The Structure of Evolutionary Theory

by Stephen Jay Gould

  Curiously, each generation of historians and biological commentators has to discover, anew and for itself — and by reading original sources rather than imbibing mythology — this general and mainstream scientific position held by a man with such idiosyncratic views on specific subjects (see Mayr, 1972, and Simpson, 1961a, for the scientists; Gillispie, 1959; Burkhardt, 1977; and Corsi, 1988, for historians). For example, Gillispie wrote in his classic article for the Darwinian centenary (1959, p. 275): “Life is a purely physical phe­nomenon in Lamarck, and it is only because science has (quite rightly) left be­hind his conception of the physical that he has been systematically misunder­stood and assimilated to a theistic or vitalistic tradition which in fact he held in abhorrence.”

  This correction allows us to see Lamarck as a key figure in and of his time — an age as rife with intellectual, as with political, ferment — and not as a painfully peripheral, and actively marginalized, oddball. In a meticu­lous analysis of French scientific thought, Corsi (1988) has placed Lamarck's views firmly amidst the debates of his age, and also demonstrated that his the­ories were not so ignored or ridiculed as tradition maintains.

  I am not arguing that Lamarck was popular in his day, only that he was contemporary. In many ways, Lamarck became his own worst enemy, and he owed his fall from favor towards obscurity as much to his own unfortunate habits as to the peculiarity of his ideas. He possessed no political skills, and could only fare badly in any match with the masterful Cuvier (in an age that must rank as the best and the worst of all political times). He continued to practice the old style of speculative system building in an increasingly empiri­cal climate. He was combative, and so self-assured, that affirmation without any documentation became his principal style of argument. Consider this claim for use and disuse from the Philosophie zoologique (1809, 1984 edi­tion, p. 108): “Nothing of all this can be considered as hypothesis or private opinion; on the contrary, they are truths which, in order to be made clear, only require attention and the observation of facts.” Lamarck's certainty ex­tended even to the maximally dubious subject of weather forecasting: “I am not submitting an opinion, but announcing a fact. I am indicating an order of [Page 174] things that anyone can verify through observation” (in Corsi, 1988, p. 59). The old story that Napoleon refused a copy of Lamarck's Philosophie zoologique is apparently true (unlike most legends in the history of science). But Napoleon's motive has not generally been recognized: he mistook the nature of the gift, thinking that he had been offered one of Lamarck's discredited volumes of weather predictions for the coming year!

  LAMARCK AS A SOURCE

  The preceding section on Lamarck as a man of his time may seem peripheral, if not wholly out of place, in a chapter on hierarchical causation in evolution­ary theory, but this theme holds a definite place in the logic of my presen­tation. Such a diffuse and comprehensive idea as evolution can claim no sin­gle initiator or unique starting point. The search for precursors in ancient Greece, while overextended (Osborn, 1894), rests upon a legitimate founda­tion. But Lamarck holds a special place as the first to transcend footnote, pe­ripheral commentary, and partial commitment, and to formulate a consistent and comprehensive evolutionary theory — in Corsi's words (1988, p. xi) “the first major evolutionist synthesis in modern biology.”

  Moreover, even in a book parochially skewed to British and American evolutionary theory, Lamarck still merits the status of an ultimate source. Ger­man and French biologists could cite a variety of references from their in­digenous movements of Naturphilosophie (Oken, Meckel, or Goethe himself, for example) and the revolutionary times of the Age of Reason (Buffon, Maupertuis, Diderot, and a host of largely forgotten Enlightenment figures). England could boast a few precursors (including Darwin's grandfather Eras­mus), but no strong movements. Ironically, as Darwin, Wallace, and all the great mid-century evolutionists acknowledged, Lamarck instigated both ma­jor treatments of evolutionary thought in English before 1859: first, the accu­rate and extensive, if negative, presentation of Lamarck's system by Charles Lyell in the first four chapters of The Principles of Geology, Volume 2 (1832); and second, the anonymously published (1844) Vestiges of the Natural His­tory of Creation. The author of that scandalous and widely debated book, the Scottish publisher Robert Chambers, acknowledged Lamarck, via Lyell, as his major source of inspiration.

  I have (see Chapter 1) rejected Hull's genealogical approach to the defini­tion of theories, but I certainly defend this criterion (almost tautologically, I suppose) for the tracing of influences. Of Lamarck's foundational impact on English evolutionary thought, Hull (1985, p. 803) writes: “Darwin first con­fronted a detailed explication of the species problem in the context of Lyell's refutation of Lamarck in his Principles of Geology . . . Others like Spencer and Chambers were converted by reading Lyell's refutation of Lamarck; still others like Wallace and Powell were led to entertain the possibility of evolu­tion by reading Chambers.” We cannot, in short, view Lamarck as an oddity, cast aside by his own contemporaries, and irrelevant except as a whipping boy ever since. And we must acknowledge that Lamarckism, properly de­fined, forms a coherent and innovative system in the context of its own time. [Page 175]

  Lamarck's active centrality provides a foundation to my historical argu­ment because his theory, as presented in the next section, rests upon the con­cept of hierarchy, with distinct causes at two primary levels. Lamarck's hier­archy differs radically in form and logic from any acceptable modern version; indeed, I shall reject the basis of Lamarck's notion as an important compo­nent in developing the modern interpretation.

  Lamarck's concept became Darwin's context. In perhaps the most widely quoted of all his letters, Darwin wrote to Hooker on January 11, 1844 (in F. Darwin, 1887, volume 2, p. 23): “I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) im­mutable. Heaven forfend me from Lamarck nonsense of a 'tendency to pro­gression,' 'adaptations from the slow willing of animals,' etc.! But the conclu­sions I am led to are not widely different from his; though the means of change are wholly so. I think I have found out (here's presumption!) the sim­ple way by which species become exquisitely adapted to various ends.”

  Hierarchy has resided at the heart of evolutionary theory from' the very beginning, despite a temporary eclipse during the rally-round-the-flag pe­riod of strict Darwinism at the middle of the 20th century. When the Beagle docked at Montevideo, Darwin received his most precious item of mail — volume two of Lyell`s Principles of Geology. His joy at this gift, and his careful study of the contents, are well attested. This volume began with a long and careful exposition of Lamarck's theory, fairly but negatively described by Lyell. Darwin formulated his focal concept of small-scale change, based on organismal advantage as the mechanism (by extrapolation) for all evolu­tion, as an explicit denial of Lamarck's hierarchy of causes. I believe that Darwin correctly rejected an untenable theory of hierarchy based on distinct causes for different levels, but that (in a historically portentous example of the cliche about babies and bathwater) he carried a good thing too far by dismissing the general concept entirely. I conceived this book — The Structure of Evolutionary Theory — both as a celebration of Darwin's exemplary tough­ness, and as a call for the reinstitution of causal hierarchy, properly refor­mulated.

  LAMARCK'S TWO-FACTOR THEORY: SOURCES FOR THE

  TWO PARTS

  In the short period of 1797 to 1800, beginning with the Directory in power and culminating in Napoleon's coup d'etat of 18th Brumaire year VIII (No­vember 9,1799), Lamarck became an evolutionist and constructed the major features of his theory. Scholars have identified many sources as Lamarck's pri­mary impetus — his developing views on spontaneous generation, his work on living and fossil shells (Burkhardt, 1977), the implications of his unconven­tional theories in physics and chemistry (Corsi, 1988). But I wish to present the logic, rather than the ontogeny, of his final and completed argument.

  Lamarck's
evolutionary system attempts to marry two sets of ideas, each embodying a primary module of his conceptual world. These two sets com­mingle at their edges, but their distinction establishes the basis of Lamarckism, [Page 176] and provides a hierarchical context for this first comprehensive attempt to formulate an evolutionary theory.

  The first set: environment and adaptation

  The first set focuses on adaptation, and links this key attribute of organisms to the history of environments, the general pace of change, and the intimate relationship between physical and biological worlds through time. (Corsi, 1988, grants primacy to this set in the ontogeny of Lamarck's developing ideas; I accept this assertion but note that the same set, curiously, becomes secondary in the logic of Lamarck's fully formulated argument.) The frame­work can be entered in several places, with the rest of the edifice following by implication from a few basic premises. Lamarck's views on extinction provide a good beginning.

  In opposition to his colleague Cuvier, and acting as a major source of their estrangement, Lamarck denied that true extinction (defined as termination of genealogical lines) could occur — though he allowed an exception for large quadrupeds wiped out by human predation. (Cuvier, on the other hand, embraced extinction both as the foundation of geological ordering, and as a cardinal indication that animals cannot evolve to match changing environ­ments.) Yet, as a molluscan paleontologist, Lamarck knew that the morphol­ogies of organisms within major taxonomic groups changed in an orderly manner through time. Evolution of outward form, with consequent preserva­tion of lineages from extinction, represents the only alternative to termina­tion of lineages followed by creation of new and different morphologies.

  Lamarck far out-Lyelled Lyell in his commitment to uniformitarian geol­ogy (an ironic fact, given Lyell's lambasting of Lamarckian biology in his own treatise on geological uniformity). Lamarck's geological volume, the Hydrogeologie of Year X (1802), may strike us as bizarre in several particular as­sertions; but his general commitment to uniformity cannot be denied as a primary feature. Lamarck would admit no causes not now observable in op­eration; in particular, no paroxysms or catastrophes beyond the range of modern effects. He adopted Hutton's rigidly ahistorical vision (see Gould, 1987b) and postulated a geological history ruled by aqueous erosion (hence the title of his work). Cycles of construction and erosion unfold so many times, and in so similar a manner, that individual moments lose any distinct­ness, given past and future repetition of their features. Ocean waters carve mountains and continents (though Lamarck made an exception for volcanoes built by magmas). Currents tend to flow from east to west, and continents therefore erode on their eastern borders and accrete by deposition at their western edges. In a sense, therefore, continents undergo a slow westward march around the globe. This curious circumnavigation has occurred several times during the earth's extended history. But why doesn't the process yield directionality as erosion wears continents down to flat plains permanently be­low water? Lamarck countered with his distinctive mineralogical thesis: all rocks arise as ultimate products of organic deposition. Erosion may break continents into tiny comminuted grains, bits, and dissolved material; but as [Page 177] long as organisms maintain their steady state of abundance, these raw materi­als will be taken up again and redeposited as new rocks fashioned from the products of life.

  In summary, if lineages could not become extinct, if climate and geology changed in a continuous and insensibly gradual manner throughout geologi­cal time, and if the forms and functions of organisms always matched the fea­tures of their local environments, then gradual, adaptive evolution becomes a logical necessity.

  But by what mechanism will this ineluctable evolution occur? In particular, since steady, continuous alteration of environment provides the impetus for organic change, how does information flow from new environments to mod­ify the old forms of organisms? Lamarck's answer to this riddle — building only one corner of his complete system — invokes the familiar ideas that later generations would call “Lamarckism” when the rest of his edifice had been forgotten. Lamarck begins by formulating the central principle of his func­tionalist credo — the counterintuitive statement, later embraced by Darwin as well, that form follows function as the order of life's history. When we con­template any adaptation of an organism, and consider the intricate correla­tion of form with function, we naturally assume (or so Lamarck asserts) that form comes first, and that function can only follow. (God makes a wing, and a bird can then fly, to cite a nonevolutionary example.) But Lamarck ad­vanced the paradoxical reverse order as his key premise: new habits lead to altered structures.*

  In Lamarck's proposed mechanisms, environment changes first. Indeed, environment changes slowly and continuously on our uniformitarian planet. “Every locality,” Lamarck writes (1809, p. Ill), “itself changes in time as to exposure, climate, character and quality, although with such extreme slow­ness, according to our notions, that we ascribe it to complete stability.” Or­ganisms must accommodate to these changes by alterations in their habits — chewing with greater strength if the food gets tougher, moving more vigor­ously if the temperature gets colder. These altered habits, if long sustained, must feed back upon the organism in the guise of altered morphology or physiology — a thicker beak to crack the harder seeds, longer hair on a tougher skin to resist the cold.

  At this point in the argument, the famous “Lamarckian” theory of inheri­tance finally enters. As many scholars have documented, “soft,” or “La­marckian” inheritance represented the folk wisdom of Lamarck's time, and cannot be regarded as an innovation of the Philosophic zoologique. There­fore, the restriction of “Lamarckism” to this relatively small and non-distinc­tive corner of Lamarck's thought must be labeled as more than a misnomer, [Page 178] and truly a discredit to the memory of a man and his much more compre­hensive system. In any case, the changes wrought by new habits during an organism's lifetime can be passed directly to offspring in the form of altered heredity. Soft inheritance may have been the standard belief of the time, but Lamarck certainly recognized its crucial and particular role in his system. He wrote with his characteristic lack of doubt (1815, in Burkhardt, 1984, p. xxix): “The law of nature by which new individuals receive all that has been acquired in organization during the lifetime of their parents is so true, so striking, so much attested by the facts, that there is no observer who has been unable to convince himself of its reality.” Lamarck abstracts his idea of inher­itance as two principles, usually printed in italics in his texts to emphasize their importance, and known ever since as:

  • use and disuse

  • the inheritance of acquired characters (1809, volume 1, p. 113)

  Even if this theory of inheritance ranked as folk wisdom of the day, Lamarck's revolutionary statement, one of the great transforming insights in the history of human thought, resides in the preceding principle that trans­lates this mode of inheritance into a theory of evolution — the triggering of change in form by prior alterations in behavior. Lamarck clearly recognized the central role of this claim, for he always cited this counterintuitive se­quence of causality — from altered environments to changed habits to modi­fied form — as the linchpin of his entire system. In the Philosophie zoologique, he quotes his own earlier statement from the Recherches of 1802: “It is not the organs, that is to say, the nature and shape of the parts of an animal's body, that have given rise to its special habits and faculties; but it is, on the contrary, its habits, mode of life and environment that have in course of time controlled the shape of its body, the number and state of its organs and, lastly, the faculties which it possesses” (1809, p. 114). Lamarck then makes his threefold causal chain — environment to habits to form — even more explicit (1809, p. 126): “This is a fact that can never be disputed; since nature shows us in innumerable other instances the power of environment over habit and that of habit over the shape, arrangement and proportions of the parts of animals.”

  Causality might run from alte
red environment to changed organism, but Lamarck insisted that he did not view organisms as passive writing slates, ripe for inscription by the modifying hand of environment. Environmental change translates to adaptation of form only through the intermediary of or­ganic action expressed, in higher creatures at least, as altered habits: “What­ever the environment may do, it does not work any direct modification what­ever in the shape and organization of animals. But great alterations in the environment of animals lead to great alterations in their needs, and these al­terations in their needs necessarily lead to others in their activities. Now if the new needs become permanent, the animals then adopt new habits which last as long as the needs that evoked them” (1809, p. 107).

  These statements about the responses of animals to “felt needs” (Lamarck [Page 179] used the word besoins) left Lamarck open to charges of mystical vitalism when distorted for rhetorical purposes by Cuvier, or approached with over­wrought caution by Darwin. One might caricature this part of Lamarck's sys­tem by saying that a giraffe felt a need for a long neck, stretched ever so hard, and then passed the results of these successful efforts directly to offspring. But a fair assessment of Lamarck's actual words shows that he advocated no inef­fable willing, but only the commonplace idea that a change in environment can, in an almost mechanical way, elicit an organic response in terms of al­tered habits: “Variations in the environment induce changes in the needs, habits, and mode of life of living beings . . . these changes give rise to modi­fications or developments in their organs and the shape of their parts” (1809, p. 45).