And, apparently, we were wrong. By a set of processes collectively called “lateral gene transfer” (LGT for short), individual genes and short sequences of genes can move from one bacterial species to another. For two reasons, these transfers may challenge Linnaean logic in a serious way. First, LGT does not seem to respect taxonomic separation. That is, genes from genealogically distant bacterial species seem to enter a host species with no greater difficulty than genes from closely related species. Second, the process occurs frequently enough to preclude dismissal as a rare and peculiar exception to the prevailing Linnaean rule of strict branching with no subsequent amalgamation. (If only a percent or two of bacterial genomes represented imports from distant species by LGT, we could view the phenomenon as a fascinating anomaly that does not degrade the primary signal of Linnaean reality. But, at least for some species, LGT may be sufficiently common to flash a primary signal of its own. In E. coli, the familiar bacillus found in all human guts, for example, 755 of 4,288 genetic units [about 18 percent of the entire genome] record at least 234 events of lateral genetic transfer during the last 100 million years.)
Professional evolutionary biologists have been puzzled and excited by these discoveries about LGT. But the word has hardly filtered through to the interested public—an odd situation given the status of LGT as a challenge to one of our most basic assumptions about the nature and fundamental topology of evolution itself, not to mention the foundation of Linnaean logic as well! Perhaps most of us just don’t care about invisible bacteria, whereas we would sit up and take notice if we heard that LGT played a major role in the evolution of animals. Or perhaps the issue strikes most people as too abstract to command the same level of attention that we heap and hype upon such events of minimal theoretical interest as the discovery of a new carnivorous dinosaur larger than Tyrannosaurus. But I would not so disrespect the concerns of public understanding. Properly explained, the theoretical challenge of LGT to some truly fundamental views about the nature of evolution and classification should be fascinating to all people interested in science and natural history.
To put the matter baldly, if LGT plays a large enough role in bacterial evolution to overcome the Linnaean signal of conventional branching without subsequent joining, then binomial logic really doesn’t work. An honest diagnosis could not then recommend a simple remedy of some minor repairs or small plaster patches—for the Linnaean system would truly be broken by the collapse of this central theoretical prerequisite. The hierarchical basis of Linnaean logic demands that life’s history develop as a tree, without amalgamation of branches once a lineage originates in independence. But if LGT dominates the composition of bacterial genomes, then trees cannot express the topology of evolutionary relationships, because the pathways of life then form a meshwork, as bacteria evolve by importing genes from any position, no matter how evolutionary distant, on the genealogical net.
But don’t just trust the words of this expert on land snails (in the realm of fat, furry things) and diligent essayist on subjects beyond his genuine expertise. Consider these measured words from a technical article by the leading researcher on the subject (W. Ford Doolittle of Dalhousie University, Halifax, writing in the June 25, 1999, issue of Science, a special report on evolution from America’s leading journal for professional scientists). Doolittle wrote, in an article titled “Phylogenetic classification and the universal tree”:
If “lateral gene transfer” can’t be dismissed as trivial in extent or limited to species categories of genes, then no hierarchical universal classification can be taken as natural. Molecular phylogeneticists will have failed to find the “true tree,” not because their methods are inadequate or because they have chosen the wrong genes, but because the history of life cannot properly be represented as a tree.
Do not lament for the spirit of Linnaeus. Yes, his dreams about the discovery of a universal system suffered two sequential blows—first, soon after his death, when scientists recognized that his logic could only work for organisms, and not for rocks and all the rest of the natural world. And, second, as discovered only in the last decade, when Linnaean taxonomy encountered a strong biological challenge from the frequency of lateral gene transfer, the ultimate tree-buster, in the substantial domain of bacteria, albeit not so strongly expressed in our own world of multicellular life (although the sequence of the human genome, published in February 2001, does reveal some important bacterial “immigrants” as well).
As a truly great scientist, Linnaeaus understood the central principle that honorable error, through overextension of exciting ideas, “comes with the territory”—and that theories gain both strength and better definition by principled limitations upon their realm of legitimate operation. Moreover, as the modern founder of the truly noble science of taxonomy, Linnaeaus also understood that all classifications must embody passionate human choices about the causes of order—in short, theories that must be subject to continuous revision and correction of error—and cannot only be conceived as passive descriptions of objective nature on the philatelic model.
Thus, taxonomies must express both concepts and percepts—and must therefore teach us as much about ourselves and our mental modes as about the structure of external nature.14 Surely Linnaeaus, of all people, comprehended this fundamental and ineluctable interrelationship of mind and nature, for when he composed, at the very beginning of Systema Naturae, his formal description of his newly crowned species, Homo sapiens, he linked us (in various editions)—in only one case correctly, as we now know—with three other mammals: monkeys, sloths, and bats. For each of these three, Linnaeus penned a conventional and objective description in terms of hairiness, body size, and number of fingers and toes. But for Homo sapiens, he chose the path of terseness and wrote just the three Latin words of another familiar motto. Notnatura non facit saltum this time, but the foremost intellectual challenge of classical wisdom: Nosce te ipsum—Know thyself.
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Abscheulich! (Atrocious)
REVOLUTIONS CANNOT BE KIND TO PROMINENT AND UNRE-constructed survivors of a superseded age. But the insight and dignity of vanquished warriors, after enough time has elapsed to quell the immediate passions of revolt, often inspire a reversal of fortune in the judgment of posterity. (Even the most unabashed northerner seems to prefer Robert E. Lee to George McClellan these days.)
This essay details a poignant little drama in the lives of three great central European scientists caught in the intellectual storm of Darwin’s Origin of Species, published in 1859. This tale, dormant for a century, has just achieved a vigorous second life, based largely on historical misapprehension and creationist misuse. Ironically, once we disentangle the fallacies and supply a proper context for understanding, our admiration must flow to Darwin’s two most prominent opponents from a dispersed and defeated conceptual world: the Estonian (but ethnic German) embryologist and general naturalist Karl Ernst von Baer (1792–1876), who spent the last forty years of his life teaching in Russia, and the Swiss zoologist, geologist, and paleontologist Louis Agassiz (1807–1873), who decamped to America in the 1840s and founded Harvard’s Museum of Comparative Zoology, where I now reside as curator of the collection of fossil invertebrates that he began.
Meanwhile, our justified criticism must fall upon the third man in this topsy-turvy drama, the would-be hero of the new world order: the primary enthusiast and popularizer of Darwin’s great innovation, the German naturalist Ernst Haeckel (1834–1919). Haeckel’s forceful, eminently comprehensible, if not always accurate, books appeared in all major languages and surely exerted more influence than the works of any other scientist, including Darwin and Huxley (by Huxley’s own frank admission), in convincing people throughout the world about the validity of evolution.
I willingly confess to hero-worship for the raw intellectual breadth and power of three great men: Darwin, who constructed my world; Lavoisier, because the clarity of his mind leaves me awestruck every time I read his work; and Karl Ernst von Baer, who lived
too long and became too isolated to win the proper plaudits of posterity. But T. H. Huxley, who ranks fourth on my personal list, regarded von Baer as the greatest pre-Darwinian naturalist of Europe, and I doubt that any expert with the detailed knowledge to render judgment about general brilliance and specific accomplishments would disagree.
As the leading embryologist of the early nineteenth century, von Baer discovered the mammalian egg cell in 1827 and then, in 1828, published the greatest monograph in the history of the field: Entwickelungsgeschichte der Thiere (The developmental history of animals). He then suffered a mental breakdown and never returned to the field of embryology. Instead he moved to St. Petersburg in 1834 (a common pattern for central European scientists, as Russia, lacking a system of modern education, imported many of its leading professors in scientific subjects from abroad). There he enjoyed a long and splendid second career as an Arctic explorer, a founder of Russian anthropology, and a geomorphologist credited with discovering an important law relating the erosion of riverbanks to the earth’s rotation.
Von Baer’s theories of natural history allowed for limited evolution among closely related forms, but not for substantial transformation between major groups. Moreover, he held no sympathy for Darwin’s mechanistic views of evolutionary causality. Darwin’s book shook the aged von Baer from decades of inactivity in his old zoological realm—and this great man, whom Agassiz, in his last (and posthumously published) article of 1874, would call “the aged Nestor of the science of Embryology,” came roaring back with a major critique entitled Über Darwins Lehre (On Darwin’s Theory).
In a second article written to criticize a brave new world that often disparaged or even entirely forgot the discoveries of previous generations, von Baer made a rueful comment in 1866 that deserves enshrinement as one of the great aphorisms in the history of science. Invoking Louis Agassiz, his younger friend and boon companion in rejecting the new theory of mechanistic evolution, von Baer wrote:
Agassiz says that when a new doctrine is presented, it must go through three stages. First, people say that it isn’t true, then that it is against religion, and, in the third stage, that it has long been known [my translation from the German original].
Ernst Haeckel (1834–1919), with his characteristic mixture of gusto and bluster, fancied himself a Darwinian general, embattled in Agassiz’s first two stages and unfurling the new evolutionary banner not only for biological truth, but for righteousness of all stripes. In 1874 he wrote in his most popular book, Anthropogenie (The Evolution of Man):
On one side spiritual freedom and truth, reason and culture, evolution and progress stand under the bright banner of science; on the other side, under the black flag of hierarchy, stand spiritual slavery and falsehood, irrationality and barbarism, superstition and retrogression. . . . Evolution is the heavy artillery in the struggle for truth.
Men of large vision often display outsized foibles as well. No character in the early days of Darwinism can match Haeckel for enigmatic contrast of the admirable and the dubious. No one could equal his energy and the volume of his output—most of high quality, including volumes of technical taxonomic description (concentrating on microscopic radiolarians, and on jellyfishes and their allies), not only theoretical effusions. But no major figure took so much consistent liberty in imposing his theoretical beliefs upon nature’s observable factuality.
I won’t even discuss Haeckel’s misuse of Darwinian notions in the service of a strident German nationalism based on claims of cultural, and even biological, superiority—a set of ideas that became enormously popular and did provide later fodder for Nazi propagandists (obviously not Haeckel’s direct fault, although scholars must bear some responsibility for exaggerated, but not distorted, uses of their arguments—see D. Gasman, The Scientific Origins of National Socialism: Social Darwinism in Ernst Haeckel and the German Monist League [London: MacDonald, 1972]). Let’s consider only his drawings of organisms, supposedly a far more restricted subject, imbued with far less opportunity for any “play” beyond sober description.
I do dislike the common phrase “artistic license,” especially for its parochially smug connotation (when used by scientists) that creative humanists care little for empirical accuracy. (After all, the best artistic “distortions” record great skill and conscious intent, applied for definite and fully appropriate purposes; moreover, when great artists choose to depict external nature as seen through our eyes, they have done so with stunning accuracy.) But I don’t know how else to describe Haeckel, who was, by the way, a skilled artist and far more than a Sunday painter.
Haeckel published books at the explicit interface of art and science—and here he stated no claim for pure fidelity to nature. His Kunstformen der Natur (Artforms of Nature), published in 1904 and still the finest work ever printed in this genre, contains one hundred plates of organisms crowded into intricate geometric arrangements. One can identify the creatures, but their invariably curved and swirling forms so closely follow the reigning conventions of art nouveau (called Jugendstil in Germany) that one cannot say whether the plates should be labeled as illustrations of actual organisms or primers for a popular artistic style.
But Haeckel also prepared his own illustrations for his technical monographs and scientific books—and here he did claim, while standard practice and legitimate convention also required, no conscious departure from fidelity to nature. Yet Haeckel’s critics recognized from the start that this master naturalist, and more than competent artist, took systematic license in “improving” his specimens to make them more symmetrical or more beautiful. In particular, the gorgeous plates for his technical monograph on the taxonomy of radiolarians (intricate and delicate skeletons of single-celled planktonic organisms) often “enhanced” the actual appearances (already stunningly complex and remarkably symmetrical) by inventing structures with perfect geometric regularity.
This practice cannot be defended in any sense, but distortions in technical monographs cause minimal damage because such publications rarely receive attention from readers without enough professional knowledge to recognize the fabrications. “Improved” illustrations masquerading as accurate drawings spell much more trouble in popular books intended for general audiences lacking the expertise to separate a misleading idealization from a genuine signal from nature. And here, in depicting vertebrate embryos in several of his most popular books, Haeckel took a license that subjected him to harsh criticism in his own day and, in a fierce brouhaha (or rather a tempest in a teapot), has resurfaced in the last two years to haunt him (and us) again, and even to give some false comfort to creationists.
We must first understand Haeckel’s own motivations—not as any justification for his actions, but as a guide to a context that has been sadly missing from most recent commentary, thereby leading to magnification and distortion of this fascinating incident in the history of science. Haeckel remains most famous today as the chief architect and propagandist for a famous argument that science disproved long ago, but that popular culture has never fully abandoned, if only because the standard description sounds so wonderfully arcane and mellifluous—“ontogeny recapitulates phylogeny,” otherwise known as the theory of recapitulation or, roughly, the claim that organisms retrace their evolutionary history, or “climb their own family tree” to cite an old catchphrase, during their embryological development. Thus the gill slits of the early human embryo supposedly repeat our distant ancestral past as a fish, while the transient embryonic tail, developing just afterward, marks the later reptilian phase of our evolutionary ascent. (My first technical book, Ontogeny and Phylogeny [Harvard University Press, 1977], includes a detailed account of the history of recapitulation—an evolutionary notion exceeded only by natural selection itself for impact upon popular culture. See essay 8 for a specific, and unusual, expression of this influence in the very different field of psychoanalysis.)
As primary support for his theory of recapitulation, and to advance the argument that all vertebrates may be
traced to a common ancestor, Haeckel frequently published striking drawings, showing parallel stages in the development of diverse vertebrates, including fishes, chickens, and several species of mammals from cows to humans. The accompanying figure (on page 310) comes from a late, inexpensive, popular English translation, published in 1903, of his most famous book, The Evolution of Man. Note how the latest depicted stages (bottom row) have already developed the distinctive features of adulthood (the tortoise’s shell, the chick’s beak). But Haeckel draws the earliest stages of the first row, showing tails below and gill slits just under the primordial head, as virtually identical for all embryos, whatever their adult destination. Haeckel could thus claim that this near identity marked the common ancestry of all vertebrates—for, under the theory of recapitulation, embryos pass through a series of stages representing successive adult forms of their evolutionary history. An identical embryonic stage can only imply a single common ancestor.
To cut to the quick of this drama: Haeckel exaggerated the similarities by idealizations and omissions. He also, in some cases—in a procedure that can only be called fraudulent—simply copied the same figure over and over again. At certain stages in early development, vertebrate embryos do look more alike, at least in gross anatomical features easily observed with human eyes, than do the adult tortoises, chickens, cows, and humans that will develop from them. But these early embryos also differ far more substantially, one from the other, than Haeckel’s figures show. Moreover, Haeckel’s drawings never fooled expert embryologists, who recognized his fudgings right from the start.
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