This objection may appear strong to those who believe in the indefinite possibility of change of forms in organized bodies, and think that during a succession of ages, and by alterations of habitudes, all the species may change into each other, or one of them give birth to all the rest. Yet to these persons the following answer may be given from their own system: If the species have changed by degrees, as they assume, we ought to find traces of this gradual modification. Thus, between the Palaeotherium and the species of our own days, we should be able to discover some intermediate forms; and yet no such discovery has ever been made. Since the bowels of the earth have not preserved monuments of this strange genealogy, we have a right to conclude, That the ancient and now extinct species were as permanent in their forms and characters as those which exist at present; or at least, that the catastrophe which destroyed them did not leave sufficient time for the production of the changes that are alleged to have taken place (1818, p. 119).
Darwin, as we all know, did not challenge Cuvier's literal description, but argued that a woefully imperfect record had failed to preserve insensibly graded intermediates in almost all cases. And Darwin, with his characteristic honesty, also admitted that his entire system depended upon the validity of this approach to the fossil record: “He who rejects these views on the nature of the geological record, will rightly reject my whole theory” (1859, p. 342).
In another confessional paragraph in the same mode, Darwin wrote of his debt to Lyell: “He who can read Sir Charles Lyell’s grand work on the Principies of [Page 492] Geology, which the future historian will recognize as having produced a revolution in natural science, yet does not admit how incomprehensibly vast have been the past periods of time, may at once close this volume” (1859, p. 282). Of course, we now acknowledge the immensity of time and therefore continue to open the Origin. We also dismiss Cuvier's argument that lack of recorded intermediacy precludes evolution.
But Cuvier's claim retains force in a more restricted sense: the earth's measured history does not provide enough time to dissolve the appearance of mass extinction into gradual change masked by an imperfect record. Geology therefore does run “too fast” to explain these crucial episodes by extended gradualism through an imperfect record — and if catastrophic mass extinctions are, as many paleontologists now argue (see Chapter 12), more frequent, profound, rapid, and different in their effects than we had previously admitted, then natural selection in Darwin's accumulative mode, with biotic competition as a primary source of order, may become seriously demoted in relative frequency among the causes of macroevolutionary pattern. In this sense, the pace of geological change remains a vital subject for evolutionary theory, and Cuvier's last line still sounds a valid warning. For evolutionists committed to biotic competition by wedging as the primary source of macro-evolutionary pattern, new data on mass extinction may “not leave sufficient time for the production of the changes that are alleged to have taken place.”
Fortunately, we may proceed beyond conjecture in trying to discern Darwin's personal response to geological systems that threatened the efficacy of natural selection as an adequate source for the larger pattern of life's history. Cuvier died while Darwin worked on the Beagle and Lyell thrust his metaphorical sword (at least to Darwin's satisfaction) through the heart of traditional catastrophism. But Darwin could not rest easy, in full confidence that geological change would always plod along at optimal slowness for natural selection. For a formidable challenge, similar in broad concept to Cuvier's but very different in overt claims, greatly worried Darwin during the last fifteen years of his life — the “odious spectre” of Lord Kelvin. Darwin's widely misunderstood response proves, once again, how clearly he had pondered and assimilated the logic of his theory — and how much he required the slowness and uniformity of geological change.
DARWIN'S GEOLOGICAL NEED AND KELVIN'S ODIOUS SPECTRE
The familiar story of Lord Kelvin's incursion into geology has usually been recounted as a morality play. In basic outline, arrogant physics invades, but beleaguered natural history holds the line and triumphs, ultimately in a twist of delicious irony. I won't dispute this basic outline, but an attention to detail does compromise the traditional moral message, while also providing a striking example of Darwin's geological commitments.
In 1866, William Thomson, the future Lord Kelvin, published one of the most arrogant documents in the history of science — a one-paragraph paper (with an appended calculation) boldly entitled “The 'Doctrine of Uniformity' [Page 493] in Geology Briefly Refuted.” This manifesto became the stalking horse in Kelvin's 40-year campaign to refute the substantive uniformities of rate and state (see pp. 479–484) by arguing that the earth's limited age did not provide enough time for explanations based solely on the accumulation of small effects produced by causes acting at current rates. Over the years, Kelvin developed a set of arguments, yielding broad ranges rather than precise ages, and based on limits to the age of both the sun (derived from estimates of meteoric influx), and of the earth (derived from outflow of heat and rotational slowing by tidal friction). Kelvin originally favored a date of some 100 million years (with an upper bound at 400 million), but he refined his estimates downward as the years passed, and finally settled upon a limited span of only 10-30 million years (at least for the duration of a solidified outer crust).
In Kelvin's most famous argument (and sole subject of the 1866 note), measurements of interior heat restrict the earth's age on the assumption that outflow represents continued cooling from an initially molten state. By measuring rates of outflow, we should be able to set an outer limit of maximal age for the origin of life by specifying the initial time of formation for a solid planetary surface. (In practice, such a calculation must remain highly uncertain due to in homogeneities of planetary composition and our own ignorance about the earth's interior.) More importantly, the entire argument rests upon an assumption that no sources of novel heat exist, and that all current efflux must therefore represent a residual flow from the original fireball of an initially molten planet. When the discovery of radioactivity revealed an engine of new heat, Kelvin's argument collapsed. In the delicious irony mentioned above, the same force that dethroned Kelvin's limited duration soon provided a clock to measure the earth's actual age — and the billions favored by many geologists triumphed over Kelvin's long campaign for restriction. (Kelvin lived into the age of radioactivity, but never publicly acknowledged his defeat. Lord Rutherford tells an interesting story of an early lecture that he delivered in 1904 on determining the age of the earth by radioactive decay — see Gould, 1985c. Rutherford spotted the aged Kelvin in his audience and realized that he was “in for trouble.” “To my relief,” Rutherford writes, “Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye and cock a baleful glance at me. Then a sudden inspiration came, and I said Lord Kelvin had limited the age of the earth, provided no new source of heat was discovered. That prophetic utterance refers to what we are now considering tonight, radium!”)
At this point in the conventional morality play, the story becomes a homily in the Manichean mode: The elegant mathematics of an arrogant physicist expires on the Achilles' heel of a false assumption. The humble and patient observers of nature, who always knew, in the bones of their rich empirical experience, that Kelvin must be wrong, but who dared not oppose such a powerful foe, triumph in the end. Empiricism wins the day: “speak to the earth, and it shall teach thee” (Job 12:8). In this canonical version, Darwin stands with his fellow geologists and biologists, forging a common front of natural historians against an intruder with no feel for the empirics of history. But this [Page 494] version requires revision to the point of near reversal — for the battle lines have been drawn falsely for two prominent reasons that have largely disappeared from historical memory.
First, Kelvin's efforts did not inspire fear and ridicule among most natural historians. The great majority of
biologists and geologists welcomed his attempt to replace a vague feeling about immensity with actual limits. Moreover, most naturalists regarded Kelvin's figure of 100 million years as quite generous and fully sufficient to render geological history at any rate suggested by the empirical record. (Only later, in the 1890's, as Kelvin revised his estimate drastically downward, did significant numbers of geologists demur, arguing that their record could not fit into the time now allotted. Darwin's persistent but idiosyncratic opposition, documented below as the chief claim of this section, has been mistaken as a general consensus of geologists, thus leading to the main error of the canonical version.) In fact, many natural scientists breathed a sigh of relief at the amplitude of Kelvin's early allotments. To cite one particularly astute observer from general culture, and to illustrate the diffusion of a common impression of sufficiency, Mark Twain stated in his famous essay, “The damned human race” (largely written as a satirical response to A. R. Wallace's argument for intrinsic human meaning in the cosmos, as justified by an early version of the anthropic principle): “According to these [Kelvin's] figures, it took 99,968,000 years to prepare the world for man, impatient as the Creator doubtless was to see him and admire him. But a large enterprise like this has to be conducted warily, painstakingly, logically.”
Second, Kelvin declared no general warfare against a hidebound science of geology; the implications that he drew from his own estimates of the earth's age contained little to offend most earth scientists. Kelvin did haughtily dismiss one style of argument carelessly pursued by many geologists, particularly the most committed of uniformitarians: the treatment of time as so vast that no practical limit could be placed upon any process — a kind of heuristic eternity, if you will. Most geologists accepted Kelvin's chastisement on this point, and happily altered their language because they felt unthreatened by Kelvin's estimate. One hundred million years seemed quite sufficient to accomplish any observed or inferred geological work.
Kelvin saw right through the shaky basis of Lyell’s conceptual edifice: the conflation of substantive and methodological meanings of uniformity. Spatio-temporal invariance (uniformity) of natural law must be assumed as a basis for scientific inquiry into the past, but uniformity of state (non-directionalism of earth's history) cannot be inferred as a consequence, and can, moreover and to the contrary, be directly refuted by nature's own invariable laws. On this particular issue, Lyell could not have encountered a tougher opponent than the man who had formulated the second law of thermodynamics! The very constancy of the second law, and the attendant vector of time that follows as a result, smashed any hope for a long-term, steady-state in the earth's physical appearance. In other words, uniformity of law disproved uniformity [Page 495] of state. (Eddington later gave elegant expression to this argument by describing Kelvin's second law of thermodynamics as “time's arrow.”)
Diplomatically perhaps, Kelvin attacked Hutton's earlier version of the conflation (via Playfair's famous exegesis of 1802), rather than Lyell's contemporary presentation. Kelvin quotes Playfair's (1802) most famous lines: “The Author of nature has not given laws to the universe, which, like the institutions of men, carry in themselves the elements of their own destruction. He has not permitted in His works any symptoms of infancy, or of old age, or any sign by which we may estimate either their future or their past duration.” Kelvin responds by demolishing this false rationale for a non-directional earth (Thompson, 1868, p. 2): “Nothing could possibly be further from the truth than this statement. It is pervaded by a confusion between 'present order,' or 'present system,' and 'laws now existing' — between destruction of the earth as a place habitable to beings such as now live on it, and a decline or failure of law and order in the universe.”
Later, in discussing the secular slowing of the earth's rotation due to tidal friction imposed by the moon (a trend now supported empirically by evidence from daily and yearly growth lines of fossil organisms), Kelvin strongly asserts that even a small directional effect, measured as seconds of slowing per century, strongly compromises the entire Lyellian system: “It is quite certain that a great mistake has been made — that British popular geology at the present time is in direct opposition to the principles of natural philosophy . . . There cannot be uniformity. The earth is filled with evidences that it has not been going on forever in this present state, and that there is progress of events towards a state infinitely different from the present” (1868, p. 16).
Kelvin criticized Lyell more directly for his claim that heat lost by radiation into space can always be reconstituted from other sources (chemical and electrical) — for such an article of non-directional faith violates the second law of thermodynamics: “These statements are directly opposed to the general principle of the dissipation of energy: and the hypothesis which they suggest is very inconsistent with our special knowledge of the conduction and radiation of heat, of thermoelectric currents, of chemical action, and of physical astronomy” (1868, p. 231).
Kelvin then invokes his second law to identify the main vector of physical change through time: as entropy and disorder increase, the energy of most causes must diminish. Therefore, most geological processes must have acted with substantially more vigor on the early earth, leading to an expectation for more rapid biological change at this time, if changes in the physical world potentiate biological evolution.
I earnestly beg Professor Huxley, and those in whose name he speaks, to reconsider their opinion, that the secular cooling of the earth and of the sun “had made no practical difference to the earth during the period of which a record is preserved in stratified deposits.” There is, surely, good ground for Sir Roderick Murchison's opinion that metamorphic causes [Page 496] have been more active in ancient times than at present, because of more rapid augmentation of temperature downwards below the earth's surface; and it cannot be reasonably urged that a hotter sun is not a probable explanation of the supposed warmer climate of the Paleozoic ages (1868, p. 230).
Kelvin's argument brings us to the nub of Darwin's objections. Darwin's strong opposition to Kelvin, a reaction that could almost be described as fear and loathing, has often been recorded, but rarely understood. To appreciate Darwin's intense reaction, we must focus upon the geological prerequisite for his account of evolution — a steady and intermediate rate of change, enough to prod, but not too much to overwhelm, natural selection. Kelvin's directional geology did not invoke the paroxysmal specter of traditional catastrophism. But he had raised, in many ways, an even greater threat — for his challenge operated as a double-edged sword to attack natural selection from both sides of Darwin's geological needs.
A question of time (too little geology)
Kelvin's estimate of time seemed generous to most naturalists, even to supporters of natural selection like Wallace, and to Darwin's self-appointed spokesman, T. H. Huxley. But Darwin envisioned natural selection as working so slowly, especially in its progressive mode of extended biotic competition, that any talk about limitation made him intensely nervous — for too little time could be equated with insufficient geological impetus for evolutionary change. Darwin had shown his hand in calculating a greatly exaggerated 300 million years for the denudation of the Weald (see p. 153), a figure that he expunged with embarrassment from later editions of the Origin (see Burchfield, 1975, pp. 70-72). Darwin had also urged readers to close his volume if they could not accept Lyell's views on the “incomprehensibly vast” time available for natural selection.
Kelvin's 100 million years sounded sufficiently long, but Darwin harbored deep doubts, especially in the light of Kelvin's related argument for a vector of diminishing rates of physical change through time. Kelvin's directional vector clashed with the most important item in Darwin's apologetics for the imperfection of the geological record. Darwin had been troubled, for example, by the abrupt appearance of so many complicated anatomies in the Cambrian explosion: “The case at present must remain inexplicable;
and may be truly urged as a valid argument against the views here entertained” (1859, p. 308). Darwin concluded that Precambrian seas had “swarmed with living creatures” not yet found as fossils. But the anatomical distance from the first living molecule to a trilobite certainly exceeded all later change from Cambrian forms to modern organisms. Following the gradualistic premise that amounts of change provide a rough measure of time, Darwin concluded that most of the earth's history had passed before the Cambrian explosion: “If my theory be true, it is indisputable that before the lowest Silurian stratum was deposited [Cambrian of modern terminology], long periods elapsed, as long as, or [Page 497] probably far longer than, the whole interval from the Silurian age to the present day” (1859, p. 307).
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