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Since The closing years of this millennium will be marked by growing concern and respect for ethnic and cultural pluralism - "the separate and equal station to which the laws of nature and of nature's God entitle them" - "a decent respect to the opinions of mankind" (to continue the quotation from a dead white American male) should also impel us to chronicle the striking differences among our academic communities. The disparities in content between arts and sciences have been much discussed and lamented, most notably by CP Snow in his Two Cultures, but few have documented, or even mentioned, the striking differences (by no means trivial or superficial) in style and manner of presentation.

Consider, for example, the important academic forum of oral presentations at professional meetings, an essential launching pad for nearly all scholarly careers. The two major differences between scientific and humanistic styles of presentation strike me as wondrously ironic. In stereotypes well known to all, scientific talks may possess empirical content, but usually fail for want of linguistic grace or skill in communication, while humanists, at their best, will at least dazzle with thoughts "ne'er so well expressed," even if the 10,000th analysis of Shakespeare's 100th sonnet fails to present anything truly novel in content. Yet - and hence my judgment of irony - the two major differences between professions show superior intuition among scientists about use of language and style of communication.

First, humanists almost invariably read their papers from a written manuscript (and almost always badly, with head buried in text and bland inflexion quite unsuited for oral presentation). Scientists hardly ever read; we think through the order or logic of the argument, make notes, and then speak extemporaneously. Extemporaneous speech is so much more compelling of attention than the bland and spiritless style of most readers. And most importantly, written and spoken English are utterly different languages - and, humanists, above all, should know this.

As a second difference between talks of scientists and humanists, scientists nearly always show slides (or visual material in some other form), while humanists usually rely on text alone (with some striking and obvious exceptions, like art history, where simultaneous use of two slide projectors has become de rigueur). Slide projectors are always, and automatically, provided for any scientific talk. I never think about asking for one; I simply assume that the machine will be there.

Why do scientists grasp the importance of visual imagery, while most humanists accept the hegemony of the word? Scholarly publication in the humanities generally degrades imagery and in many ways. Many thick tomes have no pictures at all - not even a likeness of a central figure in a narrative. But visual imagery is central to our lives. Speaking biologically, primates are the quintessentially visual animals among mammals (a glance at the standard "homunculus" image of the human brain shows how much of the cerebral cortex serves our visual system). Much of our judgment in social matters, particularly our emotional feelings, depends upon images. From a scholar's point of view, much can be learned from the study of imagery (including its neglect). Since humanists take words as their explicit stock- in-trade, they scrutinize texts with intense care and invest most of their attention in removing biases and clarifying arguments. Since iconography is usually seen as superfluous, motives that attend the choice and form of images are less conscious than those of scientists - and therefore underlying personal and social biases become exposed in the pictures that we use.

I am particularly intrigued by the subject of "canonical icons": the standard imagery attached to key concepts of our social and intellectual lives. Nothing is more unconscious, and therefore more influential through its subliminal effect, than a standard and widely used picture for a subject that could, in theory, be rendered visually in a hundred different ways, some with strikingly different philosophical implications. The shock of seeing nonstandard imagery can be revealing: we instantly realise how constraining the canonical icon had been, though the limitation had never before crossed our mind. I know no other subject so distorted by canonical icons than my own profession - evolution and the history of life: the image we see reflects social preferences and psychological hopes, rather than paleontological data or Darwinian theory. This theme of constraint by standard pictures is particularly important in science, where nearly every major theory has a characteristic icon.

The Most serious and widely pervasive of all misconceptions about evolution equates the concept with some notion of progress, usually inherent and predictable, and leading to a human pinnacle. Yet neither evolutionary theory nor life's actual fossil record supports such an idea. Darwinian natural selection only produces adaptation to changing local environments, not any global scheme of progress.

We can interpret local adaptation as "improvement" in a particular circumstance (the hairier elephant that becomes a woolly mammoth does better in ice- age climates), but a historical chain of sequential local adaptations does not accumulate to a story of continuous progress. Moreover, for each local adaptation achieved through increasing complexity by some definition, another equally successful local solution evolves by "degeneration" of morphology or behaviour. (Consider only the numerous parasites that, protected from the rigours of external environments, become little more than bags of feeding and reproductive tissue attached to the bodies of their hosts; yet the parasites have as much prospect of evolutionary success as the hosts). As for the fossil record, its pattern of nearly three billion years of exclusively unicellular life, followed by the introduction of nearly all major multicellular groups in a single episode lasting five million years (the famous "Cambrian explosion" of 535-530 million years ago), grants little credence to any idea of slow and steady advance.

At the very most, one might say that a few lineages have expanded into the originally empty sphere of anatomical elaboration (since life had to arise at the lower limit of its conceivable, preservable complexity - that is, as tiny, simple, single cells). But, without question, these earliest and simplest cells, the bacteria and their allies, remain the most abundant, widespread, and successful of all living things. And if one insists on multicellular animal species, some 80 per cent of them are insects, and these enormously successful creatures have not shown any pervasive vectors of improvement over the past 300 million years.

This conceptual problem has pervaded evolutionary biology ever since Darwin. The very word "evolution", as a description of biological change through time, entered our lexicon through Herbert Spencer's more general usage (for cosmology, economics, and a host of other historical disciplines) in the service of his firm belief in "universal progress, its law and cause". Darwin himself had consciously avoided the word in the first edition of The Origin of Species, preferring to describe biological change as "descent with modification". Taking an uncommon position among 19th-century biologists, he did not interpret evolutionary change as inherently progressive.

Thus, the false equation of evolution with progress records a socio-cultural bias, not a biological conclusion, and one hardly needs great insight to locate the primary source of this bias in our human desire to view ourselves as the apex of life's history, ruling the earth by right and biological necessity. This fundamental misconception of evolution is strongly abetted by one of the most pervasive of all canonical icons for any scientific concept - the march, or ladder, of evolutionary progress.

The standard form of this icon - largely a staple of popular culture in cartooning and advertising, but not absent from professional textbooks and museum exhibits - shows a linear sequence of advancing forms, shown either globally, running from an amoeba to a white male in a business suit (thus recording another form of iconographic bias) or, more parochially, as moving from a stooped ape to an upright human. Such a single sequence is, of course, a parody.

Most reasonably well-educated people understand that evolution is not simply a single, advancing line. But the caricature works because it epitomizes, by simplification and exaggeration to be sure, the essence of what many people understand by evolution: in a word, progress. I wonder if any other scientific concept or theory is so well and immediately recognised (though in this case almost perversely misinterpreted) by a canonical icon.

Consider, for example, a favourite of the computer industry. In a Toshiba advertisement, in order to convey the message that its products have become smaller and cheaper over time, a series of images are shown, starting from a stooped chimp weighted down by a cumbersome, antiquated computer, evolving eventually into an upright, white male in a business suit with a portable desktop computer tucked neatly under his arm. The power of the icon is perhaps shown best by numerous parodies that never fail to be immediately comprehensible. In a Frank and Ernie cartoon, for example, the standard sequence runs left to right, from a fish in the sea up a hill to Frank at the summit, who holds a fishing rod over the cliff to the far right, and is just about to hook a fish identical with the starting image at extreme left.

One Might dismiss the pop-culture versions as pure misconceptions of a scientifically illiterate mass culture, mistakes that would not be made so readily by well-educated people or by scientifically sophisticated non-professionals. But the closest version we have of evolutionary iconography intended for a more sophisticated culture makes exactly the same errors - more subtly but at the same time even more pervasively. High culture's version comprises series of paintings for the history of life in geologically sequential order, one for the Cambrian, one for the Ordovician, etc. In other words, we are not viewing single scenes of a selected moment in prehistoric life, but representations of life's history expressed as a series in proper geological order.

This genre could only have originated in the mid-19th-century, for two reasons. First, no adequate reconstructions of fossil vertebrates existed before Cuvier's seminal work of 1812. Second, the geological time scale was not well worked out until the 1840s or 1850s. As a result of limited market and restricted time, the high-culture iconography of sequential painting for life's history is small and manageable. One need not take a sample from a large statistical universe; one can actually survey all major examples for common characteristics and differences.

As a primary conclusion to be drawn from a survey of all influential series, we find no essential variation at all. The same misconceptions are encoded in eerily common ways into all examples - a stunning case for the power of canonical iconography to maintain narrowly prejudicial notions about a subject. The first influential series of lithographs was produced by Edouard Riou (1833-1900), also Jules Verne's lithographer, for a famous book on the history of life by Louis Figuier (1819-1894) - La Terre avant le Deluge, first published in 1863.

Until the current generation, 20th-century portrayals of the history of life were dominated by the great American artist-naturalist Charles R Knight (1874-1953), who virtually owned the genre from the 1920s until his death. (Knight did almost all the major murals in American institutions - New York's American Museum of Natural History, Chicago's Field Museum, and Los Angeles's Tar-pits museum, for example.) Then, in the Fifties, a Czech duo of artist Zdenik Burian and paleontologist Joseph Augusta published a series of wonderful folio books filled with paintings in colour - the first real rival to Knight's hegemony.

The domination of this iconographic tradition by the fallacious theme of progress is even more striking than in the familiar ladders of pop culture imagery - both because the particular pictures, without exception, show the same sequence (leading, at least passively, to the notion that such scenes represent the history of life, rather than one pathway among hundreds of potential and undepicted alternatives), and because greater subtlety of presentation masks the iconographic bias. Both the bias and its invariance can be illustrated by comparing Figuier's original series of 1863 with the most prominent of 20th-century examples, Charles R Knight's series, painted for National Geographic magazine in 1942 and entitled Parade of Life Through the Ages.

The bias of progress has led all these artists to paint the history of life as a progressive sequence leading from marine invertebrate to Homo sapiens. Diversification and stability, the two principal themes of natural history, are entirely suppressed, and the tiny, parochial pathway leading to humans stands as a surrogate for the entire history of life. The world of invertebrates occupies the first long stretch of life's geological history, but, in an initial display of pervasive prejudice, invertebrates receive only two or three plates (out of 30 to 60 in total). I would not object so strongly to the scarcity of plates showing "invertebrates only" if subsequent paintings continued to include invertebrates along with newly risen vertebrates. But as soon as fishes evolve, we never see an invertebrate again (except in the background, and then only occasionally).

How can such a narrow view be justified? Invertebrates didn't stop evolving when the history of fishes began. Four hundred million years of invertebrate history are simply expunged from the conventional representation of life through the ages. Fish fare no better. As soon as terrestrial vertebrates appear, artists never again show a fish. But fishes make up more than half of all vertebrate species today, and most of their evolution occurred after terrestrial vertebrates arose. For example, nearly all modern fishes belong to the Teleosti, or higher bony fish. But teleosts didn't evolve until well after the origin of amphibians and reptiles. So this most important of all events in the evolution of vertebrates, the source of more than half of all living vertebrate species, goes entirely unrecorded. Is this the history of life - or just a disconnected sequence of animals judged "highest" because, in genealogy or complexity, they closely approach humans through time - a prejudiced perspective indeed?

The canonical sequence then continues from early amphibians to dinosaurs, usually depicted in mortal combat. A canonical plate from the time of dinosaurs also serves as the rule-proving exception. Although no fish are shown after terrestrial vertebrates arise, convention permits another marine scene dated during the reign of dinosaurs - though the only animals depicted are marine reptiles (ichthyosaurs, plesiosaurs, and mosasaurs), never fish. In other words, one may draw members of "highest" groups that return to ancestral environments, but never the ordinary, and supposedly superseded, forms of those realms. And so the sequence continues on its familiar route from dinosaurs, to mammals, through to humans. The hegemony of conventional imagery is so complete that the pictures moves on through its exceptionless order no matter what the stated philosophy of the artist.

Darwin correctly noted that evolution presents two fundamental problems with potentially different solutions (and certainly, I might add, with disparate iconographies): anatomical change within lineages (solved by Darwin with the principle of natural selection), and diversification of species, or increase in the number of lineages. Darwin called this second issue the "principle of diversity" and he developed no satisfactory solution until the middle of the 1850s.

The problem of diversity is so topologically distinct from the problem of transformation that a different iconography must be employed for basic illustration. Just as the ladder provides a canonical icon for transformation misconstrued as progress, the same error of falsely equating evolution with progress yields a canonical icon for diversification: the cone of increasing diversity. This icon is less familiar to the general public, for it does not appear either as a popular version like the ladder or as a more sophisticated genre like paintings of prehistoric life.

In the cone of increasing diversity, the history of a lineage begins with a single trunk (the common ancestor) and then moves gradually, smoothly, and continually upward and outward, occupying more and more space as the number of branches (species) grows. But why should such an icon be called biased? What alternative could be suggested? Evolutionary theory demands a common ancestor for related forms, so the tree must emerge from a single trunk at its base. (I accept this argument and regard the common trunk as required by theory, not imposed as a sociocultural bias.)

The biases rather emerge from the canonical shape of such trees above their common trunk - and thus I refer to the canonical icon as a "cone" of diversity. Nothing in theory requires a smooth upward and outward flow for the tree, the feature that sets the tree's shape as an inverted cone or funnel. This arbitrary cone owes its canonical form to several subtle effects of progressivist bias as applied to diversity (rather than to anatomy as in the ladder). First of all, the cone shape requires that the early history of a lineage be composed of only a few major branches, and these must then represent primitive precursors of later forms, thus implying a predictable expansion from limited initial diversity.

Secondly, and more pervasively, the bias in this canonical icon rests upon a conflation in the meaning of axes. The horizontal axis represents morphology, and greater spread of the tree therefore records expansion in number of species and their adaptations. The vertical axis is supposed to record time alone, so higher branches on the tree should represent greater geological youth. But, with the ladder almost inevitably in mind, a higher position on the tree easily becomes conflated with anatomical progress - and the cone of diversity folds back into the ladder of pro-gress, and the two icons overlap in meaning.

If anyone doubts that the cone is a biased icon, consider the first historically important tree of life (bark and all) ever published - Ernst Haeckel's version of 1866. Haeckel conflates time with progress on the vertical axis, and his tree founders on the logical and pictorial impossibility of adequate representation, at least so long as the cone's dictates are obeyed and the top layer of the tree must therefore spread widest. The bias of progress requires that you place your "highest" creatures in the top layer because you view this as indicating maximal advance. The cone dictates that this level must bear the most branches. But suppose that the "highest" group is not diverse and contains only a few species. How can you spread them so thin?

Haeckel encounters this insoluble dilemma because he takes a conventional view and regards mammals as superior beings - so he grants them exclusive residence in the top layer of the tree. But mammals are a small group of only 4,000 species or so, and Haeckel, to fill the space, must make fine distinctions, with full branches (and numerous sub-twigs) for whales, carnivores and, inevitably in the centre, primates. But insects, representing almost a million described species, must all occupy a single unbranched twig (more than halfway down at the left) because, as "primitive" forms, they have to be fitted into a lower level of the tree (with much less room on the cone) and must, moreover, share this limited space with other lesser creatures.

Alternatives to such misleading images exist, but the unconscious hegemony of canonical iconography has generally prevented their consideration and the canonical icons have therefore continued to constrain our thinking, for pictures are such powerful guides to our theorising.

Imagine a grass field with most stems mowed and just a few flowering profusely. While this icon may circumvent (and almost invert) the traditional canonical cone, it does not capture the most philosophically radical concept arising from our modern study of life's early multicellular history - the notion that most losses occurred by the luck of the draw rather than by the predictable superiority of a few founding lineages, and that any particular lineage still alive today (including our own) owes its existence to the contingency of good fortune. All our canonical icons are based upon the opposite notion of progress and predictability, and therefore preclude proper consideration of contingency as the major force affecting the directions of life.

If icons are central to our thought, not peripheral frills, then the issue of alternative representation becomes fundamental to the history of changing ideas in science (and even to the notion of scientific progress!).

How shall we draw the geometry of contingency? How else may we draw the history of life, so that we may come closer to meeting our ancestors face to face and may even probe pictorially into our own psyches to release the potential thoughts that lie even too deep for tears? !


A longer version of this essay appears in 'Hidden Histories of Science', by Stephen Jay Gould, Oliver Sacks, Jonathan Miller, Daniel J Kevles and RC Lewontin (Granta pounds 7.99), which is published tomorrow on 6 January