight. Phenomena such as ice ages and, later, continental drift were admitted on the supposition that environmental changes associated with them, although major and widespread, occurred in a gradual fashion. Also, paroxysmal events such as earthquakes and volcanic eruptions were included (after all, they were known to occur), without disturbing the impression that uniformitarianism was more or less synonymous with gradualism, since these were thought to be too localized to be a direct cause of species extinction (Gould, 1988; Hallam, 1989). /G/radualism, of course, required a long time-scale for the history of the Earth, whereas catastrophism could make do with a much shorter time-scale. It is now generally believed that the Earth is about 4,600 million years (Myr) old, and that living organisms have been present on Earth for over 3,000 Myr. Because of this long time-scale, an apparently abrupt change in the geological or fossil record could have taken place over a period of anything up to a million years. The possibility that such a break might have been an indication of a genuine catastrophe was increasingly ignored. /THE DISREGARDED COSMIC DIMENSION / /W/ith the establishment of the myth of a dichotomy between scientific uniformitariansm and unscientific catastrophism, it became difficult for anyone to put forward new catastrophist ideas. The problem was compounded by a lack of appreciation of the potential dangers to the Earth from outside. Many in the ancient world, including Plato, had been prepared to accept the possibility of cosmic catastrophes, but Aristotle believed that the Earth was segregated from the heavens, which were perfect and unchanging. Aristotle's views were adopted by the Christian philosopher, Thomas Aquinas, in the thirteenth century, and remained influential for several hundred years afterwards. When Thomas Jefferson, president of the USA, who included palaeontology amongst his many interests, was told in 1807 that two Yale scientists were claiming that mereorites had recently struck the Earth at Weston, Connecticut, he replied: / It is easier to believe that two Yankee Professors would lie, than that stones would fall from heaven./ (Jefferson, 1807) /E/ventually it became established that meteorites came from space, but observed falls of stones were on too small a scale to do more than cause a little damage, and frighten a few people and animals. A calf was killed by a meteorite in Ohio in 1860, and a dog in Egypt in 1911, but there is no authenticated case of any human dying in this way. /O/n the other hand, explosion craters were being discovered which could be taken to be evidence of larger impacts. Attention focused on the 1.2 km diameter crater in Arizona from 1886, but the first serious investigation concluded it had been formed by a steam explosion related to volcanic activity. One of the reasons for excluding an impact was that there was no evidence of a meteorite of appropriate size being buried within the crater. Also, it was approximately circular, whereas it was expected that an oval crater would have been punched out by a meteorite arriving at a typical angle. Not until the 1960s was it appreciated that a large bolide would explode on impact, producing a circular crater of much greater diameter than that of the impacting missile. More than 130 impact craters on the Earth's surface are now known, including three of around 100 km diameter, and there are likely to be many more still undiscovered or unrecognised. An impact crater of this size would have been caused by a bolide of around 5 km diameter (Albritton, 1989; Gehrels, 1994; Steel, 1995). /F/rom what astronomers now know about the incidence of asteroids in Earth-crossing orbits, it has been calculated that several asteroids of at least 1 km diameter are likely to strike the Earth every million years. Each of these would have an impact energy equivalent to at least a million megatons of TNT, which is 50 million times greater than the energy of the atomic bomb dropped on Hiroshima. Looking to even larger impacts, in the past 500 Myr, the Earth is likely to have been struck by several asteroids with diameters greater than 10 km and impact energies in excess of 100 million megatons. Cometary nuclei may possibly be lighter than asteroids of similar size, but are likely to be travelling faster relative to the Earth, so could be just as devastating on impact. The evidence suggests that around one third of all impact craters may be attributed to comets (Clube and Napier, 1990; Gehrels, 1994; Steel, 1995). /I/n the event of a 10 km object striking the Earth, bolide and rock would be instantly vaporized, and a crater around 180 km in diameter would be formed within seconds. If, for example, the bolide hit Milton Keynes, the crater would stretch from Nottingham in the north to London in the south, and include Birmingham, Oxford and Cambridge. This huge crater would be lined with molten rock, and an intense fireball would rise through the atmosphere, producing a violent, scorching wind. The fireball would fuse atmospheric nitrogen and oxygen to form nitrogen oxides, which would later react with water to form nitric acid. Similarly, sulphuric acid might be produced from burning plant material. Thus acid rain strong enough to destroy shells of creatures living in surface waters would be likely to fall in the aftermath of an impact. Dust would spread through the upper atmosphere, obscuring the light of the Sun for a long period of time, disrupting food chains by preventing photosynthesis, and cooling the Earth, possibly for years. Similar damaging consequences would result from an impact in an ocean, except that in this case, the water vapour ejected into the upper atmosphere might cause temperatures to rise rather than fall, because of a greenhouse effect (Clube and Napier, 1990; Gehrels, 1994; Steel, 1995). /S/o, could such an impact have been responsible for one of the discontinuities in the fossil record already referred to, such as the mass extinctions of species which apparently occurred at the end of the Permian and Cretaceous Periods? In the light of present-day astronomical knowledge it has to be considered a distinct possibility. However, in the absence of such knowledge, coupled with the conviction that the ideas of the catastrophists had all been discredited, it was generally thought for most of the present century that the discontinuities were actually artifacts of an imperfect fossil record, as Lyell had claimed. If we had the complete picture, we would be able to see that an apparent episode of mass extinction was simply the false juxtaposition of two stages, widely separated in time, of the normal gradual process of species turnover, based on competition. The case for the catastrophist alternative was pursued by the Russian-born psychoanalyst, Immanuel Velikovsky, amongst others (Velikovsky, 1956), but with little effect on mainstream scientific thought. /H/owever, in 1962, the American, Norman Newell, in his address as retiring President of the Paleontological Society, argued that the mass extinctions at the end of some geological periods were real events, rather than artifacts of the fossil record. The old species could not have been out-competed by the new, because there was an appreciable delay between the extinctions and the appearance of the replacement species. Nevertheless, he thought the process was a gradual one, each mass extinction episode lasting anything up to 6 Myr, and the cause was probably nothing more exciting than a fall in sea level, which seemed to have occurred at the end of several geological periods (Newell, 1962). /I/t soon emerged that a fall in sea level could be related to a slowing down of continental drift, and a reason for this could be seen at the end of the Permian Period, 250 Myr ago, when the biggest mass extinction of all took place. At that particular time, all the different continents happened to come together to form a single supercontinent, Pangaea, which would have reduced activity for a while, and led to a fall in sea level. Even apart from that, the coming together of the continents would have led to reduced habitat areas for creatures living in shallow coastal waters, and greater seasonality and destruction of isolated environments for land animals, explaining many of the extinctions (Stanley, 1987). However, whether it could explain all was another matter. Also, this was a unique event, so what about the mass extinctions at other times? /I/n particular, what about the mass extinctions at the end of the Cretaceous Period, 65 Myr ago? Almost a quarter of all known families of animals died out at this time, including all the dinosaurs, the dominant land animals of the previous 100 Myr. No land animal larger than 25 kg, the size of a labrador dog, survived into the next Period, the Tertiary. Also to disappear were the winged reptiles, the large marine reptiles and the ammonites (Raup, 1986; Stanley, 1987). /I/n 1956, the palaeontologist, M.W. de Laubenfels, suggested that the extinction of the dinosaurs at this time might have been caused by heat associated with the impact of a large meteorite, but this hypothesis was almost completely ignored. /A/lmost 20 years later, in 1973, the Nobel Prize-winning chemist, Harold Urey, made a similar suggestion in Nature, blaming a comet rather than a meteorite, but the outcome was the same. As the palaeontologist, David Raup, wrote in 1986, after his conversion to catastrophism: / Once again silence reigned ... [The] Urey proposal was too far out to be heard against the backdrop of the Lyell paradigm - that is too incredible even to register, much less argue about. I have no recollection myself of having known about the paper at the time. If I had known about it, I would no doubt have found fault./ (Raup, 1986) /S/imilarly, there was very little reaction in 1978 when the eminent British astrophysicist, Sir Fred Hoyle, together with his Sri Lankan colleague, Chandra Wickramasinghe, suggested that the extinctions at the end of the Cretaceous Period had been caused by the close passage of a giant comet. /D/espite the increased knowledge about the threat from space, catastrophism was making very little progress. Stephen Jay Gould explained the situation in the following words: /Geologists, like all folk, have their prejudices. They prefer causes emanating from their own domain, the earth. Since Lyell's day they have been trained to view major change as the accumulation of small inputs based on processes that can be observed in the relatively calm geological present. These preferences have combined to give cataclysmic extraterrestrial theories a poor shake./ (Gould, 1983) /N/evertheless, in the light of the growing astronomical evidence, he continued: / Many geologists, including myself, have long found themselves in the uncomfortable position of viewing extraterrestrial catastrophes as inherently plausible but rooting strongly against them. For what good is a theory, even a correct theory, that can generate no confirming evidence?/ (Gould, 1983) /CATASTROPHISM RE-BORN/ /T/he breakthrough came when a paper claiming to provide evidence was published in Science in 1980. The Cretaceous-Tertiary boundary in limestone formations throughout the world is marked by a thin clay layer, just a centimetre or so in thickness, and the palaeontologist, Walter Alvarez of Berkeley, had invited his father, the Nobel Prize-winning physicist, Luis Alvarez, to help him investigate this layer at Gubbio in Italy. The significant finding was a high concentration of the noble metal, iridium, normally almost completely absent from the Earth's crust. Similar results were then found at the Cretaceous-Tertiary boundary in Denmark and New Zealand. Iridium in such high concentrations could only have come from space, or from the Earth's core, released by volcanic activity, and there were no obvious indications of the latter at the sites in question. Hence the Alvarez group proposed that the Cretaceous extinctions had been caused by the impact of a 10 km asteroid, the resulting explosion scattering iridium-rich dust from the bolide around the world (Alvarez et al, 1980). /N/ot many people were convinced, at least in the first instance. The British palaeontologist, Beverley Halstead, wrote: / The asteroid or giant meteor explanation has the great popular appeal of high drama and curiously coincided with the release of a `disaster' film on the same theme. Such theories are certainly an advance on invoking the wrath of a Deity but not very much./ (Halstead, 1981) /F/ollowing in the dubious tradition of Lyell and Geikie, he presented the alternative to catastrophism as being inherently much more scientific: /The other brand of theory involves a careful consideration of all the evidence that can be accumulated, drawing both from biology and geology. These more synthetic theories tend to be less exciting but are more likely to approximate to what actually occurred./ (Halstead, 1981) /N/evertheless, the scientific evidence for the impact hypothesis rapidly accumulated. By the end of 1983, more than 50 sites had been identified with high iridium levels at the Cretaceous-Tertiary boundary. Other evidence of impact, including tektites (glass spheres), shocked quartz grains, diamonds and stishovite (a form of silica), which have been found at known impact craters, were also identified in the boundary clay (Raup, 1986; Hallam, 1989; Gehrels, 1994; Glen, 1994). /H/owever, these might also have resulted from volcanic activity, which is known to have occurred in extensive fashion at the end of the Cretaceous Period. The lava of the Deccan Traps in India, 10,000 cubic kilometres of it, were laid down at this time. A single volcanic eruption, that of Krakatoa in 1883, released energy equivalent to about 50 megatons of TNT, so the far more extensive vulcanism of the Late Cretaceous must have had what can only be regarded as a catastrophic effect upon the environment (Hallam, 1989; Glen, 1994). /S/oon, the major debate was about which of two catastrophist mechanisms, one involving an extraterrestrial impact, the other widespread volcanic activity, was responsible for the bulk of the Late Cretaceous extinctions. Arguments still raged about whether these had been largely instantaneous, or had taken place over many thousands of years. The latter would seem to support the volcano hypothesis, and extensive vulcanism undoubtably occurred. On the other hand, there might have been a shower of impacts, perhaps because of the break-up of a giant comet, as suggested by the British astronomers, Victor Clube and Bill Napier, or the extinctions might have been due to a mixture of impacts, volcanic eruptions and perhaps other factors. The volcanic eruptions could even have been triggered by one or more impacts (Clube and Napier, 1990; Glen, 1994; Palmer, 1994). /W/hat still held the impact hypothesis back was the absence of a large crater of suitable age. Large tektites found around central America indicated that the impact, if there had been one, may have happened in that region, and a 180 km diameter crater of precisely the right age was eventually identified in 1990 at Chixculub, in the Yucat n Peninsula of Mexico. The impact had apparently occurred in shallow water off what was then the coast, and the crater was covered with sediment before becoming part of the peninsula (Gehrels, 1994; Palmer, 1994; Steel, 1995). /T/he precise sequence and inter-relationship of the various events which took place at the end of the Cretaceous still need to be sorted out, but catastrophist mechanisms were undoubtably involved, and changed the course of evolution in a significant fashion. As with other mass extinction episodes, the old picture of progression through competition no longer fits the evidence. Instead, there is extinction of one group of species followed, after a gap, by radiation of another into the vacant ecological space. Possibly in normal times, natural selection acts largely to maintain the status quo, whereas after a mass extinction there are greater opportunities for new variants to become established. There is clearly no evolutionary grand design by which the `primitive' dinosaurs inevitably gave way to `advanced' mammals. It seems very unlikely that the small, furry mammals alive in the Late Cretaceous would ever have displaced the dinosaurs, had the dinosaurs not died out under the harsh conditions of the time. The small, furry mammals may have survived simply because they were small and furry, after which they could diversify without serious competition, and eventually give rise to Homo sapiens, our own species. As the American palaeontologist, Niles Eldredge, wrote in 1991: / Dinosaurs had their undeservedly low reputation because of their supposed loss in the battle for ecological space with none other than ourselves: mammals. We now know otherwise: Dinosaurs did just fine, filling ecological space the world over for 145 million years, yielding just enough space for us mammals to cling to ... It is now abundantly obvious that it took the eradication of the dinosaurs to give mammals a chance to take over./ (Eldredge, 1991) /EPILOGUE/ /F/inally, let us compare what is said about the Late Cretaceous extinctions in two British Natural History Museum books on dinosaurs, published 14 years apart. In the first, dating from 1979, the author, Alan Charig stressed that the extinctions "were not as immediate and as sudden as some people like to believe". He then looked at a whole range of possible causes for the death of the dinosaurs, starting with the ones he considered most sensible, such as changes in temperature and rainfall due to the effects of continental drift. Then he listed ones which he thought less sensible, such as shrinking brains and slipped discs. After that he continued: /Among the even less likely causes suggested for the death of the dinosaurs are poison gases, volcanic dust, meteorites, comets, sunspots, God's will, mass suicide (like lemmings!) and wars./ (Charig, 1979) /I/n the second book, written by Tim Gardom and Angela Milner in 1993, the very first words of the relevant chapter are: / Some catastrophic event wiped out the dinosaurs. Was it an asteroid collision? Or a climate change brought about by massive volcanic eruptions? Buried under tonnes of rock, the evidence is elusive and controversial. Yet it all points to the dinosaurs' involvement in one of the most disastrous mass extinctions in the Earth's history./ (Gardom and Milner, 1993) /T/he issues are still left open, but the change in attitude from the previous book is very apparent. Catastrophist explanations, like gradualist ones, are now being treated on their individual merits. Without question, catastrophism is once again alive and well. /REFERENCES/ Albritton, C.C., Catastrophic Episodes in Earth History, Chapman and Hall, London and New York, 1989. Alvarez, L.W., Alvarez, W., Asaro, F. and Michel, H.V., Extraterrestrial cause for the Cretaceous-Tertiary extinction, Science 208 (1980) 1095-1108. Bowler, P.J., Evolution - The History of an Idea, University of California Press, 1989. Charig, A., A New Look at the Dinosaurs, British Museum (Natural History), 1979, p. 151. Clube, V. and Napier, W., The Cosmic Winter, Basil Blackwell, Oxford, 1990. Corsi, P., The Age of Lamarck, University of California Press, 1988. Cuvier, G., 1796. Quoted in Burkhardt, R., The Spirit of System, Harvard University Press, 1977, p. 129. Darwin, C., On the Origin of Species, John Murray, London, 1859. (Quotations from Penguin Edition, pp. 341, 444 & 448.) Dawkins, R., The Blind Watchmaker, Longman, London, 1986, p. 241.? Desmond, A. and Moore, D., Darwin, Penguin Books, Harmondsworth, 1992. (Quotation, p. 614). Eldredge, N., The Miner's Canary, Virgin Books, London, 1991, p. 102. Gardom, T. and Milner, A., The Natural History Museum Book of the Dinosaurs, Virgin Books, London, 1993, pp. 78-79. Gehrels, T. (ed.), Hazards due to Comets and Asteroids, University of Arizona Press, Tucson, 1994. Geikie, A., 1897. Quoted in Gould, 1988 (op. cit.), p. 67. Geology Today, 1973. Quoted in Gould, 1988 (op. cit.), p. 68. Glen, W. (ed.), The Mass Extinction Debates, Stanford University Press, California, 1994. Gould, S.J., Hen's Teeth and Horse's Toes, Norton, New York and London, 1983, p. 322. Gould, S.J., Time's Arrow, Time's Cycle, Penguin Books, Harmondsworth, 1988. (Quotation, p. 119). Hallam. A., Great Geological Controversies, Oxford University Press, 2nd edn., 1989. Halstead, L.B., Dinosaurs, Blandford Press, Poole, 1981, p. 162. Halstead, L.B., Hunting the Past, Hamish Hamilton, London, 1982, p. 10. 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Ruse, M., Darwinism Defended, Addison-Wesley, Reading, Mass., 1982, p. 13. Stanley, S.M., Extinction, Scientific American Books, New York, 1987. (Quotation, p. 234.) Steel, D., Rogue Asteroids and Doomsday Comets, Wiley, New York, 1995. Velikovsky, I., Earth in Upheaval, Gollancz, London, 1956. Whewell, W., Bridgewater Treatise, 1833. Quoted in Darwin, 1859 (op. cit.), frontispiece. Whewell, W., 1837. Quoted in Albritton, 1989 (op. cit.), p. 50; and Hallam, 1989 (op. cit.), p. 54. Young, R.M., Darwin's Metaphor, Cambridge University Press, 1985, p. 136.