The Structure of Evolutionary Theory

by Stephen Jay Gould

  (8) Correlation of biota with distance. Darwin could find no report of terrestrial mammals on islands more than 300 miles from a continent. He pre­sents the obvious evolutionary explanation for a disturbing creationist co­nundrum:

  It cannot be said, on the ordinary view of creation, that there has not been time for the creation of mammals; many volcanic islands are suf­ficiently ancient, as shown by the stupendous degradation which they have suffered and by their tertiary strata: there has also been time for the production of endemic species belonging to other classes ... why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be an­swered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across (p. 394).

  (9) Correlation with ease of access. Creatures often manage to cross shal­low water barriers between a continent and island, but fail to negotiate deep-water gaps of the same distance. [Page 111]

  (10) Taxonomic affinity of island endemics — perhaps the most obvious point of all: why are the closest relatives of island endemics nearly always found on the nearest continent or on other adjacent islands?

  Any honorable creationist, after suffering such a combination of blows, all implicating a history of evolution as the only sensible coordinating explana­tion, should throw in the towel and, like a beaten prizefighter, acknowledge Darwin as the Muhammad Ali of biology.

  DISCORDANCE (DISSONANCE OF ONE). Consilience works as a cumulative argument for inferring history from objects and phenomena, rather than di­rectly from sequences. You develop a line of attack, list numerous points, and then close in for the kill. But the empirical world often fails to provide such a bounty of evidence. Often, scientists must reason from a single object or situ­ation — just the thing itself, not a network of arguments suitable for a broad consilience. Can history be inferred from such minimal information?

  Thinkers, like soldiers, often show their true mettle in greatest adversity. I am particularly attracted by Darwin's approach to method 4, and have often cited his arguments in these “worst cases” as my primary illustration of his genius (Gould, 1986) — for Darwin met his greatest difficulty, and then not only devised a resolution, but also developed an argument of power and range. In other words, he turned potential trouble into one of his greatest strengths.

  To infer history from a single object, Darwin asserts, one must locate fea­tures (preferably several, so the argument may shade into method three) that make no sense, or at least present striking anomalies, in the current life of the organism. One must then show that these features did fit into a clearly inferable past environment. In such cases, history — as expressed by preserva­tion of signs from the past — provides the only sensible explanation for mod­ern quirks, imperfections, oddities, and anomalies.

  Darwin structured the Origin of Species as a trilogy. The first four chapters lay out the basic argument for natural selection. The middle five treat dif­ficulties with the theory, and ancillary subjects that must be incorporated or explained away (rules of variation, nature of geological evidence, instincts, hybridism, and general objections). The final five chapters present the grand consilience by summarizing evidence for evolution itself — not so much for natural selection as a mechanism — from a broad range of disparate fields: ge­ology*, geographic variation, morphology, taxonomy, embryology, and so forth.

  The last part of the trilogy features method four. One might almost say that chapters 10-14 constitute one long list of examples for inferring history [Page 112] from the oddities and imperfections of modern objects. (This arrangement of the last part struck me with particular force, as I reread the Origin before writing this book, and realized that the introductory paragraph for almost ev­ery new subject — from geographic variation to rudimentary organs — explic­itly restates the general argument for method four.) Of course, the rest of the Origin also abounds with cases of method four, beginning as usual with ex­amples from domestication. (Darwin argues that the chicks of wildfowl hide in grass and bushes to give their mother an opportunity for escape by flight. Domesticated chickens retain this habit, which no longer makes sense “for the mother-hen has almost lost by disuse the power of flight” — p. 216.)

  Of subjects treated in this final part of the Origin's trilogy, rudimentary or­gans represent, almost by definition, the “holotype” of method four. Darwin's definition, in the first sentence of his discussion, emphasizes this theme — “organs or parts in this strange condition, bearing the stamp of inutility” (p. 450). Nature tries to give us a history lesson, Darwin argues in some frus­tration, but we resist the message as inconsistent with received wisdom about natural harmony: “On the view of each organic being and each separate or­gan having been specially created, how utterly inexplicable it is that parts, like the teeth in the embryonic calf or like the shrivelled wings under the sol­dered wing-covers of some beetles, should thus so frequently bear the plain stamp of inutility! Nature may be said to have taken pains to reveal, by rudi­mentary organs and by homologous structures, her scheme of modification, which it seems that we wilfully will not understand” (p. 480). What else but imprints of history can explain rudimentary organs? Darwin ridicules the special pleading of creationist accounts as fancy ways of saying nothing at all. “In works on natural history rudimentary organs are generally said to have been created 'for the sake of symmetry,' or in order 'to complete the scheme of nature;' but this seems to me no explanation, merely a restatement of the fact. Would it be thought sufficient to say that because planets revolve in elliptic courses round the sun, satellites follow the same course round the planets, for the sake of symmetry, and to complete the scheme of nature?” (p. 453). Always searching for analogies with a short-term human history that we cannot deny, Darwin compares rudimentary organs with silent let­ters, once sounded, in the orthography of words: “Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but be­come useless in the pronunciation, but which serve as a clue in seeking for its derivation” (p. 455).

  Darwin continues the same argument as an underpinning for all discus­sions on other aspects of organic form. He introduces morphology as “the most interesting department of natural history, [which] may be said to be its very soul” (p. 434) and continues immediately with an example of method four: “What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pat­tern, and should include the same bones, in the same relative positions” (p. 434).

  Similarly, the section on embryology begins with an example of method [Page 113] four — the branchial circulation in young bird and mammalian embryos as indications of a “community of descent” with an aquatic past. This common condition in embryonic frogs, birds, and mammals cannot reflect design for current function: “We can not, for instance, suppose that in the embryos of the vertebrata the peculiar loop-like course of the arteries near the branchial slits are related to similar conditions, — in the young mammal which is nour­ished in the womb of his mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog under water” (p. 440).

  The key argument of the section on taxonomy makes the same point in a different form: if animals had experienced no history of change, and were cre­ated in accord with current needs and functions, then why should similar an­atomical designs include creatures of such widely divergent styles of life? Dar­win writes, in the opening paragraph of his discussion on taxonomy: “The existence of groups would have been of simple signification, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely differ­ent in nature; for it is notorious how commonly members of even the same subgroup have different habits” (p. 411).

  These arguments strike us as most
familiar when based on organic form, but fewer evolutionists recognize that method four also under girds Darwin's two chapters on biogeography (11 and 12). Darwin uses dissonance between organism and dwelling place as the coordinating theme of these chapters: the geographic distributions of organisms do not primarily suit their current cli­mates and topographies, but seem to record more closely a history of oppor­tunities for movement. Again, Darwin presents the basic argument in his first paragraph (p. 346): “In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be ac­counted for by their climatal and other physical conditions.”

  Example tumbles upon example throughout these two chapters. Darwin notes that northern hemisphere organisms of subarctic and north temperate climes maintain far closer taxonomic similarity than the current geographic separation of their continents would imply. He therefore interprets these like­nesses as vestiges of history — preserved expressions of the glacial age, when these climatic bands stood further to the north, near the Arctic Circle where all northern continents virtually touch (p. 370). He also finds too much organic similarity for the modern range of climatic differences along lines of longi­tude from north to south poles, and he again implicates the climax of glacial ages as a time of formation (with modern persistence as a vestige), when even a subarctic species might migrate in comfort, on a cold earth, across the equa­tor from north to south along a single line of longitude. Invoking a complex and graphic metaphor for history, Darwin writes of disjunct distributions on opposite hemispheres, and of geographic refugia at high altitudes of lower latitudes between these endpoints:

  The living waters may be said to have flowed during one short period from the north and from the south, and to have crossed the equator; but [Page 114] to have flowed with greater force from the north so as to have freely in­undated the south. As the tide leaves its drift in horizontal lines, ... so have the living waters left their living drift on our mountain summits, in a line gently rising from the arctic lowlands to a great height under the equator. The various beings thus left stranded may be compared with savage races of man, driven up and surviving in the mountain fastnesses of almost every land, which serve as a record, full of interest to us, of the former inhabitants of the surrounding lowlands (p. 382).

  Everyone cites the Galapagos in a virtual catechism about Darwin's evi­dence for evolution, but few biologists can state how he invokes these islands in the Origin. Most textbooks talk about a diversity of finches, each beauti­fully adapted to available resources on different islands, or of variation in tor­toise carapaces from place to place. Both these stories exemplify both diversi­fication and current adaptive value — but Darwin speaks not a word about either case in the Origin!

  In fact, Darwin invokes the Galapagos primarily as an extended example of method four applied to biogeography: These islands house many endemic species, necessarily created in situ according to his opponents. But why then should all these endemics bear close relationship with species on the nearby American mainland? A creationist might say that God fits creatures to im­mediate circumstances, and that the Galapagos Islands, located so near America, must resemble America in environment, and therefore be best suited to house species of the same basic design. But now we grasp the beauty of the Galapagos as an almost uncannily decisive natural experiment for the influ­ence of history. These islands do lie close to America, but could scarcely resemble the mainland less in climate, geology and topography — for the Galapagos are volcanic islands in the wake of a cool current that even permits access to the northernmost species of penguin! Therefore, if the Galapagos endemics resemble American species, they must be recording a history of acci­dental transport and subsequent evolutionary change — not similar creations for similar environments. Darwin's brilliant argument deserves citation in extenso:

  Here almost every product of the land and water bears the unmistakable stamp of the American continent. There are 26 land birds, and 25 of these are ranked by Mr. Gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to American spe­cies in every character, in their habits, gestures, and tones of voice, was manifest… why should this be so? Why should the species, which are supposed to have been created in the Galapagos Archipelago, and no­where else, bear so plain a stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the sev­eral classes are associated together, which resembles closely the condi­tions of the South American coast: in fact there is considerable dissimi­larity in these respects. On the other hand, there is a considerable degree [Page 115] of resemblance in the volcanic nature of the soil, in climate, height, and size of islands, between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The in­habitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America. I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Is­lands would be likely to receive colonists . . . from America; and the Cape de Verde Islands from Africa; and that such colonists would be lia­ble to modifications — the principle of inheritance still betraying their original birth place (pp. 397-399).

  Finally, in rereading the Origin, I was struck by another, quite different, use of the argument from imperfection — one that had entirely escaped my notice before. Darwin showed little sympathy for our traditional and venerable at­tempts to read moral messages from nature. He almost delighted in noting that natural selection unleashes a reign of terror that would threaten our moral values if we tried — as we most emphatically should not — to find ethical guidelines for human life in the affairs of nature. But I hadn't realized that he sometimes presents the apparent cruelties of nature as imperfections pointing to evolution by natural selection — imperfections relative to an inappropriate argument about morality to be sure, but imperfections that trouble our souls nonetheless, and may therefore operate with special force as suggestive argu­ments for evolution:

  Nor ought we to marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect; and if some of them be abhorrent to our ideas of fitness. We need not marvel at the sting of the bee causing the bee's own death; at drones being produced in such vast numbers for one single act, and being then slaughtered by their sterile sisters; at the astonishing waste of pollen by our fir trees; at the instinctive hatred of the queen bee for her own fertile daughters; at ichneumonidae feeding within the live bodies of caterpillars; and at other such cases. The won­der indeed is, on the theory of natural selection, that more cases of the want of absolute perfection have not been observed (p. 472).

  I may have burdened readers with too much detail about Darwin's argu­ments for inferring history, but method inheres in this extended madness. My general argument holds that the Origin should be understood as a book en­compassing two opposite, but complementary, poles of science at its best and most revolutionary — first, as a methodological treatise proving by example that evolution can be tested and studied fruitfully; and second, as an intellec­tual manifesto for a new view of life and nature. As a methodological treatise, the Origin focuses upon the palpable and the small — arguing that uniformitarian extrapolation into geological scales can render all evolution. We may therefore avoid any appeal to “higher” forces that cannot be studied directly because they work only in the untestable immensity of deep time, or occur so [Page 116] rarely that we can entertain little hope for direct observation during the short span of human history. The disabling Lamarckian paradox — what is impor­tant can't be studied; and what can be studied isn't important — therefore dis­appears, and evolution becomes, under Darwin's system, a worki
ng science for the first time. These features of methodology potentiate Darwin's theoreti­cal overview (as we shall see in the next section), and therefore contribute in­dispensably to what may legitimately be called the essence of Darwinism, the sine quibus non for a Darwinian view of nature. This book argues that we can define such a set of basic commitments, but then maintains that these commitments have become inadequate in our times.

  Darwin as a Philosophical Revolutionary

  THE CAUSES OF NATURE'S HARMONY

  Darwin and William Paley

  In November 1859, just a week before the official publication date of the Ori­gin, Darwin wrote to his neighbor John Lubbock* “I do not think I hardly ever admired a book more than Paley's 'Natural Theology.' I could almost formerly have said it by heart” (in F. Darwin, 1887, volume 2, p. 219).

  The Reverend James McCosh receives my vote for the most interesting among a largely forgotten group of late 19th century thinkers who played a vital role in their own time — liberal theologians friendly to evolution (though not usually to Darwin's philosophy), and who prove that if any warring camps can be designated in this realm, the combatants surely cannot be la­beled as science vs. religion (see Gould, 1999b), but rather as expressions of a much deeper struggle between tradition and reform, or dogmatics and open­ness to change. McCosh doesn't even merit a line in the Encyclopedia Britannica, though he did serve as president of Princeton University, where he had a major influence on the career of Henry Fairfield Osborn and other important American evolutionists.

  In 1851, McCosh published an article entitled “Typical Forms” in the North British Review. Hugh Miller, the self-taught Scottish geologist and gen­eral thinker, called this article “at once the most suggestive and ingenious which we have almost ever perused,” and urged McCosh to expand his argu­ment to an entire volume. McCosh accepted this advice and, in collaboration with George Dickie, published Typical Forms and Species Ends in Creation in 1869. The Greek inscription on the title page — typos kai telos (type and purpose) [Page 117] — epitomizes the argument. McCosh holds that God's order and benev­olence may be inferred from two almost contradictory properties that reside in tension within all natural objects — “the principle of order” and “the prin­ciple of special adaptation.” (These two principles persist in Darwin's formu­lation under the names “Unity of Type” and “Conditions of Existence” — 1859, p. 206, for example (see my extensive treatment of this passage on pp. 251–260), where their fundamental character merits upper case designa­tions from Darwin.) McCosh defines his first principle as “a general plan, pattern, or type, to which every given object is made to conform”; and his second as a “particular end, by which each object, while constructed after a general model is, at the same time, accommodated to the situation which it has to occupy, and a purpose which it is intended to serve” (1869, p. 1). (If we call these two principles “anatomical ground plan” and “adaptation” we will be able to make the appropriate evolutionary translation without difficulty.)