Purpose and Desire Page 10

  Today, we look upon Darwin’s pangenesis adventure as a quaint anomaly, something akin to a revered ancestor’s flirtations with a secret society that involved funny hats and odd rituals in secret rooms. What could the old coot have been thinking? we ask ourselves as we indulgently shake our heads. The joke is on us, though, because in proposing pangenesis, Darwin saw, more clearly than we typically bother to do today, the essential truth that Lamarck also saw: that evolutionary and physiological adaptation somehow shared a common foundation. In case we are tempted to dismiss pangenesis as a curious anomaly, it’s noteworthy that Darwin published this idea nine years after the first publication of On the Origin of Species and so represents Darwin’s mature thinking.

  Pangenesis was not exactly well received by Darwin’s contemporaries, however. Although pangenesis provided a theory for the matter of “telegony” (literally reproduction at a distance, or the tendency of a parent to influence the traits of a lineage many generations hence), as exemplified by Lord Morton’s mare, the evidence for it was nevertheless mostly circumstantial. Even though Darwin had made the best circumstantial case he could make in The Variation of Animals and Plants Under Domestication, his case did not stand up well to experimental test.18 For example, Darwin’s own half-cousin, Francis Galton, took pangenesis to test by transfusing blood between rabbits of different coat colors to see whether the supposed blood-borne gemmules from rabbits of one color could affect the coat colors of subsequent generations of pups. Galton couldn’t find any effect, leading him to reject pangenesis. This caused some hard feelings in the extended Darwin family because Galton had undertaken and published this rebuke without consulting his famous cousin.

  So negative were the reactions to pangenesis that Darwin eventually likened his idea to being a “mad dream.”19 But he never really disavowed the idea, mostly because of all that pesky evidence he had compiled. There had to be some explanation for it all, and until there was a better one, pangenesis was the explanation that he had found convincing. As an aside, pangenesis seems lately to be making a comeback. We now know, for example, that DNA and cells from embryos can enter the body and organs of the mother and affect her subsequent offspring, just as Darwin’s gemmules were supposed to have done.20

  In telling this story, there is a broader point I wish to make, however. Darwin was driven to propose pangenesis because he recognized, again more than we do today, the essential tension that sits at the heart of the evolutionary idea: between adaptation, which implies the ability to change according to conditions, and heredity, which is the opposite of change, that is, the legacy of the past imposing itself on the future. We should take a moment to let that sink in. Lamarck, for example, was all about change—about forces that push organisms and their lineages in grand arcs of greater complexity and ever-improving fit to environments. Cuvier, for his part, was all about resistance to change, lineages and species held in place by the glue of the conditions for existence that bind all of an organism’s many parts to one another—the unchanging legacy of one generation to the next. Today, we conflate evolutionism with Darwinism rather than Lamarckism or Cuvierism because Darwin was able to see, in ways that neither Lamarck nor Cuvier did, that a coherent theory of evolution must reconcile these contradictory ideas, that the tension between adaptation and heredity had to be resolved somehow.

  As it turns out, Darwin was unable to resolve it either, so the essential dilemma remains and has shaped much of the subsequent history of evolutionary thought. Its practitioners would argue that we resolved the dilemma in about the mid-twentieth century, as Michael Ghiselin did in the thumbnail history with which we began this chapter. The implication is that we now have in hand the coherent theory of evolution that Darwin started but failed to complete, and that theory is thoroughly mechanist, materialist, and purposeless. But the story, as we shall see in the next few chapters, is more complicated than that, mixing in roughly equal measures of brilliant insight, unresolved contradiction, and bedazzling sleight of hand. In the end, the task of reconciling the tension between adaptation and heredity remains incomplete.

  Before continuing our journey, I want to put Darwin and Bernard into some context, for each was, in his own way, reflective of the Hobson’s choice of modern science. Let me present the Hobson’s choice another way: when we study nature in a scientific way, should we approach it as Romantic idealists or as Enlightenment rationalists? Must we be one or the other? Cannot we be both?

  We saw in Claude Bernard’s life how he straddled those two worlds, mixing clear-eyed rationalism with Romanticism: homeostasis is, after all, a very idealist conception, very much a Romantic idea. Darwin, for his part, similarly straddled both worlds, one foot grounded solidly in evidence apprehensible by the senses but the other stuck in the essentially Romantic idea of adaptation, of the ineffable coherent organism. The two men’s lives pose for us an uncomfortable question: would we today consider either Darwin or Bernard scientists? I suspect that the answer would be “no,” given the difference between the mythic characters both are widely portrayed to be, and the way they actually were. Ultimately, both have suffered the same fate at our hands. Just as Bernard has had his Romantic roots cut out from under him as physiology cast its lot solely with mechanism, so too has Darwin seen the vitalist and Romantic core of his theory reamed out by his successors. The intellectual legacies of both men have been impoverished by our ministrations.

  6

  The Barrier That Wasn’t

  In the authoritative opinion of the indefatigable Ernst Mayr, August Weismann was the greatest evolutionary thinker of the nineteenth century, save Darwin himself (Figure 6.1).1 Among other things, he helped launch twentieth-century biology on its all-consuming quest for the material nature of the gene. In so doing, he set biology’s trajectory toward its twentieth-century materialist triumphs and, as it turns out, toward its looming crisis.

  Today, Weismann’s reputation is mostly pegged on his decisive purge of the dreaded taint of Lamarckism from evolutionary biology, where only pure Darwinism is allowed to rule. Because of Weismann, we now regard Lamarckism as a quaint philosophy with a ruffled collar that we need no longer take seriously. We keep it around mainly for nostalgia, because it might have inspired Darwin in his thinking. But once Darwin was done with it, we have permission to put it back on the bric-a-brac shelf with all the other mental clutter we keep around for sentimental reasons.

  The real story, as you might expect, is quite different. In the post-Origin nineteenth century, Lamarckism ran rampant on the evolutionary stage and formed the main scientific opposition to the Darwinian idea. So serious was the Lamarckian opposition that by the end of the century, Darwinism was looking rather tattered, sinking into what has come to be known as the crisis of Darwinism.2 In Darwinism’s epic story, it was August Weismann who rode in on a white horse and saved the day, not through forcefulness of opinion or prestige of position, but with brilliantly clear reasoning and simple experiment. When Weismann was done with it, Lamarckism lay impaled and gasping on the Darwinian pickets, never to rise again. From that point on, evolutionary thinking could be neatly divided into two types: Darwinist and wrong.

  Figure 6.1

  August Weismann (1834–1914).

  That’s the story we tell ourselves anyway. As usual, quite a bit of contrarian detail lurks within that comfortable myth, detail that we commonly regard as best forgotten. It is unnecessary to take Lamarckism seriously, we often tell students, because we have known since Weismann that Lamarckism is just plain wrong. Why delve into it? To do so would be like including a lesson on buggy whip technique in a driver’s ed course: what could possibly be the point? There is a point, nevertheless.

  August Weismann was born in Frankfurt into a solidly middle-class family. He was infatuated with butterfly and caterpillar collecting as a youth (inspired by his piano teacher), but his family’s upwardly mobile aspirations drove him to pursue a career in medicine. He did not find medical practice much to his liking, and soon after completing his medical degree, he left for the more verdant fields of academic medicine. There, he began studying embryonic development and thinking about the new theory of cells that was then sweeping biology.* An eye disease eventually drove him away from the microscope and laboratory bench, but to such a man as Weismann, near-blindness was but a trifle—something that simply was not allowed to stand in his way. So he shifted his attention to thinking about the relationship between embryos and evolution and devising experiments to test his thoughts.

  In the early post-Origin world, embryos were ground zero for the debate over Darwinism, because Lamarckism had found its surest foothold in embryonic development. To the embryologists, embryos were the parchment on which the history of evolution was written. New species, genera, phyla—all arose from subtle variations of embryonic development. New species were not so much new things as new recipes for how to build an organism. Imagine how a lean-to might differ from a pup tent or a tipi or a yurt. Each type of shelter differs in form—they are different species of shelter—but at root, they differ in the steps one follows to assemble poles, stakes, lines, and sheets into a shelter. So it is with embryos: new forms of organism come from how and when the embryo folds, grows, and pinches itself into new forms. Evolution, to the embryologists, was the historical record of how these basic building processes elaborated into new organisms: shuffle the steps, add a new step here, delay another step there, and you have a new organism. To those who looked upon evolution as the emergence of new forms of organisms, it was difficult to see how Darwin’s model of new species arising by natural selection could possibly work. Where is the competition and struggle for existence in the protected and inexorable development of the embryo? Darwin himself had no good answer for this rhetorical question: in fact, his only answer was not Darwinian at all, but his Lamarckian model of pangenesis.

  Weismann is supposed to have cut through all this by building a wall, what has come to be called the Weismann barrier. Weismann himself described it as the “segregation of the germ line,” which requires some explanation, of course. Let’s start with the “line” part. All embryonic development is a story of lineages, a family tree of sorts that represents lines of descent from progenitors (Figure 6.2). In the case of the embryo, the ultimate progenitor is the single fertilized egg, the zygote. The zygote divides, producing two descendants, which themselves divide, and their descendants divide, ad infinitum, producing the multitude of descendant cells that compose the mature organism. Each cell of the body can therefore trace a line of descent back to that single zygote, just as I can trace the Turner lineage in North America back to the unfortunate John Turner who sailed on the Mayflower to the New World.*

  As for the “germ line” part, Weismann proposed that among the multitudinous lineages of cells within the developing embryo, two distinct categories of cell lineages stand out. On the one hand is the somatic line, or soma, which builds the body and all the physiological functions that come with it. On the other hand is the germ line, whose cells segregate themselves from the soma early in embryonic development and eventually become the gametes—the ova and sperm (Figure 6.3).

  Figure 6.2

  Lines of descent in the development of the embryo.

  The soma and germ line are components of embryonic development, to be sure, but both also play into evolution in distinctive ways. The soma is the essential vehicle that nurtures and readies the gametes for reproduction, but the soma—so resplendent in our eyes—is merely the supporting actor for the real stars of the evolutionary show, the gametes. Once the gametes have been ushered through their role, the soma shuffles offstage and dies.

  There is another important distinction between germ line and soma, and this defines the “segregation” part. To produce the mature organism, the somatic lineages must differentiate, that is to say, they must specialize to perform specialized functions. So, for example, one lineage may specialize to produce muscle cells, another will specialize differently to produce bone cells, other lineages will specialize as brain cells, skin cells, etc.* To Weismann, such differentiation could never be allowed to happen in the germ line, because that would compromise the fidelity of the soma across generations. The undifferentiated germ line was what ensured that mouse parents would produce mouse children and not, say, sparrow children. The germ line therefore had to be kept pristine, walled off from the tumultuous fecundity of its somatic siblings. The wall that keeps soma and germ line apart is Weismann’s barrier (see Figure 6.3).

  The Weismann barrier is important because it throws an entirely new wrinkle into the central paradox of Darwinism: the tension between adaptation (which requires change) and heredity (which opposes change). As we have seen, Darwin reconciled this tension with his theory of pangenesis, which allows physiological adaptation in the soma to affect the germ line through the gemmules. This is a very Lamarckian view, because both the organism and its lineage share a common language of adaptation, of the organism’s “many little lives” negotiating their passage through time, mediated by the traffic in gemmules between the body’s various parts, and across generations, as suggested by the stripy offspring of Lord Morton’s mare (Chapter 5). Weismann’s barrier drove a wedge through all that. Now, physiological adaptation in the soma could no longer affect the germ line. No longer could evolution be the outcome of a turbulent and raucous conversation between soma and germ line. Now, the germ cells were elevated to be the organism’s cloistered royalty, never to be sullied by mingling with the grubby and grasping lumpen soma. Evolutionary change could come about only through changes in those pristine germ cells. The “many little lives” of the soma, for their part, were reduced to something akin to serfdom. Like serfs, the soma’s fleeting existence on the other side of the Weismann barrier mattered not a jot to the cloistered royalty within.

  Figure 6.3

  Segregation of the germ line from the soma by the Weismann barrier. The continuity of the germ plasm is ensured by the production of new gametes from the germ line. The somatic lineages die.

  Weismann didn’t just think Lamarckism into oblivion: he was supposed to have killed it off with a brilliantly simple experiment. What happens, Weismann asked, to a lineage of animals if the soma is modified, generation upon generation? If the soma’s experience informed the germ line, as Darwin and his Lamarckian rivals said it should, persistent modification of the soma should eventually show up in the lineage. So, Weismann took a colony of mice and amputated their tails. He bred these newly tailless mice with one another, amputated their offspring’s tails, bred them and amputated their tails in turn, and so on, for five generations. In the end, the tails in the fifth generation of mice had not shortened, not even a whit.* The result was clear: continual modification of the soma had left no heritable imprint on the lineage. That left the germ line as the only possible avenue of inheritance, and therefore of evolutionary change. With his amputated mouse tails, Weismann had wrought a decisive experimental disproof of the Lamarckian doctrine of the inheritance of acquired characters. For this, Weismann’s experiment has come to be touted as one of the greatest biology experiments of all time.3

  Weismann’s experiment, however, was nothing of the sort. First, the idea of a mutilation experiment to test Lamarckism was not original to him: such experiments had been common throughout the late nineteenth century. Many of these experiments were carried out by animal breeders, who were inspired by the experience of their counterparts in plant husbandry. Horticulturists often saw that grafts take on characteristics of their root stock, and animal breeders naturally wanted to know whether they could apply similar art to their own hybrid creations. These attempts were generally failures. Weismann was familiar with this dismal record, so he was very confident what the result of his mutilation experiment with mice would be.4 Why do the experiment then? It’s a fair question to ask. Weismann also was familiar with the numerous folktales circulating at the time that acquired characteristics could be heritable, and he found them all wanting. He cites a claim, for example, that a race of tailless cats in Germany had arisen from a female whose tail had been lost when it “was said” to have been crushed under a wagon wheel. The problem there, of course, was the circumstantial and hearsay “evidence,” never mind the unknown father of the tailless line. In another, better documented case of supposed acquired feline taillessness, from the Black Forest village of Waldkirch, the culprit turned out to be the pet Manx tomcat of the village parson’s English wife, which, as tomcats do, had surreptitiously sired an extensive lineage of offspring with diminished tails.5

  There is also a certain nagging . . . illogic . . . to Weismann’s experiment, which seems quite at odds with Ernst Mayr’s assessment of him as a brilliant evolutionary thinker. Let’s start with the experiment’s core assumption: that mutilation of the body is somehow akin to the normal process of adaptation that Lamarckians thought guided evolution. Who believes that? No one I know, nor were there many takers of the proposition in Weismann’s day. Nor is the division of germ line from soma always clear-cut: sometimes the segregation occurs very early in embryonic life, sometimes later, sometimes not at all. Why make segregation of the germ line so important? Germ lines are also a peculiarity of animals, leaving vast swathes of the Earth’s biota uncovered by Weismann’s doctrine of the segregation of germ lines: you can’t segregate something that doesn’t exist.

  Weismann knew these details very well—he was a superb naturalist and embryologist—so it seems a little strange that we are asked to believe he would pin so much on such a flimsy demonstration. If anything, Weismann’s serial murine mutilation was not so much an experiment as it was a rhetorical flourish in an ongoing debate over something else entirely. By elevating Weismann’s experiment into something it was not, we have lost sight of the point Weismann was trying to make. That point was not so much whether Lamarckism or pangenesis or the origins of lineages of tailless cats was true or not, but what the nature of heredity was. What Weismann did was to cast heredity into an entirely new framework, which colors how we think about it to this day.