The Structure of Evolutionary Theory

by Stephen Jay Gould

  Thus, Kelvin's 100 million years since the initial consolidation of the earth's crust implied substantially less than 50 million years for the entire fos­sil record since the Cambrian explosion. The situation only worsened as Dar­win pondered Kelvin's views. If early changes had been so rapid and intense, then the earth must have spent most of its history just “calming down” enough to enter a realm where natural selection might work. Even more of Kelvin's limited time must therefore be allotted to a pre-Darwinian earth, and even less to a severely restricted later world amenable to natural selection as the cause of life's pattern. In January 1869, Darwin wrote to James Croll: “Notwithstanding your excellent remarks on the work which can be effected within a million years, I am greatly troubled at the short duration of the world according to Sir W. Thomson, for I require for my theoretical views a very long period before the Cambrian formation” (in F. Darwin and Seward, 1903, volume 2, p. 163). And, more graphically, Darwin wrote to A. R. Wallace in 1871: “I can say nothing more about missing links than I have said. I should rely much on pre-Silurian times; but then comes Sir W. Thom­son like an odious spectre” (in Marchant, 1916, Letters of Wallace, volume 1, p. 268).

  Lest we construe Darwin's anxiety as exaggerated, consider the corroboration that he received for his distress right from the horse's mouth. Kelvin him­self addressed Darwin's greatest fear in his own direct and succinct way. The great physicist did not dispute evolution per se, but Kelvin argued that his own limitations upon time had effectively debarred natural selection as an important mechanism. Kelvin, following a cultural tradition that Darwin had transcended, believed that some spiritual force must be guiding the progress of evolution, if only because geology provided insufficient time for producing the observed order by mechanical processes. As Burchfield observes (1975, p. 73): “Kelvin was convinced that the complexity of life bore witness to the work of a Creative Intelligence. He was equally convinced that whereas natu­ral selection would require almost endless time, divine guidance would enable evolution to produce the diversity of life in a relatively short period. Thus as far as evolution was concerned, his arguments for limiting the earth's age were also proofs of design in nature.” Kelvin wrote: “A correction of this kind [on the duration of geological time] cannot be said to be unimportant in reference to biological speculation. The limitation of geological periods, im­posed by physical science, cannot, of course, disprove the hypothesis of trans­mutation of species; but it does seem sufficient to disprove the doctrine that transmutation has taken place through 'descent with modification by natural selection'” (Thompson, 1868, p. 222).

  A question of direction (too much geology)

  Thus, the vector imposed by Kelvin's Second Law — intense energy of early causes, diminishing continuously through time — troubled Darwin by com­pressing the fossil record into a restricted realm of adequate calmness. But the [Page 498] same vector also challenged Darwin by stressing the importance of early ra­pidity itself. Restricted time implied too little geology when considered in terms of Darwin's need for an intermediate amount of environmental change. But the rapid change on Kelvin's early earth implied too much geology on the flip side of impermissibility. Intense heat flows and high rates of volcanism and erosion on an early earth raised the specter of traditional catastrophism, with its paroxysms of mass extinction. This argument about rapidity opposed Darwin at both essential poles of practice and theory (see Chapter 2). Could a process like natural selection really operate effectively on a planet en­gulfed in such perpetual turmoil? Must other causes of biological change be postulated for the early earth — a particularly distressing prospect for Darwin, who yearned, above all, to establish a temporally invariant and fully general account of evolution. Moreover, the prospect of such a prominent vector clashed with Darwin's Lyellian vision of an earth operating with sufficient constancy of change that the study of modern causes would suffice for ex­plaining the past as well. Kelvin, astute as ever, had explicitly proclaimed the victory of “catastrophism” more for its explanation of a vector of diminish­ing intensity than for its theory of paroxysms (1868, pp. 231-232). And Dar­win, who had so assiduously explored the ramifications of all major ideas in natural history, rejected the directionalist aspect of catastrophism as firmly as he dismissed the paroxysmal claim.

  Yet Darwin could not escape the directionalist implication, and he eventu­ally compromised on this single point alone. Putting his best face on adver­sity, Darwin added a passage to later editions of the Origin. He now ac­knowledged the vector of diminishing intensity in change, admitted higher rates of evolution on an early earth, and made peace with Kelvin's temporal restrictions by awarding evolution an ontogeny with a speedy childhood: “It is, however, probable, as Sir William Thompson [sic] insists, that the world at a very early period was subjected to more rapid and violent changes in its physical conditions than those now occurring; and such changes would have tended to induce changes at a corresponding rate in the organisms which then existed” (Origin, 1872, 6th edition).

  In a reciprocal move, Darwin also quietly dropped the following passage of his first edition, with its opposing claim for slower initial rates of evolution — no doubt with regret, for this original argument had embodied his favored theme of biotic control: “During early periods of the earth's history, when the forms of life were probably fewer and simpler, the rate of change was proba­bly slower; and at the first dawn of life, when very few forms of the simplest structure existed, the rate of change may have been slow in an extreme de­gree” (1859, p. 488).

  If all evolutionists had reacted to Kelvin in the same way, we might have learned something about conflicts across scientific disciplines, but little about Darwin's distinctive view. However, in a fascinating and little known aspect of this familiar story, Darwin stood virtually alone in loathing Kelvin's re­striction — and his reaction therefore reveals some important implications of his own strict selectionist logic applied with firm gradualist commitments. [Page 499] Darwin, as I have emphasized throughout, was dogged and relentless, fiercely honest and logical in his thinking. He wrestled with every major difficulty, working and reworking, fretting and fretting again, until he achieved closure or at least understood why a solution eluded him. He often became obsessed with problems (levels of selection, for example) that his supporters either didn't grasp at all, or didn't understand as sources of interest or trouble. Sometimes, as with Kelvin, he probably worried an issue too far, but when we grasp the source of his exaggerated concern, we understand the logic of his theory in a more complete way.

  Darwin's opposition to Kelvin has been well recorded, but the missing piece in this historical puzzle, and the key for revising the false but canonical version of this story as a morality play, lies in Darwin's loneliness. I have cited the approbation of most geologists for Kelvin's efforts, but when we note the similar acquiescence of Darwin's two major English supporters — Huxley and Wallace — then the story becomes even more interesting (and the falsity of the conventional version even more apparent).

  Huxley devoted his 1868 presidential address for the Geological Society of London to defending this profession against Kelvin's charges. Yet by Huxleyan standards — as the greatest literary polemicist (equalled, perhaps, by Buffon) in the history of biology — this particular address packs little punch. Huxley does not assert a distinctive geological way of thought against Kel­vin's unwelcome intrusions. Instead, he simply accepts Kelvin's claims, and defends geology only against Kelvin's characterizations. No one, he argues, not even Lyell (who had, by then, abandoned uniformity of state by admitting vectors in the history of life), maintains so strict and comprehensive a view of uniformity. In fact, the old dichotomy of uniformity vs. catastrophe has largely been swept aside, with both views yielding to an “evolutionism” based on slow, continuous and directional change on an ancient earth.

  Huxley argues that neither of Kelvin's two major claims contradicts this new, evolutionist synthesis. Geologists can accept Kel
vin's directionalism be­cause uniformitarianism has abandoned any former flirtation with the doc­trine of an earth in steady state. Geologists would be distressed by a truly seri­ous limitation upon time, but 100 million years provides more than sufficient amplitude for any legitimate geological purpose.

  Coming to his key point, Huxley allows that some people (he does not mention his friend Darwin by name) feel a tug between Kelvin's dates and a greater age supposedly implied by the extreme slowness of evolutionary change. But this feeling, Huxley assures us, cannot be defended. We can only assess the speed of evolution by calibration against elapsed geological time. Previous assertions of extreme slowness flowed from geological convictions about immensity — as no purely biological data exist for a truly independent calibration. If Kelvin has now demonstrated that time must be shorter, we can only conclude that evolution has generally been faster.

  But, it may be said that it is biology, and not geology, which asks for so much time — that the succession of life demands great intervals; but this [Page 500] appears to me to be reasoning in a circle. Biology takes her time from geology. The only reason we have for believing in the slow rate of the change in living forms is the fact that they persist through a series of deposits, which, geology informs us, have taken a long while to make. If the geological clock is wrong, all the naturalist will have to do is to modify his notions of the rapidity of change accordingly. And I venture to point out that, when we are told that the limitation of the period during which living beings inhabited this planet to one, two, or three hundred million years requires a complete revolution in geological speculation, the onus probandi rests on the maker of the assertion, who brings forward not a shadow of evidence in its support (1869, year of publication of 1868 ad­dress, in Huxley, 1894, pp. 328-329).

  And so Huxley, bowing to the physicists, concludes that geology has devel­oped no legitimate reason for discomfort with Kelvin's dates, while biology has similarly failed to offer valid objection. Therefore, a little terminological misunderstanding, and a minor battle over professional turf, can be resolved into the sweetness and light of intellectual agreement.*

  Huxley's acquiescence may not surprise us. After all, given his attraction to saltationist ideas, Huxley had never shared Darwin's commitment to gradual­ism and the attendant need for such ample geological time. But when we learn that A. R. Wallace, a stouter defender of natural selection than Darwin himself, also readily accepted Kelvin's dates, then we can better sense the idio­syncrasy of Darwin's concern. Wallace ventured even further than Huxley. He published several letters in Nature on the age of the earth and the measure­ment of geological time (1870, 1892, 1893, 1895a and b). Wallace willingly accepted Kelvin's dates, and he presented an ingenious argument to explain why some biologists had been fooled into believing that evolution proceeded so slowly.

  Working from a theory by James Croll linking ice ages to changes in the earth's orbit, Wallace claimed that the last 60,000 years had experienced extraordinary climatic stability due to unusually low orbital eccentricity. Before then, and probably for most of geological time, orbital eccentricity had been more pronounced, prompting climatic fluctuations in local areas, attendant deaths and migrations of faunas, and a greatly accelerated rate of evolution. As an unfortunate consequence of our generally valid uniformitarian method of measuring current rates and extrapolating backwards, we developed the [Page 501] false impression from our recent (and unusual) climatic stability that evolu­tion must always proceed with imperceptible slowness. But the general pace of natural selection can be much faster, matching the usual rate of climatic fluctuation. When we recognize this “slower change of species since the gla­cial epoch than at any former period” (1870, p. 455), we can correct our false allegiance to evolution's perpetual sloth, “thus allowing us to suppose change of form in the organic world to go on more rapidly than we had before thought possible” (1870, p. 455).

  Wallace then proposes a correction to Lyell’s estimates of time based on turnover in molluscan species. He obtains an age of 24 million years for the base of the Cambrian, proclaims himself in agreement with Kelvin's 100 mil­lion years since the solidification of the earth's surface (leaving some 75 mil­lion years for the Precambrian interval that Darwin had proclaimed longer than all subsequent time), and urges Darwin to accept this framework with good grace and confidence in natural selection:

  These figures will seem very small to some geologists who have been accustomed to speak of “millions” as small matters; but I hope I have shown that, so far as we have any means at present of measuring geolog­ical time, they may be amply sufficient. Taking Sir William Thomson's allowance of a hundred million years for the time during which the earth can have been fit for life, it yet allows Mr. Darwin, for the process of development from the primordial germ, three times as many years anterior to the Cambrian epoch as have elapsed since that date, an amount of time which, I believe, will fully satisfy him (1870, pp. 454-455).

  I find an ironic similarity between this tale of Darwin and his intellectual brethren, and the story of Solomon and the baby claimed by two women. The true parent loves her child so fiercely that she would rather give her offspring to an imposter than to see the infant dismembered in mock compromise. The false mother feels no such protective love, thus permitting Solomon's wise identification. I do not wish to push the analogy too far. Huxley and Wallace cannot be called false parents, but the child of natural selection had not origi­nated in their womb (or, in Wallace's case, at least not for so long a gestation, or so slow and prideful a growth). They had not nurtured this idea through so many years of thought, through such struggles of upbringing, to build such an extensive and coherent edifice of logic and implication. They could there­fore compromise, sacrifice a bit here, and trim a bit there, to preserve the entity, however tarnished. But, for Darwin, any departure from full integrity became unthinkable. Darwin also understood, however, that intransigence compro­mises the spirit of science, and that all good thinkers must make and admit mistakes. For this reason, Darwin separated his core commitments (the “es­sences” of my treatment) from postulates more easily compromised. To aban­don, or even seriously to mitigate, any of the core commitments might be equated with the unacceptable solution of dividing the child. A conviction about the generally slow and steady character of geological change, a prerequisite [Page 502] for the third leg on my tripod of Darwinian essentials, lay among the core commitments — for natural selection could not shape macroevolutionary pattern without this auxiliary support from another profession. We can only understand the intensity, and the loneliness, of Darwin's reaction against Kel­vin when we properly read his response as a cry from the heart of his entire system.

  [Page 503]

  CHAPTER SEVEN

  The Modern Synthesis as

  a Limited Consensus

  Why Synthesis?

  A rose may retain its fragrance under all vicissitudes of human taxonomy, but never doubt the power of a name to shape and direct our thoughts. The evolutionary consensus that became such a bulwark of orthodoxy by the time of the Darwinian centennial celebrations of 1959 featured no recognized name in its early days. Fisher offered no general designation for his genetically revivified Darwinism in 1930, nor did Haldane in 1932. Dobzhansky, begin­ning the second wave of integration in 1937, proposed no label, either for the theoretical center or for the general movement.

  The accepted name emerged later, and without conscious intent. The “synthetic theory,” or the “modern synthetic theory” — in many ways an oddly uninformative and overly broad name — derives from the title of a book writ­ten by the grandson of Darwin's most effective defender: Evolution, The Modern Synthesis, published by Julian Huxley in 1942. (Historian of science B. Smocovitis (1996) points out that, as a general goal among scholars, syn­thesis enjoyed a great vogue during these years, especially as a central theme for measuring intellectual maturity, as expressed in the “unity of science” movement expounded by positivist
philosophers of the Vienna Circle and supported by biological pundits like J. H. Woodger. Ernst Mayr, for example, strongly supported the unity of science movement early in his career, but changed his mind when he began to fear that misplaced claims for grander synthesis would bury natural history in a reductionist scheme to uphold the primacy of physics and chemistry.)

  Huxley obviously felt that the morphology of evolutionary consensus could best be described as a synthesis — that is, a gathering together of previ­ously disparate elements around a central core. Following Smocovitis's argu­ment on the favorable Zeitgeist for “synthesis” provided by the “unity of sci­ence” movement, note how Huxley places his chosen name within this wider context by extolling the general virtues of synthesis.

  Biology in the last twenty years, after a period in which new disciplines were taken up in turn and worked out in comparative isolation, has become a [Page 504] more unified science. It has embarked upon a period of synthesis, until today it no longer presents the spectacle of a number of semi-inde­pendent and largely contradictory sub-sciences, but is coming to rival the unity of older sciences like physics, in which advance in any one branch leads almost at once to advance in all other fields, and theory and experi­ment march hand-in-hand. As one chief result, there has been a rebirth of Darwinism (1942, p. 26).

  Thus we evolutionists have, ever since, called our central theory by a name fashioned for a largely superseded set of concerns. In another sense, however, Huxley chose his name to emphasize a particular scenario for evolutionary science — and a characterization of this history as a “synthesis” seems both reasonable and accurate. Huxley viewed the synthesis, in an interpretation that became common in his time and has remained conventional ever since, as a two-stage process of integration around a renewed Darwinian core. He em­phasized this Darwinian center at the close of the passage cited above, and then offered praise, in purpler prose, to “this reborn Darwinism, this mutated phoenix risen from the ashes of the pyre kindled by men so unlike as Bateson and Bergson” (1942, p.28).