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The I ? a n d a s ~ Thumb

MORE REFLECTIONS IN NATURAL HISTORY

STEPHEN JAY GOULD


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1 The Panda's Thumb

FEW HER 0 E S LO W E R their sights in theprime of their lives; triumph leads inexorably on, often todestruction. Alexander wept because he had no new worldsto conquer; Napoleon, overextended, sealed his doom inthe depth of a Russian winter. But Charles Darwin did notfollow the Origin of SPecies (1859) with a general defense ofnatural selection or with its evident extension to humanevolution (he waited until 1871 to publish TheDescentof;'VIan).Instead, he wrote his most obscure work, a book entitled:On the Various Contrivances by Which British and Foreign OrchidsAre Fertilized by Insects (1862).Darwin's many excursions into the minutiae of naturalhistory-he wrote a taxonomy of barnacles, a book onclimbing plants, and a treatise on the formation of vegetable mold by earthworms-won him an undeserved reputation as an old-fashioned, somewhat doddering describer ofcurious plants and animals, a man who had one lucky insightat the right time. A rash of Darwinian scholarship has laidthis myth firmly to rest during the past twenty years (seeessay 2). Before then, one prominent scholar spoke formany ill-informed colleagues when he judged Darwin as a"poor joiner of ideas . . . a man who does not belong withthe great thinkers."In fact, each of Darwin's books played its part in the grandand coherent scheme of his life's work-demonstrating thefact of evolution and defending natural selection as its pri-


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20 1 THE PAN D A 'S T HUM Bmary mechanism. Darwin did not study orchids solely fortheir own sake. Michael Ghiselin, a California biologist whofinally took the trouble to read all of Darwin's books (see hisTriumph of he Darwinian Method), has correctly identified thetreatise on orchids as an important episode in Darwin'scampaign for evolution.Darwin begins his orchid book with an important evolutionary premise: continued self-fertilization is a poor strategy for long-term survival, since offspring carry only thegenes of their single parent, and populations do not maintain enough variation for evolutionary flexibility in the faceof environmental change. Thus, plants bearing flowers withboth male and female parts usually evolve mechanisms toensure cross-pollination. Orchids have formed an alliancewith insects. They have evolved an astonishing variety of"contrivances" to attract insects, guarantee that sticky pollen adheres to their visitor, and ensure that the attachedpollen comes in contact with female parts of the next orchidvisited by the insect.Darwin's book is a compendium of these contrivances,the botanical equivalent ofa bestiary. And, like the medievalbestiaries, it is designed to instruct. The message is paradoxical but profound. Orchids manufacture their intricatedevices from the common components of ordinary flowers,parts usually fitted for very different functions. I f God haddesigned a beautiful machine to reflect his wisdom andpower, surely he would not have used a collection of partsgenerally fashioned for other purposes. Orchids were notmade by an ideal engineer; they are jury-rigged from alimited set of available components. Thus, they must haveevolved from ordinary flowers.

Thus, the paradox, and the common theme of this trilogyof essays: Our textbooks like to illustrate evolution withexamples of optimal design-nearly perfect mimicry of adead leaf by a butterfly or of a poisonous species by apalatable relative. But ideal design is a lousy argument forevolution, for it mimics the postulated action of an omnipotent creator. Odd arrangements and funny solutions are theproof of evolution-paths that a sensible God would never


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24 I THE PAN D A 'S T HUM BIn a panda 's foot, the counterpart of the radial sesamoid,called the tibial sesamoid, is also enlarged, although not somuch as the radial sesamoid . Yet the tibial sesamoid sup-

ports no new digit, and its increased size confers no advan-tage, so far as we know. Davis argues that the coordinatedincrease of both bones, in response to natural selectionupon one alone, probably reflects a simple kind of geneticchange. Repeated parts of the body are not fashioned by theaction of individual genes-there is no gene " for" yourthumb, another for your big toe, or a third for your pinky.Repeated parts are coordinated in development; selectionfor a change in one element causes a corresponding modifi-cation in others. It may be genetically more complex toenlarge a thumb and not to modify a big toe, than to in-crease both together. (In the first case, a general coordina-tion must be broken, the thumb favored separately, andcorrelated increase of related structures suppressed. In thesecond, a single gene may increase the rate of growth in afield regulating the development of corresponding digits.)

The panda 's thumb provides an elegant zoological coun-terpart to Darwin's orchids. An engineer's best solution isdebarred by history. The panda's true thumb is committedto another role, too specialized for a different function tobecome an opposable, manipulating digit. So the pandamust use parts on hand and settle for an enlarged wristbone and a somewhat clumsy, but quite workable, solution.The sesamoid thumb wins no prize in an engineer's derby.I t is, to use Michael Ghiselin's phrase, a contraption, not alovely contrivance. But it does its job and excites our imagi-nation all the more because it builds on such improbablefoundations.Darwin's orchid book is filled with similar illustrations.The marsh Epipactus, for example, uses its labellum-anenlarged petal-as a trap. The labellum is divided into twoparts. One, near the flower's base, forms a large cup filledwith nectar-the object of an insect's visit. The other, nearthe flower's edge, forms a sort of landing stage. An insectalighting on this runway depresses it and thus gains en-trance to the nectar cup beyond. It enters the cup, but the


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THE PAN D A 'S T HUM B I 25Marsh Epipactis, lower sepals removed

a. Runway of labellum depressed after insect lands.D. L. CRAMER

b. Runway of labellum raised after insect crawls intocup below. D. L. CRAMER


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30 I T H E PAN D A 'S T HUM B"able to take a load." Cattle, in Latin, are veterinae.This general principle of historical science should applyto the earth as well. The theory of plate tectonics has led usto reconstruct the history of our planet's surface. Duringthe past 200 million years, our modern continents havefragmented and dispersed from a single supercontinent,Pangaea, that coalesced from earlier continents more than225 million years ago. If modern oddities are the signs ofhistory, we should ask whether any peculiar things that animals do today might be rendered more sensible as adaptations to previous continental positions. Among the greatestpuzzles and wonders of natural history are the long andcircuitous routes of migration followed by many animals.Some lengthy movements make sense as direct paths tofavorable climates from season to season; they are no morepeculiar than the annual winter migration to Florida oflarge mammals inside metallic birds. But other animals migrate thousands of miles-from feeding to breedinggrounds-with astounding precision when other appropriate spots seem close at hand. Could any of these peculiarroutes be rendered shorter and more sensible on a map ofancient continental positions? Archie Carr, world's experton the migration ofgreen turtles, has made such a proposal.A population of the green turtle, Chelonia mydas, nests andbreeds on the small and isolated central Atlantic island ofAscension. London soup chefs and victualing ships of HerMajesty's Navy found and exploited these turtles long ago.But they did not suspect, as Carr discovered by tagginganimals at Ascension and recovering them later at theirfeeding grounds, that Chelonia travels 2,000 miles from thecoast of Brazil to breed on this "pinpoint ofland hundredsof miles from other shores," this "barely exposed spire inmid-ocean. "Turtles feed and breed on separate grounds for goodreasons. They feed on sea grasses in protected, shallowwater pastures, but breed on exposed shores where sandybeaches develop-preferably, on islands where predatorsare rare. But why travel 2,000 miles to the middle of anocean when other, apparently appropriate breeding


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32 I THE PANDA 'S T H U M Bfloat to Brazil on the Equatorial Current, but how do theyget back? Carr supposes that they begin their journey bycelestial cues and finally home in by remembering the character [taste? smell?] of Ascension water when they detectthe island's wake.)

Carr's hypothesis is an excellent example of using thepeculiar to reconstruct history. I wish I could believe it. I amnot troubled by the empirical difficulties, for these do notrender the theory implausible. Can we be confident, forexample, that a new island always arose in time to replacean old one-for the absence ofan island for even one generation would disrupt the system. And would the new islandsalways arise sufficiently "on course" to be found? Ascensionitself is less than seven million years old.I am more bothered by a theoretical difficulty. If the entire species Chelonia mydas migrated to Ascension or, evenbetter, if a group of related species made the journey, Iwould have no objection, for behavior can be as ancient andas heritable as form. But C. mydas lives and breeds throughout the world. The Ascension turtles represent only oneamong many breeding populations. Although its ancientancestors may have lived in the Atlantic puddle 200 millionyears ago, our record of the genus Chelonia does not extendback beyond fifteen million years, while the species C. mydasis probably a good deal younger. (The fossil record, for allits faults, indicates that few vertebrate species survive for asmany as ten million years.) In Carr's scheme, the turtles thatmade the first trips to proto-Ascension were rather distantancestors of C. mydas (in a different genus at least). Severalevents of speciation separate this Cretaceous ancestor fromthe modern green turtle. Now consider what must havehappened if Carr is right. The ancestral species must havebeen divided into several breeding populations, only one ofwhich went to proto-Ascension. This species then evolvedto another and another through however many evolutionary steps separated it from C. mydas. At each step, the Ascension population kept its integrity, changing in lock stepwith other separate populations from species to species.But evolution, so far as we know, doesn't work this way.


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34 I T H E P A N D A 'S T H U M Bfollows the great historical principle of using the peculiar asa sign of change.I am afraid that the turtles illustrate another aspect ofhistorical science-this time a frustration, rather than aprinciple of explanation. Results rarely specify their causesunambiguously. If we have no direct evidence of fossils orhuman chronicles, if we are forced to infer a process onlyfrom its modern results, then we are usually stymied orreduced to speculation about probabilities. For many roadslead to almost any Rome.This round goes to the turtles-and why not? While Por-tuguese sailors hugged the coast of Africa, Chelonia mydasswam straight for a dot in the ocean. While the world's bestscientists struggled for centuries to invent the tools of navi-gation, Chelonia looked at the skies and proceeded oncourse.


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31 Double Trouble

NATURE MARKS Izaak Walton as a rankamateur more often than I had imagined. In 1654, theworld's most famous fisherman before Ted Williams wroteof his favorite lure: "I have an artificial minnow . . . socuriously wrought, and so exactly dissembled that it wouldbeguile any sharpsighted trout in a swift stream."An essay in my previous book, Ever Since Darwin, told thetale of Lampsilis, a freshwater clam with a decoy "fish"

mounted on its rear end. This remarkable lure has a stream-lined "body," side flaps simulating fins and tail, and aneyespot for added effect; the flaps even undulate with arhythmic motion that imitates swimming. This "fish," con-structed from a brood pouch (the body) and the clam'souter skin (fin and tails), attracts the real item and permitsa mother clam to shoot her larvae from the brood pouchtoward an unsuspecting fish. Since the larvae of Lampsiliscan only grow as parasites on a fish's gill, this decoy is auseful device indeed.I was astounded recently to learn that Lampsilis is notalone. Ichthyologists Ted Pietsch and David Grobeckerrecovered a single specimen of an amazing Philippine an-glerfish, not as a reward for intrepid adventures in the wilds,but from that source of so much scientific novelty-the localaquarium retailer. (Recognition, rather than machismo, isoften the basis of exotic discovery.) Anglerfish lure theirdinner, rather than a free ride for their larvae. They carry


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DOUBLE TROUBLE I 37epigraph the passage from Walton q u o t ~ d a b o v ~ . ) This. exquisite fake also sports eyelike spots of pigment In the nghtplace. In addition, it bears compressed filaments representing pectoral and pelvic fins along the bottom of the body,extensions from the back resembling dorsal and anal fins,and even an expanded rear projection looking for all th.eworld like a tail. Pietsch and Grobecker conclude: "The baitis nearly an exact replica of a small fish that could easilybelong to any of a number of percoid f a m i l i ~ s c o m ~ o n t?the Philippine region." The angler even npples. Its baitthrough the water, "simulating the lateral undulations of aswimming fish." .

These nearly identical artifices of fish and clam mightseem, at first glance, to seal the case for Darwinian evolution. I f natural selection can do this twice, surely it can doanything. Yet---continuing the theme of the la.st two essaysand bringing this trilogy to a close-perfection works aswell for the creationist as the evolutionist. Did not thepsalmist proclaim: "The heavens declare the glory of God;and the firmament showeth his handiwork." The last twoessays argued that imperfection carries the day for evolution. This one discusses the Darwinian response to perfection.The only thing more difficult to explain than perfectionis repeated perfection by very different animals. A fish ona clam's rear end and another in front of an anglerfish'snose-the first evolved from a brood pouch and outer skin,the second from a fin spine-more than doubles the trouble. I have no difficulty defending the origin of both"fishes" by evolution. A plausible series of intermediatestages can be identified for Lampsilis. The fact that anglerfish press a fin spine into service as a lure reflects thejury-rigged, parts-available principle that made the panda'sthumb and the orchid's labellum speak so strongly for evolution (see the first essay of this trilogy). But Darwiniansmust do more than demonstrate evolution; they must defend the basic mechanism of random variation and naturalselection as the primary cause of evolutionary change.Anti-Darwinian evolutionists have always favored the re-


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DOUBLE TROUBLE I 39can never yield anything close to perfect resemblance. Organisms cannot erase their past. Two lineages may developremarkable, superficial similarities as adaptations to a common mode of life. But organisms contain so many complexand independent parts that the chance of all evolving twicetoward exactly the same result is effectively nil. Evolution isirreversible; signs of ancestry are always preserved; convergence, however impressive, is always superficial.

Consider my candidate for the most astounding convergence of all: the ichthyosaur. This sea-going reptile withterrestrial ancestors converged so strongly on fishes that itactually evolved a dorsal fin and tail in just the right placeand with just the right hydrological design. These structures are all the more remarkable because they evolvedfrom nothing-the ancestral terrestrial reptile had no humpon its back or blade on its tail to serve as a precursor.Nonetheless, the ichthyosaur is no fish, either in generaldesign or in intricate detail. (In ichthyosaurs, for example,the vertebral column runs through the lower tail blade; infish with tail vertebrae, the column runs into the upperblade.) The ichthyosaur remains a reptile, from its lungsand surface breathing to its flippers made of modified legbones, not fin rays.

IchthyosaurCOURTESY OF THE AMERICAN MUSEUM OF NATURAL HISTORY

Koestler's carnivores tell the same tale. Both placentalwolfand marsupial "wolf' are well designed to hunt, but noexpert would ever mistake their skulls. The numerous,small marks of marsupiality are not obliterated by convergence in outward form and function.


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40 1 THE PAN D A 'S T HUM BSecond, the explanation: Darwinism is not the theory ofcapricious change that Koestler imagines. Random variation may be the raw material of change, but natural selection builds good design by rejecting most variants whileaccepting and accumulating the few that improve adapta

tion to local environments.The basic reason for strong convergence, prosaic thoughit may seem, is simply that some ways of making a livingimpose exacting criteria of form and function upon anyorganism playing the role. Mammalian carnivores must runand stab; they do not need grinding molar teeth since theytear and swallow their food. Both placental and marsupialwolves are built for sustained running, have long, sharp,pointed canine teeth and reduced molars. Terrestrial vertebrates propel themselves with their limbs and may use theirtails for balance. Swimming fish balance with their fins andpropel from the rear with their tails. Ichthyosaurs, livinglike fish, evolved a broad propulsive tail (as whales did later-although the horizontal flukes of a whale's tail beat upand down, while the vertical flukes of fish and ichthyosaursbeat from side to side).No one has treated this biological theme of repeated,exquisite design more eloquently than D'Arcy WentworthThompson in his 1942 treatise, On Growth and Form, still inp r i ~ t and still as relevant as ever. Sir Peter Medawar, a manwho eschews hype and exaggeration, describes it as "beyond comparison the finest work of literature in all theannals of science that have been recorded in the Englishtongue." Thompson, zoologist, mathematician, classicalscholar, and prose stylist, won accolades as an old man butspent his entire professional life in a small Scottish university because his views were too unorthodox to win prestigious London and Oxbridge jobs.Thompson was more a brilliant reactionary than a visionary. He took Pythagoras seriously and worked as a Greekgeometrician. He took special delight in finding the abstractforms of an idealized world embodied again and again inthe products of nature. Why do repeated hexagons appearin the cells of a honeycomb and in the interlocking plates


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D O U B L E TR O U B L E 141of some turtle shells? Why do the spirals in a pine cone anda sunflower (and often ofleaves on a stem) follow the Fibon a ~ c i series? system of spirals radiating from a commonpo.mt can be VIewed either as a set of left- or right-handedspIrals. Left and right s ~ i r a l s are not equal in number, butrepresent two consecutIve figures of the Fibonacci series.The Fibonacci series is constructed by adding the previoustwo n,:,mbers to form the next: 1, 1,2,3,5,8, 13,21, etc.~ h e pm.e cone may, for example, have 13 left spirals and 21nght spIrals.) Why do so many snail shells, ram's horns, andeven the path of a moth to light follow a curve called thelogarithmic spiral?

Thompson's answer was the same in each case: theseabstract forms are optimal solutions for common problems.They are evolved repeatedly in disparate groups becausethey are the best, often the only, path to adaptation. Triangles, parallelograms, and hexagons are the only planefigures that fill space completely without leaving holes.~ e x a g o n s are often favored because they approximate aCIrcle an.d .maximize area within relative to the supportingwalls (mmlmum c o n ~ t r u c t i o ? for greatest storage of honey,~ o r example). The FI?o?acCl ~ a t t e r n emerges automatically _m any system of radlatmg spIrals built by adding new elements at the apex, one at a time in the largest space available. T ~ e logarithmic spiral is the only curve that does notchange ItS shape it grows in size. I can identify the abstract Thompsoman forms as optimal adaptations, but tothe . l ~ r g e r m e t ~ p h y s i c a l issue of why "good" form oftenexhIbits such sImple, numerical regularity, I plead only ignorance and wonder.So far, I have only spoken to half the issue embodied inthe problem of repeated perfection. I have discoursed onthe "why." I have argued that convergence never renderstwo complex organisms completely identical (a circumstance that would strain Darwinian processes beyond theirr e a s o ~ a b l e power) and I have tried to explain close repeatsa.s optImal adaptations to common problems with few solutIOns.

But what about the "how?" We may know what the fish


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42 I T H E P A N D A 'S T H U M Bof Lampsilis and the lure of the anglerfish are. for, but howdid they arise? This problem becomes partlc,:,larly a c u ~ e when the final adaptation is complex and pecuhar but bUiltfrom familiar parts of different ancestral function. If t ~ e angler 's fishlike lure required 500 entirely separate ~ o d l f i cations to attain its exquisite mimicry, then how did theprocess begin? And why did it continue, unless sO.me nonDarwinian force, cognizant of the final goal, drove It on? Ofwhat possible benefit is step one a l o ~ e Is a f i v e - h u n d ~ e d t h of a fake enough to inspire the cunoslty of any real Item?D'Arcy Thompson's answer to this problem was overextended but characteristically prophetic. He argued that organisms are shaped directly by physical forces acting uponthem: optima of form are nothing more than t ~ e n a t u r ~ l states of plastic matter in the presence of a p p r o p n a ~ e physIcal forces. Organisms jump suddenly from one optimum toanother when the regime of physical forces alters. We nowknow that physical forces are too weak, in most cases, tobuild form directly-and we look to natural selection instead. But we are derailed if selection can only act in apatient and piecemeal way-step by sequential step to buildany complex adaptation.I believe that a solution lies in the essence ofThompson'sinsight, shorn of his unsubstantiated claim that physicalforces shape organisms directly. Complex forms are oftenbuilt 'by a much simpler (often a very simple) system ofgenerating factors. Parts are connected in intricate waysthrough growth, and alteration of one may resoundthrough the entire organism and change it in a variety ofunsuspected ways. David Raup, of Chicago's Field Museumof Natural History, adapted D'Arcy Thompson's insight toa modern computer, and showed that the basic forms ofcoiled shells-from nautiloid to clam to snail-can all begenerated by varying only three simple gradients g r o w t ~ . Using Raup's program, I can change a garden-vanety snailinto a common clam by modifying just two of the threegradients. And, believe it or not, a peculiar genus of modern snails does carry a bivalved shell so like a conventional


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D O U B L E T R O U B L E I 43

In these computer-drawn figures (they are not real snails, despitethe similarities), a form (right) looking much like certain clams canbe converted into a "snail" (left figures) simply by decreasing therate at which the generating ellipse increases as the "shell" growsand by increasing the rate of translation of this ellipse down theaxis of coiling. All these figures are drawn by specifying just fourparameters. PHOTO COURTESY OF D. M. RAUPclam's that I gasped when I saw a snail's head poking outbetween the valves in a striking close-up movie.

This closes my trilogy on the issue of perfection andimperfection as signs of evolution. But the entire set isreally an extended disquisition on the panda's "thumb," a.s i ~ g l e , co.ncrete object that spawned all three essays, despIte theIr subsequent wanderings and musings. Thethumb, built of a wrist bone, imperfect as a sign of history,constructed from parts available. Dwight Davis faced thedilemma of potential impotence for natural selection if itmust work step by countless step to make a panda from abear. And he advocated D'Arcy Thompson's solution ofreduction to a simple system of generating factors. Heshowed how the complex apparatus of the thumb, with allits muscles and nerves, may arise as a set of automaticconsequences following a simple enlargement of the radial~ e s a m o i d bone. J:Ie then argued that the complex changesIn form and functIon of the skull-the transition from omnivory to nearly exclusive munching on bamboo-could bee x p r ~ s s e ~ as consequences of one or two u ~ a e r l y i n g m ~ d l f i c a t l O n s . He concluded that "very few genetic mechanIsms-perhaps no more than half a dozen-were involved in the primary adaptive shift from Ursus [bear] to


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