Lyell's theory of climate - University of Arizona - Lyell's Theory of Climate.pdfLyell's Theory of Climate DOV OSPOVAT University of Nebraska ... And his conception of the geological

Lyell's Theory of Climate

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University of Nebraska Lincoln, Nebraska

It has been clear, since the Hooykaas's analysis was published nearly twenty years ago, that the principle of the uniformity of nature, as interpreted by Charles Lyell, was not merely, as Lyell implied, a "principle of reasoning" which geologists had to emply in order to be scientific. Lyelrs conception of the principle of uniformity embodied also a hypothesis about the nature and pattern of geological change - a theory of the earth. 1 Some aspects of Lyelrs theory of the earth are well known. He belieVed that geological change has usually been gradual, rather than episodic. He proposed an indefinite, but very long, geological time scale. And his conception of the geological past was ahistoric; the earth, Lyell thought, undergoes no linear development through time. 2 What has not been well known is exactly how Lyell envisioned earth history. In a general way, the ahistoric character of Lyelrs theory supplies an answer: in the very long view, the indefinitely distant past and future, the earth undergoes no directional change. But what was Lyell's interpretation of the relatively short view, the known geological record with which geologists were primarily concerned?

1. R. Hooykaas, Natural Law and Divine Miracle: The Principle of Uniformity in Geology, Biology and Theology, 2nd ed. (Leiden: E. J. Brill, 1963). Subsequently many others have made the same point. See especially the following articles by Martin Rudwick: "The Principle of Uniformity," review of Natural Law and Divine Miracle (! st ed., 1959), by R. Hooykaas, Hist. Sci., 1 (1962), 82-86, "The Strategy of Lyell's Principles of Geology," Isis, 61 (1970), 5-33; "Uniformity and Progression: Reflections on the Structure of Geological Theory in the Age of Lyell," in Perspectives in the History of Science and Theology, ed. Duane H. D. Roller (Norman, Okla.: University of Oklahoma Press, 1971); "Caricature as a Source for the History of Science: De la Beche's Anti-Lyellian Sketches of 1831," Isis, 66 (1975), 534-560. See also Stephen J. Gould, "Is Uniforrnitarian- ism Necessary?" Amer. Jour. Sci., 263 (1965), 223-228.

2. "A-historic" is Hooykaas's description. On this aspect of Lyelrs theory, see, in addition to the works cited in n. 1, R. Hooykaas, "The Parallel between the History of the Earth and the History of the Animal World," Archives ]nternationales d'Histoire des Sciences, 10 (1957), 3-18; Catastrophism in Geology, Its Scientific Character in Relation to Actualism and Uniformitarianism (Amsterdam: North-Holland Publishing Co., 1970); Martin Rudwick, "A Critique of Uniformitarian Geology: A Letter from W. D. Conybeare to Charles Lyell, 1841," Proc. Amer. Phil. Soc., 111 (1967), 272-287.

Journalofthe History o f Biology, vol. 10, no. 2 (Fall 1977), pp. 317-339. Copyright © 1977 by D. Reidel Publishing Company, Dordrecht, Holland.

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It has also been clear, since the publication of Lyell’s Scient$c Journals on the Species Question, and especially since the appearance of Michael Bartholomew’s “Lye11 and Evolution,” that nongeological considerations, particularly the issue of man’s place in nature, played an important part in Lyell’s scientific thought.3 But it has not been clear how and to what extent these considerations shaped Lyell’s geology. The purpose of this paper is to examine closely the most important part of Lyell’s theory of the earth, his theory of climate, in order to explain what Lyell’s view of earth history was, what its origins were, and how it was related to Lyell’s “principle of reasoning*’ and to the available geological and paleontological evidence. I intend to argue that Lyell’s version of the principle of uniformity resulted from his desire to justify a highly speculative interpretation of the evidence about climate, an interpretation whose purpose was to preserve man’s unique status in creation.

Lyell’s theory of climate occupies a prominent position in the Principles of Geology. In the first nine editions (1830-1853), it is presented in Chapters 6-8, the central three of the five chapters which Lyell’s biographer, Leonard G. Wilson, has called “Lye& conscious effort to ‘create the science’[of geology] and to express the consequences of the uniformity of nature in the history of the earth.‘” These five chapters constitute, in fact, Lyell’s theory of the earth, and the theory of climate is its heart. As we will see, the central location of the discussion of climate is fully justified by its relationship to Lyell’s broad theoretical objectives.

The strategic importance of the theory of climate is due to Lyell’s conception of the connection between changes in the organic and inorganic worlds. Lye11 believed that environmental conditions, of which climate is the most important, determine which plants and animals exist at any time and place. The course of organic change thus depends directly on the course of climatic change. As Michael Barthol- omew has demonstrated, the question of the course of organic change

3. Leonard G. Wilson, ed., Sir Charles Lye/l’s Scientific JournaLr on rhe Species Quesrion (New Haven: Yale University Press, 1970); Michael Bartholomew, “Lye11 and Evolution: An Account of Lyell’s Response to the Prospect of an Evolutionary Ancestry for Man,” &it. J. Hist. Sci., 6 (1973), 261-303. Bartholomew contends that Lyell’s beliefs about man led him to adopt a nonprogression theory of the organic world but had no effect on the geology of the Principles (pp. 270-271,281). As will become clear below, the geological and biological aspects of Lyell’s thought cannot be separated in this fashion.

4. Leonard G. Wilson, Charles Lyek The Years to 1841: The RevoIution in Geology

(New Haven: Yale University Press, 1972), p. 278.

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was of great concern to Lye11 because of its close relationship to the question of man’s place in nature. In 1827, after reading Lamarck’s Philosophie zoologique, with its suggestion that man is part of a progressive evolution of life, Lyell, in order to escape Lamar&s conclusions, began immediately to seek alternatives to the theory of organic progress.5 Because of his belief in environmental determinism, Lye11 required a suitable theory of climate to support any new theory of the history of life. To state the matter more positively, Lyell’s belief in environmental determinism, which in the 1820’s was widely shared by his fellow naturalists, made it possible for him to use a theory of the inorganic world as the basis for a nonprogression theory of organic nature.

The features of Lyell’s theory of the earth which most sharply distinguish it from the views of his contemporaries had their origin in the interactions among Lyell’s belief in environmental determinism, his concern for man’s place in nature, and the available evidence about past climatic conditions. These three factors worked together, and it is impossible to explain the influence of one without taking into account the other two. But it was the evidence relating to climate that led Lye11 to the interpretation of earth history which he ultimately proposed. His desire to preserve man’s special place in creation initially inclined him toward a different interpretation, but this proved to be incompatible with what was then known about the climates of previous geological periods. He was thus forced to adopt a theory which was more highly speculative than he originally intended and even more speculative than historians have generally recognized+ for though his theory could account for the evidence on climate, it was not susceptible of being tested against it, or against any evidence whatever. As an aid in the construction of his theory, Lye11 formulated an equally speculative version of the principle of uniformity.

In common with most British geologists of his day, Lye11 held, as one of his most deep-seated convictions, the belief that every organism is perfectly adapted for the situation which it occupies in the economy of nature. This perfect fit between organisms and their environments was supposed to be the result of creative foresight. The organic and inorganic worlds were two parts of one divine and harmonious plan. On

5. Bartholomew, “Lye11 and Evolution,” pp. 264-265, 272-276.

6. Martin Rudwick and, especially, R. Hooykaas have pointed out the speculative elements in Lyell’s system, as did a number of his contemporaries (see nn. 1 and 2 above).

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this point, though on scarcely any others, Lye11 in the Principles of Geology and William Buckland in his Bridgewater treatise were in complete agreement. Buckland wrote, as a firm believer in organic progress, that modification of the earth’s surface represented a “general prospective adaptation to the economy of animal and vegetable life,” while plants and animals, in turn, “were constructed with a view to the varying conditions of the surface of the Earth, and to its gradually increasing capabilities of sustaining more complex forms of organic life.“l Lye11 expressed the same idea, but without any implication of progressive change. “We must suppose,” he said, “that when the Author of Nature creates an animal or plant, all the possible circumstances in which its descendants are destined to live are foreseen, and that an organization is conferred upon it which will enable the species to perpetuate itself and survive under all the varying circumstances to which it must be inevitably exposed.‘8

In the sense in which Lye11 understood it, perfect adaptation implied environmental determinism.9 It meant not merely that every form of life is well adapted to external conditions, but that it is the best adapted of all possible forms for the particular conditions under which it is destined to live. In 1863 Lye11 explained the assumption on which he had worked for over thirty years as follows: “creative power . :. has at successive geological epochs introduced new forms best suited to each area and climate.‘*is In his first species journal (1855) he was more explicit:

Of innumerable ways in which Omnipotence might fit a new species to all the present and future condit.S of its existence, there may be one which is preferable to all others. . . What is here called necessity may merely mean that it pleases the Author of Nature not simply to ordain fitness, but the greatest fitness...* i

I. William Buckland, Geology and Mineralogy considered with Reference to Natural

Theology, 2nd ed., 2 ~01s. (London: William Pickering, 1837), I, 35, 107. 8. Charles Lyell, Principles of Geology, II (London: John Murray, 1832), p. 24; see also

Principles o/Geology, III (London: John Murray, 1833), pp. 384-385; and Lyell,“Address to the Geological Society... 1837,” Proc. Geol. Sot. London, 2 (1834-1837), 517-521.

9. This was not the case for many of Lyell’s contemporaries, especially after 1830. Although environmental determinism depends on perfect adaptation, the latter does not necessarily imply the former. I have discussed environmental determinism and other interpretations of the idea of perfect adaptation in a paper read at the annual meeting of the History of Science Society in December, 1975, now in press.

10. Charles Lye& Geological Evidences of the Antiquity ofMan (Philadelphia: George W. Childs, 1863), p. 422.

I I. Wilson, ed., Lyell’s Scienrific Journals, p. 6.

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For any given set of circumstances, only one form can be the best fitted. Therefore, certain types of animal and vegetable existence are, as Lye11 put it, “required” for certain conditions, and certain conditions ‘determine” the existence of certain genera of plants and animals in different regions and at different times.12

Lyell’s most succinct statement of the principle of environmental determinism, and the one which shows most clearly its importance to his thinking, occurs in the Principles of Geology. Through the first nine editions, Lye11 insisted on the rule that similar external conditions will always produce analogous forms of life; in the early editions he also drew attention to the corollary that progressively changing conditions could be expected to produce a progressive series of life forms.‘3 It was this corollary which led Lye11 to develop his “Uniformitarian view” of earth history, designed, as he acknowledged, so that the “coming in of Man” might be “viewed apart.“14 Had Lye11 not been committed to environmental determinism, a theory of nonprogression in the organic world alone would have served his purposes. But as it was, his desire to argue that the forms of life which have successively appeared on earth do not constitute a progressive series leading toward man presented Lye11 with the task of constructing a theory of the inorganic world as well, one which would yield a nonprogressive series of climatic conditions.

The problem of climate arose for Lye11 in an acute form only after his reading of Lamarck in 1827. Until then, Lye11 believed in organic progress. Despite some reservations, he affirmed, in 1826, that “the general inference to be deduced from observed facts” is “that in ascending from the lowest to the more recent strata, a gradual and progressive scale could be traced from the simplest forms of organization to those more complicated, ending at length in the class of animals most related to man.“i5 At the same time Lye11 argued in favor of the generally accepted view of the changes which have occurred in the earth’s climate. Paleontological evidence, he said, pointed unmistakably toward a revolution in the temperature of the earth. Fossil animals and plants alike proved that the Northern Hemisphere had formerly had a

12. Charles Lyell, Principles of Geology, 5th ed., 2 ~01s. (Philadephia: James Kay and Brother, 1837). II, 459-460. The fiith London edition was not available to me. The passages cited occur in chap. 24 of book IV, in a section which is, in effect, asupplement to the discussion of the fossil record in chap. 9 of vol. 1 of the 1st ed.

13. Charles Lyell, Principles of Geology, 1 (London: John Murray, 1830), pp. 146-147. 14. Wilson, ed., Lye& Scientific Journals, p. 223.

15. [Charles Lyell], “Transactions ofrhe Geological Society of London.” Quart. Rev.. 34 (1826), 513.

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tropical climate and had subsequently cooled. The causes of “so surprizing a revolution” were a mystery which astronomy might some- day solve, Lye11 thought; geology, however, could provide no explanation. ‘6

Within four years Lye11 had changed his mind on geology’s ability to solve the problem and had himself proposed a geological explanation for the shift from a hotter to a cooler climate. The impetus which moved Lye11 to take up the question of climate was, I believe, his rejection of the theory of organic progress in 1827. Michael Bartholomew has described in detail Lyell’s reaction to Lamarck’s theory of evolution,r7 and I do not intend to repeat all that he has said. The essential point is that as soon as Lye11 had finished reading the Philosophie zoologique, he declared his intention of combating progressionism and sketched briefly a theory of nonprogression. He wrote the paleontologist Gideon Mantell:

1 am going to write in confirmation of ancient causes having been the same as modern, and to show that those plants and animals which we know are becoming preserved now, are the same as were formerly. E.g., scarcely any insects now, no lichens, no mosses, &c., ever get to places where they can become imbedded in strata. But quadrupeds do in lakes, reptiles in estuaries, corals in reefs, fish in sea, plants wherever there is water, salt or fresh, &c. &c. Now have you ever in Lewes levels found a bird’s skeleton or any cetacea? If not, why in Tilgate and the Weald beds? In our Scotch marl, though water birds abound in those lakes, we meet with no birds in the marl; and they must be at least as rare as in 01 freshwater formations, for they are much worked and examined. You see the drift of my argument-ergo, mammalia existed when the oolite and coal, &c., were formed.18

Lyell, in this first attempt to formulate a nonprogression theory, is proposing a steady-state view of the organic world. Those “higher” animals which exist now, but are only infrequently preserved, also existed in former ages.

In 1826, Lye11 had argued, on the basis of the principle that similar conditions always produce similar forms of life, that the tropical

16. Ibid., p. 528. 17. Bartholomew, “Lye11 and Evolution,” pp. 272-277. 18. Katherine Lyell, ed., L(fe, Lerrers and Journals of Sir Charles Lyell, Barr., 2 ~01s.

(London: John Murray, 1881), I, 169. See also Lye& letter to his father, Nov. 14, 1827, quoted in Wilson, Charles Lye/l, p. 182: “There was everything but Man even so far back as the Oolite.”

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character of the faunas and floras of ancient deposits was indisputable evidence for marked climatic change. 19 As long as Lye11 accepted the fact of organic progress, which constituted a corresponding change in the organic world, his argument was sound. But if he now wished to defend the steady-state hypothesis that there has been no significant change in the organic world, he would have to reconsider the question of climatic change as well.

During his travels in France and Italy in 1828 and 1829, Lye11 did indeed try to convince himself that the evidence for climatic change was not as conclusive as he had claimed in 1826. His efforts were unsuccess- ful, however. Instead of reasons for rejecting his former beliefs, he found further evidence for the tropical character of the ancient climate. He wrote John Fleming early in 1830:

As a staunch advocate for absolute uniformity in the order of Nature, I have tried in all my travels to persuade myself that the evidence [for climatic change] was inconclusive, but in vain. I am more confirmed than ever, and shall labour to account for vicissitudes of climate, not to dispute them.20

In the Principles of Geology, before giving his own account of climatic change, Lye11 devoted a chapter (Chapter 6) to a summary of the evidence that change had occurred. He recounted again the paleontological evidence which he had cited in 1826 - the tropical character of the ancient faunas and floras of the Northern Hemisphere. And he added to this new evidence which he had gathered himself in Italy. Where fossil shells in recent strata belonged to species still living in the Mediterranean, Lye11 found that the fossils exceeded in size the living representatives of the species:

Many of them are common to the Subapennine hills, to the Mediter- ranean, and to the Indian Ocean. Those in thefossilstate, and their living analogues from the tropics, correspond in size; whereas the individuals of the same species from the Mediterranean are dwarfish and degenerate, and stunted in their growth, for want of conditions which the Indian Ocean still supplies.21

19. Lyell. “Transactions of the Geological Society of London,” pp. 525-528. 20. K. Lyell, ed.. Life. Lerrers and Journals, I, 260. Fleming at this time was denying the

validity of the evidence for climatic change. John Fleming, “On the Value of the Evidence from the Animal Kingdom, Tending to Prove that the Arctic Regions Formerly Enjoyed a Milder Climate Than at Present,” Edinburgh New Phil. J.. 6 (1829), 277-286.

21. Lyell. Principles, 1. 94-95.

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“This evidence,” he said, “amounts to demonstration” of a formerly hotter climate.22

Lye11 was unable to accommodate such facts within a steady-state theory, which would, at most, allow for local fluctuations of temperature around a constant mean. He felt it necessary to adopt fully the interpretation which had long been placed on the evidence: from the Carboniferous period to the present there has been a change, in one direction, from a hotter to a cooler climate, over the whole of the Northern Hemisphere.23 Since for Lye11 climatic change implied organic change, a steady-state theory of nonprogression, such as he had sketched in his letter to Mantell, was no longer tenable. Lye11 could still argue that occasionally in the past local climatic conditions might have required the existence of some of the higher animals.24 But he could not and dit not insist that there had been no significant revolutions in the history of organic nature, for the general course of organic change must have run parallel to the progressively cooler series of climatic conditions.25

The problem of climate was now defined clearly for Lyell. If he would deny organic progress, he needed a theory which could at one and the same time account for an apparently directional climatic change during the whole period with which geologists were acquainted and claim that the change was not truly directional. Lyell’s solution to this formidable problem was a very elegant and very radical theory of climate. Lye11 proposed that the decrease in temperature since the Carboniferous period appeared to be directional because it was only a segment ofagreat geological cycle.26 This was the “grand new theory of climate” which he

22. Ibid., p. 95.

23. Ibid., p. 103. When Lyell first wrote the Principles ofGeo/ogv, the Carboniferous period was the earliest period familiar to geologists.

24. This is, in effect, what Lye11 did in chap. 9 of the Principles. See below. 25. Although he does not discuss it explicitly, Leonard Wilson has apparently noticed

Lyell’s shift, between 1827 and 1830, away from a steady-state view of climate and life. Wilson, Charles LyeN, pp. 182, 284.

26. Numerous historians, foremost among them Hooykaas. have noted a cyclical component in Lyell’s geological system, without, however, discussing either its source or

its characteristics. Hooykaas, Natural Law and Divine Miracle, pp. 29-30; Catostrophism in Geo1og.r. pp. 29-30.32.43.45; Walter F. Cannon, “The Uniformitarian-Catastrophist Debate,” Isis, 5/ (1960) 38; Loren Eiseley, Darwin’s Century (Garden City, N.Y.: Doubleday and Co.. 1961), p. 114; Camille Limoges. Lo StVection NaturelIe (Paris: Presses Universitaires de France, 1970). p. 15.

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devised during the winter of 1829-30, when the first chapters of the Principles of Geology were already coming from the press.27

When Lye11 wrote the Principles of Geology, the cooling of the earth’s climate was explained, by most geologists, in terms of the theory of central heat, according to which the earth had been formed as a very hot, presumably fluid, mass and then cooled, leaving a residual heat which continues to be diminished by further cooling.28 This cooling-earth theory of climate was the very antithesis of the sort of climate theory for which Lye11 was searching. It yielded a linear series of climatic conditions, and in the minds of Lye11 and his fellow geologists it was closely bound up with the theory of organic progress which Lye11 was trying to refute.29

As an alternative to the theory of central heat, Lye11 returned to a theory of climate which he had discarded in 1826 as being an inadequate explanation, suitable only for local climatic change. At that time he had written:

Geology can only account for local fluctuations of climate in the same latitudes, by furnishing us with evidence almost conclusive, that, during the deposition of stratified rocks, changes in the distribution of land and sea were frequent and considerable. In consequence of these changes the relative extent of superficial land and water may often have differed greatly. Continents or open seas may have alternately existid at the poles or at the equator. The land, according to its varying form, would necessarily determine in particular directions warm currents from tropical towards arctic seas, or cold currents bearing floating ice from arctic towards tropical latitudes. But these causes, though far too important to be kept out of view whenever this question is considered, are essentially partial in their operation and limited in degree; whereas the phenomena indicate[a]

27. The quoted phrase is from K. Lyeli, ed., L(/ti, Lerterrs and Journals, I, 261. On the

composition of vol. I of the Principles, see Wilson, Charles LyeI/, pp. 262-293.

28. On the theory of central heat, see Philip J. Lawrence, “Heaven and Earth: The Relation of the Nebular Hypothesis to Geology,” in Hismy. Theo/og.v, and Cosmology, ed. W. Yourgrau (in press): and Cannon, “The Uniformitarian-Catastrophist Debate.”

29. The union of the cooling earth theory and progressionism is what Martin Rudwick

has called the “directionalist synthesis” (Rudwick, “Uniformity and Progression”). The

bond uniting the organic and inorganic halves of the synthesis was the principle of perfect

adaptation as interpreted by geologists in the first half of the nineteenth century.

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most signal and remarkable alteration in climate, and that co- extensive with every part of the northern hemisphere hitherto examined in America, Europe, and Asia.30

During the winter of 1829-30, Lyell reversed his opinion and concluded that changes in the distribution of land and sea could be made to account not only for local fluctuations in climate, but also for a genera1 climatic change affecting the whole earth.31

In the Principles of Geology, Lye11 argued that the revolution in climate since the Carboniferous period was due not to the cooling of the earth, but to changes in the disposition of land and water on its surface. He assumed that the relative areas of dry land and ocean always remain fairly constant, as do the mean elevation of the land and the mean depth of the sea. But earthquakes, volcanoes, and other causes, he said, continuously bring about small variations in the relative positions of land and sea. Lyell explained, as he hadin 1826, how small changes in the position of land and sea could produce local climatic change and concluded that “unless the superficial inequalities of the eartl! be fixed and permanent, there must be never-ending fluctuations in the mean temperature of every zone.“3*

The question Lye11 posed was how such local fluctuations could combine to produce “a general change of temperature.“33 Over the course of ages, he said, continuously occurring variations in land and sea have completely altered the configuration of the earth’s surface. These variations gave Lye11 the freedom to rearrange the land and sea in his imagination to produce hypothetical climatic conditions. Citing the observations of numerous geographers and explorers, particularly

30. Lyell, “Transactions of the Geological Society of London,” p. 528. 3 I, Martin Rudwick has argued that the theory of climate in the Principles of Geology

was designed to account only for “local climatic change,” because Lye11 did not admit that

there had been a general change in climate. He suggests that Lyell thought that when the “overall state of the whole globe is taken into account,” the pattern of change is “steadr- stale, exhibiting relatively minor fluctuations around a constant mean” and that in the Carboniferous period northern Europe, but not the whole globe, was hotter. As we have seen, Lyell recognized a “great fluctuation in the mean temperature qfrhe earth” (my

italics) and his theory of climate was designed to account for a “great.. refrigeration of the mean temperature in a/l lafirudes” (Lyell’s italics) (Principles, I, 140). For Rudwick’s treatment of Lye& theory of climate see “The Strategy of Lyell’s Principles,“pp. 12-13;

“Uniformity and Progression,” p. 212; The Meaning of Fossils (New York: American Elsevier, 1972), pp. 180-181.

32. Lyell, Principles, I, 115. 33. Ibid.

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Alexander von Humboldt’s study of the distribution of the earth’s surface heat, Lye11 said that there will be a general drop in temperature when high land is collected around the poles or when the oceans lie mainly between the tropics. If these conditions are reversed, the general climate of the globe will be hotter. Lye11 described the conditions which might bring about the extreme of cold of which the earth’s surface is susceptible-“the winter of the ‘great year,’ or geological cycle” he called it-and those which would bring about the extreme of heat, or summer of the great year (see Fig. 1).j4 He then urged that during the Carbonifer- ous period, conditions approaching those of the hypothetical summer of

Fig. I. Maps showing hypothetical geographical conditions which would produce the summer and winter of the “great year, or geological cycle.” These maps appeared for the

first time in the third edition of the Principles of Geology.

34. Ibid., pp. 116-124.

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the great year had actually prevailed. Geological and paleontological evidence from numerous sources indicated that formerly the Northern Hemisphere was covered by a great ocean dotted with islands. Land must then have been concentrated between the tropics. In succeeding periods, the size of the islands in the northern ocean increased and new land was introduced above the tropics. As more and more of the northern ocean was converted into land, the general temperature of the globe was lowered to its present state.35

Lyell’s method of summing up small changes over a long period of time in order to produce a general result -the essence of the gradualist dynamics for which the Principles of Geology is famous - is nowhere better illustrated than in his theory of climate. “It is by the repetition of an indefinite number of local revolutions due to volcanic and various other causes,” he said, “that a general change of climate is finally brought about.“36 But though this method enabled Lye11 to explain the shift in climate without recourse to the theory of central heat, it could not alone eliminate the directional character of the shift. For that, Lye11 required an additional assumption, which he derived from the principle of uniformity.

Lye11 argued throughout the Principles of Geology that if nature is uniform, and the scientist must assume that it is, then “causes now in operation” are sufficient to account for all past geological events. As Hooykaas and Rudwick have pointed out, Lye11 went beyond such mere actualism to the proposition that not only were past causes the same as present, but the energy with which they acted was the same as we11.3’ “No causes wharever,” he wrote Roderick Murchison in 1829, “have from the earliest time to which we can look back, to the present, ever acted, but those now acting; and . . . they never acted with different degrees of energy from that which they now exert.“38 Two elements in Lyell’s theory of the earth may be derived from this extreme form of actualism. If all past geological change is the product of causes which are the same, in kind and in energy, as those now acting, then change must generally

35. Ibid., 125-143. pp.

36. Ibid., 124. p.

37. See especially Hooykaas, Natural Law and Divine Miracle, pp. 13-14; and

Rudwick,“TheStrategyof Lyell’s Principles,“pp. 6-7, and “Uniformityand Progression,”

pp. 211-213.

38. K. Lye& ed., Life, Letters and Journals, 1, 234.

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have been accomplished gradually. 39 And if the causes continue to act with the same intensity, it may be inferred that the energy of the system does not diminish over time.

Beyond these two hypotheses, Lyell’s extreme actualism does not necessarily imply any particular pattern of geological change. It is compatible with several sorts of steady-state system: for instance, a system in which there is no significant change in the conditions at the earth’s surface; or a system in which there are only minor fluctuations around some mean state; or a system in which change in one place is balanced by change in the opposite direction elsewhere at the same time. It is compatible also with a series of directionally changing conditions. And it is compatible with a cyclical system, in which change in one direction is after a time reversed. Extreme actualism thus gave Lye11 the freedom to choose from a wide variety of systems as the evidence and his fancy might require. And it left open the possibility of developing the principle of uniformity further in accordance with the needs of the system he chose.

Lyell’s choice of a cyclical system led him to push the principle of uniformity two steps beyond extreme actualism. The first was but a short step, and not an unreasonable one, from the proposition that geological causes have maintained a constant level of energy for the entire period which geology can investigate to Lyell’s contention that these same causes have the power to return the globe to any former condition.40 The problem of climate urged Lye11 to take one additional step - leap would perhaps better describe it-and affirm that actual causes not only can, but will in fact, return the globe to former conditions of its existence. “Vicissitudes of climate of no less importance [than those since the Carboniferous] may be expected to recur in future,” he said, “if it be admitted that causes now active in nature have power, in the lapse of ages, to vary to an unlimited extent the relative position of land and sea.“4’The summer of the great year will come again, Lye11 supposed, and with it will come summertime inhabitants as well:

Then might those genera of animals return, of which the memorials are preserved in the ancient rocks of our continents. The huge

39. It seems to me most likely that gradualism was what Lyell had in mind when he referred, in this same letter to Murchison, to the geological “system” which necessarily follows from his “principle of reasoning”(ibid.). See, however, Rudwick, “The Strategy of Lyell’s Principles,” p. 7.

40. Lyell, Principles, 1, 150.

41. Ibid., p. 125.

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iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyle might flit again through umbrageous groves of tree ferns.42

In the Principles of Geology Lye11 presented these poetic lines as “speculations,” which they certainly were; but they nevertheless reflect accurately his sincere belief. At about the time they were composed he wrote as follows to Gideon Mantell:

1 will not tell you how, till the book is out-but without help from a comet, or any astronomical change, or any cooling down of the original red-hot nucleus, or any change of inclination of axis or central heat, or volcanic hot vapours and waters and other nostrums, but all easily and naturally. I will give you a receipt for growing tree ferns at the pole, or if it suits me, pines at the equator; walruses under the line, and crocodiles in the arctic circle. And now, as I shall say no more, I am sure you will keep the secret. All these changes are to happen in future again, and iguanodons and their congeners must as assuredly live again in the latitude of Cuckfield as they have done so.43

Lye11 was fond of protesting that this cyclical system was not like those of the Oriental cosmogonists whom he ridiculed in the historical chapters of the Principles of Geology, for they went to the absurd extreme of supposing that “the same identical phenomena recurred again and again in a perpetual vicissitude.“@ Lyell’s claim, though absurd enough in the eyes of his contemporaries, is more modest.45 The environmental conditions which determine the existence of a species are too complex ever to be exactly repeated.& But they may be so nearly

42. Ibid., p. 123. 43. K. Lyell, ed., Life, Letters and Journals, I, 262. 44. Lyell, Principles, I, 157. 45. The most interesting contemporary response to Lyell’s theory is the geologist Henry

De la Beche’s lithograph cartoon “Awful Changes,” recently republished by Martin Rudwick (‘Caricature as a Source for the History of Science,” p. 539). The cartoon is

subtitled “Man only found in a fossil state. -Reappearance of Ichthyosauri,” and in it Professor lchthyosaurus is portrayed lecturing on a fossil human skull to a class of ichthyosaurs, plesiosaurs, and other extinct reptiles. In his discussion of the cartoon Rudwick deemphasizes its cyclical implications, treating Lyell’s belief in the return of the iguanodon as merely a consequence of the indefinitely repeated “fluctuations” of Lyell’s long view of geological change (p. 558).

46. Wilson, ed., Lyeil’s Scientific Journals, p. 5.

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repeated as to require the existence of the same genera at distinct epochs. It is only the genus Zguanodon which will return, Lye11 believed, not particular species of iguanodon.47

Lye11 was a follower of Hutton, and like the master, he professed to see in the rocks no vestige of a beginning and no prospect of an end.4* “All these changes” are to happen not once more, but indefinitely into the future, just as they have occurred an indefinite number of times in the past. As far as the interpretation of the geological record is concerned, however, Lyell’s theory reduces to the proposition that the earth has passed through only a portion of one cycle during the period with which geologists are acquainted. In later editions of the Principles of Geology Lyell, responding to a critic, stated explicitly that this was his opinion. “It has been objected to this theory of climate,” he noted, “that there are no geological proofs of the prevalence at any former period of a temperature lower than that now enjoyed; whereas, if the causes above assigned [Lyell’s mechanism for climatic change] were the true ones, it might reasonably have been expected that fossil remains would some- times indicate colder as well as hotter climates than those now established.” Lye11 pointed out that the present distribution of land and sea suggests that the present climates are much nearer the extreme of cold than of heat:

Hence it ought not to surprise us [he said] if, in our geological retrospect, embracing, perhaps, a small part of a complete cycle of change in the terrestrial climates, we should happen to discover everywhere the signs of a higher temperature. The strata hitherto examined may have originated when the quantity of equatorial land was always decreasing, and the land in regions nearer the poles augmenting in height and area, until at length it attained its present excess in high latitudes. There is nothing improbable in supposing that the geographical revolutions immediately preceding our times had this tendency; and in that case the refrigeration must have been constant [my italics].49

The one-cycle theory clearly suited Lyell’s needs. It allowed him to believe that the change in climate since the Carboniferous period was no more directional than is the seasonal change form summer to winter in a single calendar year. This provided him at last with a suitable founda-

47. Lyell, Princigles, 1, 123; II, 76. 48. Lyell, Principles, 111. 384-385.

49. Lyell, Principles, 5th ed., (1837), 1, 144-145. A similar statement first appeared in the 3rd ed. of the Principles, 4 vols. (London: John Murray, 1834-35), I, 202-203.

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tion for a theory of a nonprogressive history of life on earth; the course of organic change is only apparently progressive, for the iguanodon, ichthyosaur, and pterodactyl may be expected to return when the cycle is completed.

Having presented in the chapters on climate his own nonprogression theory, Lye11 turned in Chapter 9 of the Principles of Geology to a critique of the prevalent theory of organic progress. A principal component of his argument against progressionism is his cyclical view of climatic change. In Lyell’s opinion, his theory of climate was strictly incompatible with the theory of organic progress. This is the significance of Lyell’s effort to “create the science” of geology in accordance with his own interpretation of the principle of uniformity; for in the form in which he created it, the theory of a progression of life leading up to man was made impossible.50

Lyell’s argument on this point, like the rest of Chapter 9, is most horribly convoluted. The gist of it is that progressionism involves the assumption, which Lye11 held to be unphilosophical, that geological change is irreversible. According to Lyell’s principles, by contrast, all change is reversible, with the single exception of human progress, to which I will return. Lye11 began his discussion by conflating progressionism with Lamarck’s doctrine of a progressive tendency in nature. He then urged that the idea of a progressive tendency is inconsistent with the rule that similar environmental conditions always produce similar sorts of plants and animals. If plants and animals are introduced onto the earth in order of increasing complexity, according to some law of progress, he said, then it is this law or order, and not the environment, which determines the existence of different forms of life at different times. Progressionism, Lye11 implied, violates the uniformity of the law of perfect adaptation to conditions.51

Actually, the progressionist geologists by no means believed in a Lamarckian progressive tendency or a law of progress. As I noted earlier, they, like Lyell, believed in perfect adaptation and the depen- dence of organic change on changes in environmental conditions. Lye11 admitted as much in a brief passage whose main purpose was to criticize progressionists for putting faith in a speculative hypothesis of climatic change, the theory of central heat:

50. For a contrary opinion, see Bartholomew, “Lye11 and Evolution,” esp. pp. 270,281. 281.

51. Lyell, Principles, 1, 146.

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We grant, indeed, that there may be a connexion between an extraordinary profusion of monocotyledonous [“simpler’1 plants, and a youthful condition of the world, if the dogma of certain cosmogonists be true, that planets, like certain projectiles, are always red hot when they are first cast; but to this arbitrary hypothesis we need not again revert.52

The problem with progressionists, then, was not that they disregarded the law of adaptation, but rather that they were, in Lyell’s view, cosmogonists, for they assumed “either that the order of nature was formerly distinct” - which was true of some, but not most, progressionists - “or that the globe was in a condition to which it can never again be reduced by changes which the existing laws of nature can bring about”-which was indeed the case; most accepted the theory of a cooling earth.” Progressionism, Lye11 was arguing, partook of the worst features of the unphilosophical approach to geology which his own work was designed to banish. Progressionism would die a natural death, he thought, as soon as geologists adopted his principle that actual causes, their energy undiminished, have the powerto return the globe to any former condition, and may in time be expected to do so.

The second major component of the argument of Chapter 9 is Lyell’s well-known critique of the progressionist interpretation of the fossil record.s4 Lyell’s discussion of the fossil record is essentially an elabora- tion of the argument he outlined to Gideon Mantel1 in the letter of 1827 quoted earlier, without, however, the implication that there has been no significant change in the organic world. The similarity between the letter and the paleontological discussion in Chapter 9 of the Principles of Geology has, understandably, led some historians to see in Lyell’s treatment of the fossil record a steady-state theory of the organic world.55 Lye11 certainly believed that local conditions might have required the existence of birds or mammals in very ancient times. But it is certain also that he recognized that great changes in living nature must

52. Ibid., p. 147. 53. Ibid., p. 150. 54. Lyell’s paleontological discussion is treated by Rudwick, “A Critique of Uniformi-

tarian Geology,” pp. 277,28 l-282; and by Peter Bowler, Fossils and Progress: Paleontolo- gy and the Idea of Progressive Evolution in the Ninereenth Cenfury (in press). I am

grateful to Dr. Bowler for allowing me to read his book in manuscript. 55. Bartholomew, “Lye11 and Evolution,“pp. 283, 286; Rudwick, “Unformity and

Progression,” p. 225; Meaning ofFossils,p. 187; “Caricature as a Source for the History of

Science,” pp. 558-559.

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have occurred as a result of the shift from summer to winter in the great cycle of climate. He wrote, for instance, that in the secondary period mammals were “very feebly represented” (the two marsupial-like jaws from Stonesfield were the only instances known), while reptiles, which were better suited to the warmer climate, predominated.56

In a letter to Richard Owen in 1851 Lye11 defined his position quite clearly. As president of the Geological Society in that year, Lye11 devoted his anniversary address to a reiteration of his criticism of the progressionist interpretation of the fossil record.57 Owen responded for the progressionists with an article in the Quarterly Review in which he attributed to Lye11 the steady-state assumption “that the vertebrate kingdom was represented by the same classes, and in the same proportion at the secondary period as at the present day.“58 This, Lye11 wrote indignantly to Owen, “is not a correct enunciation of any theory ever proposed by me.. . I have always maintained that the fauna & flora of the Earth approximated more & more nearly to that now existing, in proportion as it was more modern & strikingly so in the successive tertiary periods.” He and theprogressionistsdiffered, Lye11 saidnotover the question of whether change had occurred, but over its interpretation. He went on to say, as he had in Chapter 9 of the Principles of Geology, that the progressionist’ interpretation was “cosmological,” because it was directional, while his was not.i9

The concluding half of Chapter 9 is concerned with man, the single exception which Lye11 made to his principle of reversibility. I need not dwell long on Lyell’s discussion, for others have treated it at length.” One point only requires emphasis. Unlike the existence of other animals, and plants, man’s existence, Lye11 thought, is determined not by environmental conditions, but by the “moral ends” of the Creator.6’ Man, that is to say, is not part of the ordinary course of nature, the cyclical variation which will one day result in the return of the

56. Lyell, Princ~$des, 5th ed. (1837), II, 459. The significance of the Stonesfield mammals is discussed briefly in Rudwick, Meaning of Fossils, pp. 145, 181.

57. Charles Lyell, “Anniversary Address of the President,” Quart. J. Geol. Sot. Zmdon, 7 (1851), xxv-lxxvi.

58. Richard Owen, “Lye11 On Life and Its Successive Development,” Quart. Rev.,

89 (185 I), 436. Owen’s article is discussed in Bowler, Fossils and Progress, and in Dov Ospovat, “The Influence of Karl Ernst von Baer’s Embryology,” J. Hist. EioL, 9 (1976).

59. Charles Lye11 to Richard Owen, Oct. 9, 1851, Owen Collection, British Museum (Natural History).

60. See especially Bartholomew, “Lye11 and Evolution,” pp. 282-284. 61. Lye& Principles, I, 162-164.

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iguanodon. Man, Lye11 said, is by nature progressive, the only progressive part of God’s creation .62 It was by portraying human history and the history of the natural world as utterly dissimilar, one progressive, the other cyclical, that Lye11 achieved the original aim of his quest for a nonprogressive geological system-he made geology serve to defend man’s unique status in creation against the threat posed by Lamarck.

The period following the publication of the Principles of Geology

witnessed major developments in both geology and paleontology. But for thirty years they had virtually no effect on the structure of Lyell’s theory. In geology, for instance, a most striking innovation was the introduction of the glacial theory by Louis Agassiz. Such a revolution- ary idea about past climatic conditions as Agassiz’s theory of an ice age might have been expected to produce significant changes in Lyell’s theory of climate. Lye11 might have used the proposition that there has been a period of intense cold between the summer of the great year and the present as an opportunity to return to his position of 1827 of doubting whether there had really been a constant refrigeration of the earth’s surface since the earliest known periods. Perhaps there had been constant fluctuations instead. Lyell, however, reacted differently; he chose to incorporate the ice age into his theory of cycles. The result was a very minor change in his discussion of climate. When at the end of Chapter 6 he came to summarize the known history of terrestrial climatic conditions, Lyell, as in the first edition, described a process of constant cooling, adding only that the cooling reached “its maximum of intensity in European latitudes during the glacial epoch, or the epoch immediately antecedent to that in which all the species now contemporary with man were in being. ‘~3 Whereas in earlier editions Lye11 could locate the present age only approximately, as probably somewhere near the extreme of cold, it could now be set clearly in place, just past the turn which leads from the winter back toward the summer of the great year.

The years after 1830 were also extraordinarily fruitful in paleontological discoveries, and with each new edition of his book, Lye11 dutifully revised his discussion of organic remains to accommodate the latest findings. The new evidence which emerged was generally, though by no means wholly, favorable to the view that “higher” forms of life had been

62. Ibid., p. 156. See also Wilson, ed., Lyeli’s Scientific Journds, p. 55: TMan’s] progressive power causes him to differ wholly from all the Brutes in kind rather than in degree.”

63. Charles Lyell, Principles of Geology, 7th ed. (London: John Murray, 1847), p. 92.

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successively introduced onto the earth since the earliest period of which the rocks preserved any record.64 Lyell, however, was not dismayed. One of the great advantages of his one-cycle theory of climate and life was that it could not be tested against any sort of evidence. Lye11 could accept every piece of paleontological data which preogressionists brought forth without weakening his nonprogression theory, for in his view the new fossil discoveries meant only that paleontologists were constantly learning more about a segment of a single geological cycle. The ninth edition of the Principles of Geology (1853) still contained both the passage on the return of the iguanodon and the suggestion that the known geological record represents only part of a complete cycle of change.

Lyell’s greatest source of difficulty was not the fossil record, but his colleagues. He found no support among them for his theory of cycles. After he learned (in 1856) that even his friend Darwin was a progressionist, and moreover an advocate of the Lamarckian hypothesis, Lye11 began seriously to reconsider his position.65 In 1859 he acknowledged in his private journals that the “uniformitarian view” had not “proved true,“& and by 1863 he publicly proclaimed his acceptance of progressionism.6’

As we have seen, the strategic importance of Lyell’s theory of climate resulted from his belief in environmental determinism. Lye11 abandoned that hypothesis along with nonprogressionism in the 1860’s. In the Antiquity of Man Lye11 admitted the inadequacy of the principle of perfect adaptation, the natural theological underpinning of environmental determinism. I formerly supposed, he said, that the Creator has from time to time introduced new forms “best suited to each area and climate.” But to this Darwin and Hooker have replied that the placental mammals introduced by man into Australia have become dominant there at the expense of the endemic marsupial inhabitants.68 In the tenth edition of the Principles of Geology (1867-68) Lye11 explained that this “in some degree” precludes our attributing the peculiarity of the marsupial fauna “to the nature of the climate, soil, and vegetation of Australia.“@

64. See Owen, “Lyell-On Life and Its Successive Development”; Bowler, Fossils and

Progress; and Rudwick, Meaning of Fossils, pp. 191-200. 65. Wilson, ed., Lyell’s Scienlific Journals, beginning on p. 55. 66. Ibid., p. 223. 67. Lyell, Antiquity of Man, pp. 405, 472. 68. Ibid., pp. 422-423. 69. Charles Lyell, Principles of Geology, 10th ed., 2 ~01s. (London: John Murray, 1867-

68), II, 332.

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With environmental determinism, as well as progressionism, elimi- nated from Lyell’s system, his theory of climate suffered a corresponding decline in significance. In the tenth edition of the Principles of Geology it was pushed back to Chapters 10-13, thus failing after, rather than before, the chapter on the fossil record and man(Chapter 9), which was now a defense of the theory of organic progress. Lyell’s critique of the theory of central heat, which formerly had served as the conclusion to the three chapters on climate, was removed to Volume II, where it formed part of a discussion of geological dynamics. The reference to the return of the iguanodon was deleted, as was Lyell’s proposition that the known geological record is a part only of a single revolution in the condition of the earth’s surface. Lye11 still suggested that there was a “great cycle of terrestrial climate,” but his advocacy of the idea was only half-hearted, for the “Annus Magnus” was no longer part of Lyell’s defense of the status of man .70 After 1869 Lye11 was forced to look outside geology for scientific justification for his belief in man’s uniqueness.”

This paper has focused on only a small part of Lyellian geology. Nevertheless, I think that from it some general conclusions may be drawn about the nature and origins of Lyell’s theory of the earth, his interpretation of the principle of uniformity, and his treatment of the evidence with which he and his fellow geologists had to work. Some historians, though by no means all, have recognized a cyclical component in Lyell’s geological system. But it has not been generally understood that Lyell’s interpretation of the known geological record amounted to a one-cycle (or, more appropriately, less-than-one-cycle) theory of earth history. Nor has it been clear what were the origins of this theory. A major part of the answer to the question of origins was provided by Michael Bartholomew when he drew attention to the important role Lyell’s ideas about man played in his scientific thought. It remained to investigate Lyell’s theory of climate in order to see how Lyell’s preconceptions about man’s place in nature influenced the structure of his entire theory.

When in 1827 Lye11 first turned his mind to the problem of constructing a scientific foundation for his conviction that man occupies a special place in the scheme of creation, his preference was for a steady-state theory of the earth. Except for his belief in environmentaldeterminism,

70. Ibid., 11, 267. 71. For some indication of the directions in which Lyell’s thoughts on the subject

turned, see Wilson, ed., Lyell’s Sciendjk Journals, pp. 193-194, 227, 230-231, 242-243, 245, among others; and Lyell, Principles, 10th ed., II, 485494.

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Lye11 could have argued on paleontological grounds alone that a steady- state interpretation of the history of life was as plausible as the theory of organic progress, for the fossil evidence was, as he insisted in the Principles of Geology, inconclusive.72 But Lye11 found that in the evidence for climatic change there was no comparable ambiguity. Convinced that there had been a revolution in the temperature of the earth, Lye11 rested his defense of man on the theory of climatic and organic cycles.

Lyell’s cyclical vision of earth history required that he introduce some rather creative modifications of the principle of the uniformity of nature. The relationship between Lyell’s theory and his “principle of reasoning” is two-sided. As others have pointed out, Lye11 always wrote as if his geological system followed directly from the principle of uniformity. Part of Lyell’s theory - most notably the gradualism which characterizes his mechanism for climatic change, and his geological dynamics in general - may indeed be derived from the proposition that geological causes remain ever the same in both kind and intensity. But the essence of Lyell’s uniformitarianism, 1 suggest, is his principle that change is reversible and his belief that it will be reversed. These follow not from the principle of uniformity, but rather from the requirements of cyclism. To a considerable extent, the relationship between theory and principle was the reverse of that which Lye11 presented. It was Lyell’s solution to the problem of climate which produced his idiosyncratic transformation of the principle of uniformity, and not the principle which led to his theory of the earth.

Theories of cyclical change in geology may have many sources - an analogy with the motions of the heavens, for instance. They are, it might be argued, characteristic of the Huttonian school of geology, of which Lye11 was a prominent member;73 witness Hutton himself, or William Fitton. In Lyell’s case, however, the idea of cyclical change was a response to a particular body of data, the evidence for climatic change; a response conditioned by two of Lyell’s fundamental beliefs - his belief in environmental determinism and his belief in the uniqueness of man. The relationship of Lyell’s theory to this body of data, like its relationship to the principle of uniformity, is two-sided. In one sense, his theory rests directly on the evidence. Lye11 did not seriously distort known facts in order to suit his own ideas. In common with his contemporaries, he

72. See Bowler, Fossib and Bogress. By the 1840’s it was far less inconclusive. 73. Hooykaas, Catastrophism in Geology, pp. 4546. 74. James Hutton, “Theory of the Earth,” Trms. Roy. Sot. Eilinburgh, I(1788). 209-

304; William Fitton, “Address” (February 20, 1829), Proc. Geol. Sot. London, 1 (1826- 1833). 134.

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recognized great changes, in one direction, in both the organic and the inorganic world, during the time span with which geologists were familiar. But in order to deny the progressive character of these changes, Lye11 argued that they will in time be reversed. Here Lyell’s preconceptions led him to construct a theory of the earth out of distinctly fanciful speculations about the future, speculations which were, of necessity, based on no evidence at all.

Lye11 holds an unshakable claim to the attention of historians. He is, without doubt, a figure of major proportions in the histori of nineteenth-century scientific thought. It was Lyell, and no one else, who made the idea of gradual, continuous change through geological time a part of the mental equipment of nearly every Victorian biologist and geologist. It was Lye11 who established the idea of a vast geological time scale of the sort required for an evolutionary view of organic nature. But Lye11 could not escape the influence of all the prejudices of his age. He shared with his fellow geologists the belief that organisms are perfectly adapted by the Creator’s wise design for the conditions under which they are to live, a belief which shaped his whole approach to the problem of the history of life and, as we have seen, to the problem of the history of the earth as well. Nor could Lye11 avoid the habit, which he was quick to criticize as unphilosophical in others, of according to an untestable hypothesis the status of a well-founded theory. Lye11 deplored the fact that many of his colleagues in the 1830’s saw little continuity in the geological record and were anxious to believe that each succeeding geological period witnessed a new creation of life, which they took to be proof of God’s continued attention to the affairs of man’s world. But Lyell, for closely related reasons, persuaded himself in similar fashion that “all these changes are to happen in future again.”

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