The Structure of Evolutionary Theory

by Stephen Jay Gould

  McCosh argues that God's existence and benevolence can be inferred from either principle — from the first by the order of taxonomy, and the abstract beauty of bodily symmetry and structure; and from the second, by “adapta­tion,”* or the exquisite fit of form to function. McCosh also notes that the second, or functional, argument constitutes the “national signature” of Brit­ish thought: “The arguments and illustrations adduced by British writers for the last age or two in behalf of the Divine existence, have been taken almost exclusively from the indications in nature of special adaptation of parts” (1869, p. 6).

  The main lineage of this national tradition for “natural theology” based on the “argument from design” runs from Robert Boyle's Disquisition About the Final Causes of Natural Things (1688) and John Ray's Wisdom of God Mani­fested in the Works of the Creation (1691) in Newton's generation that pro­mulgated what historians call “the scientific revolution”; to a grand culmina­tion in William Paley's Natural Theology (1802), one of the most influential books of the 19th century; to an anticlimax, during the 1830's, in the eight “Bridgewater Treatises” (including volumes by Buckland and Whewell), es­tablished by a legacy from the deceased Earl of Bridgewater for a series of volumes “on the power, wisdom, and goodness of God, as manifested in the creation.” Critics in Darwin's circle generally referred to this series as the “bilgewater treatises.”

  Revolutions usually begin as replacements for older certainties, and not as pristine discoveries in uncharted terrain. In understanding the second pole of Darwin's genius as the uncompromising radicalism of his new philosophy for life and history, we must first characterize the comfortable orthodoxy [Page 118] uprooted by the theory of natural selection. Darwin's essential argument begins with a definition of the dominant philosophy for natural history in his day — natural theology in the Paleyan mode.

  At the outset of Chapter 4, I will say more about Paley and the alternative vision of continental natural theology (adaptationism vs. laws of form). For now, a simple statement of the two chief precepts of Paleyan biology will suffice:

  Natural theology in general. The rational and harmonious con­struction of nature displays the character and benevolence of a creating God. In the last four chapters of his book, Paley tells us what we may infer about God from the works of creation. God's existence, of course, shines forth in his works, but this we know from many other sources. More specifically (and with a Paleyan chapter for each), nature instructs us about God's personality, his natural attributes, his unity, and (above all) his goodness.

  Paley's particular version of natural theology. Natural theology has been expressed in two basic modes (see Chapter 4), one primar­ily continental (laws of form), the other mainly British (adaptationism). Paley held that God manifests his creating power in the exquisite design of organ­isms for their immediate function. We all know Paley's famous opening meta­phor: if I find a watch lying abandoned on an open field, I can conclude from the complex set of parts, all shaped to a common purpose and all well de­signed for a specific end, that some higher intelligence constructed the watch both directly and for a particular use. Since organisms show even more com­plexity and even more exquisite design, they must have been fashioned by an even greater intelligence. But fewer biologists know Paley's more specific ar­gument against the alternative version of natural theology (laws of form), as presented in his chapter 15 on “relations.” The parts of organisms exist in concert not because laws of form or symmetry demand one feature to balance another, but “from the relation which the parts bear to one another in the prosecution of a common purpose” (1803 edition, p. 296) — that is, to secure an optimal adaptation of the whole.

  At the very outset of the Origin, Darwin tells us that his explanation of evolution will stress the Paleyan problem of exquisite adaptation. He writes, in the Introduction, that we could obtain sufficient confidence about evo­lution by “reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts” (1859, p. 3). “Nevertheless,” he continues, “such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modi­fied, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration” (1859, p. 3). The explanation of adapta­tion therefore stands forth as the primary problem of evolution. Many lines of evidence prove that evolution occurred. But if we wish to learn how evolu­tion works, we must study adaptation.

  This basic Darwinian argument operates as a close copy of Paley's defense, recast in evolutionary language, for the English alternative in natural theology. [Page 119] We can infer, Paley often states, that God exists from innumerable as­pects of nature. But if we wish to know any more about the creator — his nature, his attributes, his intentions — we must study the excellence of adapta­tion via the “argument from design.” Paley writes (1803, p. 60): “When we are enquiring simply after the existence of an intelligent Creator, imperfec­tion, inaccuracy, liability to disorder, occasional irregularities, may subsist, in a considerable degree, without inducing any doubt into the question.”

  On the other hand, adaptation in the fashioning of contrivances for definite ends reveals God's nature. Paley invokes this theme as a litany in developing his initial parable of the watch and watchmaker. He cites other possible ex­planations for the origin of the watch, and then intones, after each: “Contriv­ance is still unaccounted for. We still want a contriver” (“want,” that is, in the old sense of “lack,” not the modern “desire” — p. 13). “Contrivance must have had a contriver, design, a designer” (p. 14). Later, he tells us explicitly that nature can testify to God's character and goodness only by the phenome­non of adaptation (pp. 42-43): “It is only by the display of contrivance, that the existence, the agency, the wisdom of the Deity, could be testified to his ra­tional creatures. This is the scale by which we ascend to all the knowledge of our Creator which we possess, so far as it depends upon the phenomena, or the works of nature ... It is in the construction of instruments, in the choice and adaptation of means, that a creative intelligence is seen. It is this which constitutes the order and the beauty of the universe.”

  I had never read Natural Theology straight through before pursuing my research for this book. In so doing, I was struck by the correspondences be­tween Paley's and Darwin's structure of argument (though Darwin, of course, inverts the explanation). Darwin did not exaggerate when stating to Lubbock that he had virtually committed Paley to memory. The style of Darwin's argu­ments, his choice of examples, even his rhythms and words, must often reflect (perhaps unconsciously) his memory of Paley. Consider just a few examples of this crucial linkage:

  1. Paley, like Darwin, relies upon comparison and extrapolation from arti­ficial to natural. Darwin moves from artificial to natural selection, Paley from human to animal machines. Both rely on the central argument that a common mechanism works much more powerfully in nature. Paley's words recall Dar­win's argument that natural selection, working on all parts for so much time, must trump artificial selection, which only affects the few features we choose to emphasize in the short duration of human history. “For every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of be­ing greater and more, and that in a degree which exceeds all computation” (1803, p. 19).

  2. Both men invoke the same examples. Paley compares the eye and telescope; Darwin lauds the eye as the finest example of complex natural design, and then presents an evolutionary explanation. Paley cites the swimbladder as an independent device created for life in water; Darwin illustrates homology with the tetrapod lung and proposes an evolutionary passage. [Page 120]

  3. Darwin often uses Paley's logic, sometimes against his predecessor. Paley, for example, dismisses arguments about “tendencies to order” or “princ
iples of design” as empty verbiage, explaining nothing; a true cause must be identi­fied, namely God himself. Darwin makes the same point, but cites evolution as the true cause, while branding statements about creation ex nihilo as empty verbiage. Paley writes (p. 76): “A principle of order is the word: but what is meant by a principle of order, as different from an intelligent Creator, has not been explained either by definition or example: and, without such ex­planation, it should seem to be a mere substitution of words for reasons, names for causes.”

  4. Paley discusses many themes of later and central importance to Darwin. He criticizes the major evolutionary conjectures of his day, including Buff on “interior molds,” and the idea of use and disuse. (Since I doubt that he had read Lamarck's earliest evolutionary work by 1802, Paley probably derived this aspect of Lamarck's theory from its status as folk wisdom in general cul­ture.) Paley also states the following crisp epitome of the very argument from Malthus that so struck Darwin. (I am not claiming that this passage provided a covert source for Darwin's central insight. Darwin, after all, had also read Malthus.) “The order of generation proceeds by something like a geometrical progression. The increase of provision, under circumstances even the most advantageous, can only assume the form on an arithmetic series. Whence it follows, that the population will always overtake the provision, will pass be­yond the line of plenty, and will continue to increase till checked by the dif­ficulty of procuring subsistence” (p. 540).

  This influence, and this desire to overturn Paley, persisted throughout Darwin's career. Ghiselin (1969), for example, regards Darwin's orchid book as a conscious satire on Paley's terminology and argument. Darwin called this work (1862), his next book after the Origin of Species, “On the various con­trivances by which British and foreign orchids are fertilized by insects.” Paley used the word “contrivance,” as my previous quotations show, to designate an organic design obviously well-made by an intelligent designer. But Darwin argues that orchids must be explained as contraptions, not contrivances. Their vaunted adaptations are jury-rigged from ordinary parts of flowers, and must have evolved from such an ancestral source; the major adaptive fea­tures of orchids have not been expressly and uniquely designed for their cur­rent functions.

  Now suppose, as a problem in abstract perversity, that one made a pledge to subvert Paley in the most radical way possible. What would one claim? I can imagine two basic refutations. One might label Paley's primary observa­tion as simply wrong — by arguing that exquisite adaptation is relatively rare, and that the world is replete with error, imperfection, misery and caprice. If God made such a world, then we might want to reassess our decision to wor­ship him. An upsetting argument indeed, but Darwin chose an even more rad­ical alternative.

  With even more perversity, one might judge Paley's observation as un­doubtedly correct. Nature features exquisite adaptation at overwhelming relative [Page 121] frequency. But the unkindest cut of all then holds that this order, the very basis of Paley's inference about the nature of God, arises not directly from omnipotent benevolence, but only as a side-consequence of a causal principle of entirely opposite import — namely, as the incidental effect of or­ganisms struggling for their own benefit, expressed as reproductive success. Could any argument be more subversive? One accepts the conventional ob­servation, but then offers an explanation that not only inverts orthodoxy, but seems to mock the standard interpretation in a manner that could almost be called cruel. This more radical version lies at the core of Darwin's argu­ment for natural selection. (Darwin actually employed both versions of the radical argument against Paley, but for different aspects of his full case. He in­voked oddities and imperfections as his major evidence for the factuality of evolution (see pp. 111–116). But he used the more radical version — exqui­site adaptation exists in abundance, but its cause inverts Paley's world — to construct his mechanism for evolutionary change, the theory of natural selection.)

  We all understand, of course, that the force of Darwin's radicalism extends well beyond the inversion of an explanatory order; he also undercut a pri­mary source of human comfort and solace. This book cannot address such a vital issue at any depth, but I must record the point — for this wrenching be­came so salient in subsequent human history. If the natural footprints of Paley's God — the source of our confidence in his character, his goodness and, incidentally, the only hint from nature that we should accept other revealed doctrines, in particular the idea of bodily resurrection (1803, pp. 580-581) — must be reconceived as epiphenomena of a struggle for personal success, then what becomes of nature's beauty, instruction and solace? What a bitter cup Darwin offers us, compared with Paley's sweet promise (1803, pp. 578-579): “The hinges in the wings of an earwig, and the joints of its antennae, are as highly wrought, as if the Creator had had nothing else to finish. We see no signs of diminution of care by multiplication of objects, or of distraction of thought by variety. We have no reason to fear, therefore, our being forgotten, or overlooked, or neglected.”

  But then, the man who served as the primary focus of Paley's venera­tion had also promised that the truth would make us free; and Darwin justly argued that nature cannot provide the source of morality or comfort in any case.

  Darwin and Adam Smith

  Many scientists fail to recognize that all mental activity must occur in social contexts, and that a variety of cultural influences must therefore impact all scientific work. Those who do note the necessary link usually view cultural embeddedness as an invariably negative component of inquiry — a set of bi­ases that can only distort scientific conclusions, and that should be identified for combat. But cultural influences can also facilitate scientific change, for in­cidental reasons to be sure, but with crucially positive results nonetheless — the exaptive principle that evolutionists, above all, should grasp and honor! [Page 122] The origin of Darwin's concept of natural selection provides my favorite ex­ample of cultural context as a promoter.

  The link of Darwin to Malthus has been recognized and accorded proper importance from the start, if only because Darwin himself had explicitly noted and honored this impetus. But if Darwin required Malthus to grasp the central role of continuous and severe struggle for existence, then he needed the related school of Scottish economists — the laissez-faire theorists, centered on Adam Smith and the Wealth of Nations (first published in the auspicious revolutionary year of 1776) — to formulate the even more fundamental prin­ciple of natural selection itself. But the impact of Adam Smith's economics did not strike Darwin with the force of eureka; the concepts crept upon him in the conventional fashion of most influences upon our lives. How many of us can specify a definite parental admonition, or a particular taunt of our peers, as central to the construction of our deepest convictions?

  Silvan S. Schweber (1977), a physicist and historian of science, has traced the chain of influence upon Darwin from Adam Smith's school of Scottish economists — beginning in the early 1830's, and culminating in Darwin's in­tense study of these ideas as he tried to fathom the role of individual action during the weeks just preceding his “Malthusian” insight of September 1838. I believe that Schweber has found the key to the logic of natural selection and its appeal for Darwin in the dual role of portraying everyday and palpable events as the stuff of all evolution (the methodological pole), and in overturn­ing Paley's comfortable world by invoking the most radical of possible argu­ments (the philosophical pole).

  In fact, I would advance the even stronger claim that the theory of natural selection is, in essence, Adam Smith's economics transferred to nature. We must also note the delicious (and almost malicious) irony residing in such an assertion. Human beings are moral agents and we cannot abide the heca­tomb* — the death through competition of nearly all participants — incurred by allowing individual competition to work in the untrammeled manner of pure laissez-faire. Thus, Adam Smith's economics doesn't work in economics. But nature need not operate by the norms of human morality. If the adapta­
tion of one requires the deaths of thousands in amoral nature, then so be it. The process may be messy and wasteful, but nature enjoys time in abun­dance, and maximal efficiency need not mark her ways. (In one of his most fa­mous letters, Darwin wrote to Joseph Hooker in 1856: “What a book a devil's chaplain might write on the clumsy, wasteful, blundering, low, and horribly cruel works of nature!”) The analog of pure laissez-faire can and does operate in nature — and Adam Smith's mechanism therefore enjoys its [Page 123] finest, perhaps its only, full application in this analogous realm, not in the do­main that elicited the original theory itself.