Hen’s Teeth and Horse’s Toes (11 page)

Darwin studied the sinking of “Druidical stones” at Stonehenge and the foundering of Roman bathhouses, but he found his most persuasive example at home, in his own field, last plowed in 1841:

Section through one of the fallen Druidical stones at Stonehenge, showing how much it had sunk into the ground. Scale ½ inch to 1 foot.

An original illustration from Darwin’s worm book showing the foundering of large stones by the action of worms.

In 1871, he cut a trench in his field and found 2.5 inches of vegetable mold, entirely free from flints: “Beneath this lay coarse clayey earth full of flints, like that in any of the neighboring ploughed fields…. The average rate of accumulation of the mould during the whole thirty years was only .083 inch per year (i.e., nearly one inch in twelve years).”

In various attempts to collect and weigh castings directly, Darwin estimated from 7.6 to 18.1 tons per acre per year. Spread out evenly upon the surface, he calculated that from 0.8 to 2.2 inches of mold would form anew every ten years. In gathering these figures, Darwin relied upon that great, unsung, and so characteristically British institution—the corps of zealous amateurs in natural history, ready to endure any privation for a precious fact. I was particularly impressed by one anonymous contributor: “A lady,” Darwin tells us, “on whose accuracy I can implicitly rely, offered to collect during a year all the castings thrown up on two separate square yards, near Leith Hill Place, in Surrey.” Was she the analogue of a modern Park Avenue woman of means, carefully scraping up after her dog: one bag for a cleaner New York, the other for Science with a capital S?

The pleasure of reading Darwin’s worm book lies not only in recognizing its larger point but also in the charm of detail that Darwin provides about worms themselves. I would rather peruse 300 pages of Darwin on worms than slog through 30 pages of eternal verities explicitly preached by many writers. The worm book is a labor of love and intimate, meticulous detail. In the book’s other major section, Darwin spends 100 pages describing experiments to determine which ends of leaves (and triangular paper cutouts, or abstract “leaves”) worms pull into their burrows first. Here we also find an overt and an underlying theme, in this case leaves and burrows versus the evolution of instinct and intelligence, Darwin’s concern with establishing a usable definition of intelligence, and his discovery (under that definition) that intelligence pervades “lower” animals as well. All great science is a fruitful marriage of detail and generality, exultation and explanation. Both Darwin and his beloved worms left no stone unturned.

I have argued that Darwin’s last book is a work on two levels—an explicit treatise on worms and the soil and a covert discussion of how to learn about the past by studying the present. But was Darwin consciously concerned with establishing a methodology for historical science, as I have argued, or did he merely stumble into such generality in his last book? I believe that his worm book follows the pattern of all his other works, from first to last: every compendium on minutiae is also a treatise on historical reasoning—and each book elucidates a different principle.

Darwin’s original illustration for his theory of coral reefs. Top figure: lower solid line, stage 1, a fringing reef (AB) abuts the shore line. Island sinks (level of sea rises) to upper dotted line, stage 2, barrier reef (A’) separated from sinking island by a lagoon (C).
Bottom figure: lower solid line, stage 2, barrier reef (copied from upper dotted line of top figure). Island sinks further (below level of sea) to upper dotted line, stage 3, an atoll (A”), enlarged lagoon (C’) marks previous location of sunken island.

Consider his first book on a specific subject,
The Structure and Distribution of Coral-Reefs
(1842). In it, he proposed a theory for the formation of atolls, “those singular rings of coral-land which rise abruptly out of the unfathomable ocean,” that won universal acceptance after a century of subsequent debate. He argued that coral reefs should be classified into three categories—fringing reefs that abut an island or continent, barrier reefs separated from island or continent by a lagoon, and atolls, or rings of reefs, with no platform in sight. He linked all three categories with his “subsidence theory,” rendering them as three stages of a single process: the subsidence of an island or continental platform beneath the waves as living coral continues to grow upward. Initially, reefs grow right next to the platform (fringing reefs). As the platform sinks, reefs grow up and outward, leaving a separation between sinking platform and living coral (a barrier reef). Finally the platform sinks entirely, and a ring of coral expresses its former shape (an atoll). Darwin found the forms of modern reefs “inexplicable, excepting on the theory that their rocky bases slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards.”

This book is about coral, but it is also about historical reasoning. Vegetable mold formed fast enough to measure its rate directly; we capture the past by summing effects of small and observable present causes. But what if rates are too slow, or scales too large, to render history by direct observation of present processes? For such cases, we must develop a different method. Since large-scale processes begin at different times and proceed at diverse rates, the varied stages of different examples should exist simultaneously in the present. To establish history in such cases, we must construct a theory that will explain a series of present phenomena as stages of a single historical process. The method is quite general. Darwin used it to explain the formation of coral reefs. We invoke it today to infer the history of stars. Darwin also employed it to establish organic evolution itself. Some species are just beginning to split from their ancestors, others are midway through the process, still others are on the verge of completing it.

But what if evidence is limited to the static object itself? What if we can neither watch part of its formation nor find several stages of the process that produced it? How can we infer history from a lion? Darwin treated this problem in his treatise on the fertilization of orchids by insects (1862); the book that directly followed the
Origin of Species
. I have discussed his solution in several essays (1, 4, 11 and
The Panda’s Thumb
) and will not dwell on it here: we infer history from imperfections that record constraints of descent. The “various contrivances” that orchids use to attract insects and attach pollen to them are the highly altered parts of ordinary flowers, evolved in ancestors for other purposes. Orchids work well enough, but they are jury-rigged to succeed because flowers are not optimally constructed for modification to these altered roles. If God wanted to make insect attractors and pollen stickers from scratch, he would certainly have built differently.

Thus, we have three principles for increasing adequacy of data: if you must work with a single object, look for imperfections that record historical descent; if several objects are available, try to render them as stages of a single historical process; if processes can be directly observed, sum up their effects through time. One may discuss these principles directly or recognize the “little problems” that Darwin used to exemplify them: orchids, coral reefs, and worms—the middle book, the first, and the last.

Darwin was not a conscious philosopher. He did not, like Huxley and Lyell, write explicit treatises on methodology. Yet I do not think he was unaware of what he was doing, as he cleverly composed a series of books at two levels, thus expressing his love for nature in the small and his ardent desire to establish both evolution and the principles of historical science. I was musing on this issue as I completed the worm book two weeks ago. Was Darwin really conscious of what he had done as he wrote his last professional lines, or did he proceed intuitively, as men of his genius sometimes do? Then I came to the very last paragraph, and I shook with the joy of insight. Clever old man; he knew full well. In his last words, he looked back to his beginning, compared those worms with his first corals, and completed his life’s work in both the large and the small:

At the risk of unwarranted ghoulishness, I cannot suppress a final irony. A year after publishing his worm book, Darwin died on April 19, 1882. He wished to be buried in the soil of his adopted village, where he would have made a final and corporeal gift to his beloved worms. But the sentiments (and politicking) of fellow scientists and men of learning secured a guarded place for his body within the well-mortared floor of Westminster Abbey. Ultimately the worms will not be cheated, for there is no permanence in history, even for cathedrals. But ideas and methods have all the immortality of reason itself. Darwin has been gone for a century, yet he is with us whenever we choose to think about time.

IN
1936,
TROFIM D. LYSENKO
, struggling to reform Russian agricultural science on discredited Lamarckian principles, wrote: “I am not fond of controversy in matters concerning theory. I am an ardent controversialist only when I see that in order to carry out certain practical tasks I must remove the obstacles that stand in the way of my scientific activities.”

As his practical task, Lysenko set out to “alter the nature of plants in the direction we desire by suitable training.” He argued that previous failure to produce rapid and heritable improvements in important crop plants must be laid to the bankrupt ideology of bourgeois science, with its emphasis on sterile academic theory and its belief in Mendelian genes, which do not respond directly to the prodding of breeders but change only by accidental and random mutation. The criterion of a more adequate science must be success in improved breeding.

“The better we understand the laws of development of plant and animal forms,” he wrote, “the more easily and quickly will we be able to create the forms we need in accordance with our wishes and plans.” What “laws of development” could be more promising than the Lamarckian claim that altered environments can directly induce heritable changes in desired directions? If only Nature worked this way! But she does not, and all Lysenko’s falsified data and vicious polemics budged her not one inch.

If Lysenko’s “obstacles” had been disembodied ideas alone, the history of Russian genetics might have been spared some of its particular tragedy. But ideas emanate from people, and the obstacles designated for removal were necessarily human. Nikolai Ivanovich Vavilov, Russia’s leading Mendelian geneticist and director of the All-Union Lenin Academy of Agricultural Sciences centered in Leningrad, served as a focal point for Lysenko’s attacks in 1936. Lysenko castigated Vavilov for his general Mendelian views, but any geneticist might have served equally well for such generalized target practice. Lysenko singled out Vavilov for a more specific and personal theory (and the subject of this column)—the so-called law of homologous series in variation.

Twelve years later, following the devastation of war, Lysenko had triumphed. His infamous address, “The Situation in Biological Science,” read at the 1948 session of the Lenin Academy of Agricultural Sciences, contains as the first statement of its summary what may well be the most chilling passage in all the literature of twentieth-century science.

Following another ten pages of rhetoric and invective, Lysenko concludes: “Glory to the great friend and protagonist of science, our leader and teacher, Comrade Stalin! [
All rise. Prolonged applause
.]”

Nikolai Vavilov was unable to attend the 1948 meeting. He had been arrested in August 1940 while on a collecting expedition in the Ukraine. In July 1941, he was sentenced to death for agricultural sabotage, spying for England, maintaining links with émigrés, and belonging to a rightist organization. The sentence was commuted to ten years imprisonment, and Vavilov was moved to the inner prison of the NKVD in Moscow. In October, he was evacuated to the Saratov prison where he spent several months in an underground death cell, suffering from malnutrition. He died, still a prisoner, in January 1943.

What is Vavilov’s “law of homologous series in variation,” and how did it provide Lysenko with rhetorical leverage? Vavilov published this law, the guiding principle for much of his practical work in agricultural genetics, in 1920 and revised it in 1935. It was printed in English in the prestigious
Journal of Genetics
in 1922 (vol. 12, pp. 48–89).

Vavilov was perhaps the world’s leading expert on the biogeography of wheat and other cereals. He traveled throughout the world (thereby leaving himself vulnerable to trumped-up charges of espionage), collecting varieties of plants from their natural habitats and establishing the world’s largest “bank” of genetic variation within major agricultural species. As he collected natural races of wheat, barley, oats, and millet over a large range of environments and places, he noticed that strikingly similar series of varieties could be found within the different species of a genus and often within species of related genera as well.

He collected, for example, a large number of geographical races within the species of common wheat,
Triticum vulgare
. These varied in complex sets of traits, including color of the ears and seeds, form of the ears (bearded or beardless, smooth or hairy), and season of maturation. Vavilov was then surprised and delighted to find virtually the same combinations of characters in varieties of two closely related species,
T. compactum
and
T. spelta
.

He then studied rye (
Secale cereale
), a species in a genus closely related to wheat but previously regarded as much more limited in its geographical variation. Yet, as Vavilov and his assistants collected rye throughout European and Asiatic Russia, Iran, and Afghanistan, he found not only that its differentiation matched wheat in extent but also that its races displayed the same sets of characters, with the same variations in color, form, and timing of growth.

The similarities in series of races were so precise and complete between related species that Vavilov felt he could predict the existence of undiscovered varieties within one species after finding their parallel forms in another species. In 1916, for example, he found several varieties of wheat without ligules in Afghanistan (ligules are thin membranes that grow from the base of the leaf blade and surround the stem in many grasses). This discovery suggested that varieties of rye without ligules should also exist, and he grew them from seeds collected in Pamir in 1918. He predicted that durum wheat (
Triticum durum
), then represented exclusively by spring varieties, should also have winter forms since related species do—and he found them in 1918 in an isolated region of northern Iran.

Vavilov’s observation would have engendered less controversy had he not interpreted it, indeed overinterpreted it, in a manner uncongenial with strictly Darwinian or Lamarckian views. He might have argued that his series of parallel varieties represented similar adaptations of different genetic systems to common environments that engendered natural selection in the same direction. Such an interpretation would have satisfied Darwinian preferences for random variation, with evolutionary change imposed by natural selection. (It could also have been distorted by Lysenko into a claim that environments directly altered the heredity of plants in favorable ways.)

But Vavilov proposed a different explanation more in tune with non-Darwinian (though not anti-Darwinian) themes still popular during the 1920s: he claimed that the parallel series of varieties represented identical responses of the same genetic systems, inherited in toto from species to related species. Thus, in evolutionary parlance, his series were “homologous”—hence the name of his law. (Homologies are similarities based on inheritance of the same genes or structures from a common ancestor. Similarities forged within different genetic systems by selective pressures of similar environments are called analogies.)

Vavilov argued that new species arise by developing genetic differences that preclude interbreeding with related species. But the new species is not genetically distinct from its ancestor in all ways. Most of the ancestor’s genetic system remains intact; only a limited number of genes are altered. The parallel varieties, then, represent a “playing out” of the same genetic capacities inherited as blocks from species to related species.

Such an interpretation is not anti-Darwinian because it does not deny an important role to natural selection. While each variety may represent a predictable latent capacity, its expression in any climate or geographical region still requires selection to preserve the adaptive variant and to eliminate others. But such an explanation does conflict with the spirit of strict Darwinism because it weakens or compromises the cardinal tenet that selection is
the
creative force in evolution. Random, or undirected, variation plays a crucial role in the Darwinian system because it establishes the centrality of selection by guaranteeing that evolutionary
change
cannot be ascribed to variation itself. Variation is only raw material. It arises in all directions or, at least, is not preferentially ordered in adaptive ways. Hence, direction is imposed by natural selection, slowly preserving and accumulating, generation after generation, the variations that render organisms better adapted to local environments.

But what if variation is not fortuitous and undirected but strongly channeled along certain paths? Then only a limited number of changes are possible, and they record the “internal” constraints of inheritance as much as the action of selection. Selection is not dormant; it still determines which of several possibilities reaches expression in any one climate or area. But if possibilities are strictly limited, and if a species displays all of them among its several varieties, then this range of form cannot be ascribed only to selection acting upon fortuitous variation.

Moreover, this explanation for new varieties compromises the cardinal principle of creativity for natural selection. The variations are predictable results within their genetic system. Their occurrence is almost foreordained. The role of natural selection is negative. It is an executioner only. It eliminates the variants unfit in any given environment, thus preserving the favored form that had to arise eventually.

Vavilov interpreted his law of homologous series in this non-Darwinian manner. “Variation,” he wrote, “does not take place in all directions, by chance and without order, but in distinct systems and classes analogous to those of crystallography and chemistry. The same great divisions [of organisms] into orders and classes manifest regularities and repetitions of systems.” He cites the case of “several varieties of vetches so similar to ordinary lentils in the shape, color, and size of their seeds, that they cannot be separated by any sorting machine.” He agrees that the extreme similarity in any one spot is a product of selection—unconscious selection in agricultural sorting machines. But the agent of selection was, literally in this case, only a sieve that preserved one variant among many. The proper variant existed already as a realized product of an inherited set of possibilities.

Vavilov, overenthused with his own idea, went on. He became intoxicated with the notion that his law might represent a principle of ordering that would render biology as exact and experimental as the “hard” sciences of physics and chemistry. Perhaps genetic systems are composed of “elements.” Perhaps the geographical varieties of species are predictable “compounds” that arise inevitably from the union of these elements in specified mixtures. If so, the ranges of biological form within a species might be laid out in a table of possibilities similar to the periodic table of chemical elements. Evolution might be deduced from genetic structure itself; environment can only act to preserve inherent possibilities.

He advocated an explicit “analogy with chemistry” in the concluding section of his 1922 paper and wrote: “New forms have to fill vacancies in a system.” He experimented with a style of notation that expressed varieties of a species as a chemical formula and advocated “the analogy of homological series of plants and animals with systems and classes of crystallography with definite chemical structures.” One zealous supporter commented that “biology has found its Mendeleev.”

Vavilov moderated his views during the 1920s and early 1930s. He learned that some of the parallel varieties between species are not based upon the same genes after all, but represent the similar action of selection upon different sources of variation. In these cases, the varieties are analogous, not homologous, and the Darwinian explanation must be preferred. He wrote in 1937:

Still, Vavilov continued to champion the importance and validity of his law, and he continued to advocate the analogy with chemistry in only slightly weakened form.

Unfortunately, in the deepest sense, Vavilov had left himself open to Lysenko’s polemical attack. The law of homologous series provided Lysenko with important ammunition, and Vavilov’s overextended chemical analogy deepened his troubles. Lysenko caricatured Vavilov’s law in 1936 by presenting ridiculous examples involving species too distantly related to present parallel series in Vavilov’s system: “In nature we find apple trees with round fruit, hence there must or can be trees with round pears, cherries, grapes, etc.”

Lysenko’s ideological attack was more vicious. He made two major charges involving both parts of that catchword for official Soviet philosophy—dialectical materialism. Vavilov’s law, he claimed, was undialectical because it located the source of organic change within the genetic systems of organisms themselves and not in the interaction (or dialectic) between organism and environment. Secondly, Lysenko charged that the law of homologous series was “idealist” rather than materialist because it viewed the evolutionary history of a species as prefigured in the unrealized (and therefore nonmaterial) capacity of an inherited genetic system.