Until lately I thought the most troubling book I had ever read was The End of Nature, Bill McKibben's searching meditation on humankind's definitive eclipse of our nonhuman context. That book was not primarily a catalogue of likely environmental disasters, imminent or eventual, though there was more than enough of such worrisome news. What mainly preoccupied McKibben was that we have, without forethought, passed a momentous limit. By the end of the 20th century, the scale of human activity had grown so rapidly and enormously that the nonhuman world now exists on our sufferance - not this or that niche, but the character of the whole.

The resulting loss, McKibben suggested, cannot be gauged. In the last two centuries we destroyed many of the wonders of the world: the Colorado River and its sublime canyons (very nearly including the Grand Canyon), the California redwoods, the tall-grass prairie, the bison herds, the grizzlies. Each disappearance has occasioned much anguish among those who loved those places and creatures, at least in imagination. But in each case the idea of nature survived: our confidence, however feckless, that we might disfigure the planet here or there but could never overwhelm it; that we might misuse it, or even use up this or that aspect of it, but that as a whole it would endure, essentially as it always had, mute, unmasterable, sovereign in its indifference to us.

No longer. The increase of carbon dioxide and other "greenhouse" gases in the atmosphere, and the associated long-term climate changes, are capable of altering the distribution of life forms on earth so profoundly that the distinction between natural and artificial is imperiled. If "natural" means what happens without human intervention, outside human control, then the natural world has, in effect, disappeared. Even if global industrial activity is henceforth curtailed, which is hardly likely, we have already gone too far. We may yet avoid ruining the world, but we have already shrunken it drastically, with unpredictable - and probably not very healthy - effects on our collective psyche.

McKibben's next book, Hope, Human and Wild, reported on some comparatively sane ways of living within this shrunken world. It described the resourceful city administration of Curitiba, Brazil, with its ingenious low-tech solutions to flood control, garbage collection, and public transportation; the Indian district of Kerala, comparable to the United States in literacy and life expectancy with only one-seventieth the per-capita income; and, surprisingly, the northeastern U.S., where forests and wildlife have made an unexpected recovery in recent decades and communities have begun to defend themselves against industrial predation. Whether enough people will decide soon enough, as McKibben urged his readers, "to slow down, to reduce expectations, to undevelop" is highly uncertain. Still, the book left one with a chastened, tentative hopefulness about nature's prospects.

About the human prospect, however, it is hard to feel anything but terrified after reading Enough: Staying Human in an Engineered Age. As most people know, the genetic modification of many plants and animals - not by traditional crossbreeding but by directly inserting or altering individual genes - has been attempted, with much controversy about the results. According to one eminent scientist (James Watson, who discovered the structure of DNA), we are "at the beginning of a great GM [genetically modified] plant revolution," which will "ensure that crops contain a fuller array of nutrients ... hold the key to distributing orally administered vaccine proteins ... [and] provide ways as yet unimagined to preserve the environment." According to another (Harvard professor Richard Lewontin), the new technology merely "provides a powerful tool for the control of agricultural production by monopolistic producers of the inputs into agriculture, with no ultimate advantage either to farmers or consumers and with the possibility of destroying entire national agricultural economies."

An even more fateful controversy is just getting under way. It looks, many scientists claim, more and more likely that humans too can be genetically modified by such means, whether to prevent disease or to enhance desired traits. Some diseases, like cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease, require the presence of a single defective gene. If a couple conceives several embryos in vitro, one without the disease-carrying gene can be chosen and implanted in the mother's womb. Such genetic screening is already available and is not particularly controversial.

But what if genes for IQ, physique, memory, musicality, etc. can be isolated and modified? What if a future child's character and personality can, to an initially limited but slowly increasing extent, be predetermined? Would such genetic interventions be morally right? And even if most people believe they wouldn't be, should they nevertheless be legal for those who believe otherwise?

Before approaching these questions, a factual caveat. Some scientists object that it does not look very likely that human behavior can be genetically modified. Steven Pinker, for example, writes that "not only is genetic enhancement not inevitable, it is not particularly likely in our lifetimes. ... The human brain is not a bag of traits with one gene for each trait. Neural development is a staggeringly complex process guided by many genes interacting in feedback loops." Richard Lewontin likens the genetic code to a complex ecosystem: "You can always intervene and change something ... but there's no knowing what the downstream effects will be or how it will affect the environment." Barry Commoner declares flatly: "By any reasonable measure, the findings [of the Human Genome Project] ... destroy the scientific foundation of genetic engineering."

Still, it seems wise to consider ahead of time what we should do if genetic enhancement turns out to be feasible. If we are to pass this even more momentous limit, to alter human nature as we have always known it, let us at least not do so without forethought. So: even if someday we can safely intervene in the biological mechanism of inheritance and get the results we want, ought we to do it? Enough is McKibben's answer, an unequivocal no.

In the first place, genetic enhancement is un-American. The essence of Americanism is found in that bold and noble sentence from the Declaration of Independence: "All men are created equal." As with all great truths, precisely what this statement means has been disputed. But it has been sufficiently well understood throughout the last two centuries to serve as the foundation of equality before the law and the basis of American democracy, which is perhaps humankind's grandest (however imperfect) political achievement.

Whatever "all men are created equal" may mean, exactly, it will no longer be true after a few dozen or a few hundred generations of genetic enhancement. As McKibben points out, like most technological advances in an already drastically unequal society, "designer" genes would benefit the rich far more than the poor:

They would take the gap in power, wealth, and education that currently divides both our society and the world at large, and write that division into our very biology. A sixth of the American population lacks health insurance of any kind - they can't afford to go to the doctor for a checkup. And much of the rest of the world is far worse off. If we can't afford the fifty cents a person it would take to buy bed nets to protect most of Africa from malaria, it is unlikely we will extend to anyone but the top tax bracket these latest forms of genetic technology.

These are not mere Luddite mutterings. According to Princeton geneticist Lee Silver, a prominent advocate of the new technology, "emotional stability, long-term happiness, inborn talents, increased creativity, healthy bodies - these could be the starting points chosen for the children of the rich," while "obesity, heart disease, hypertension, alcoholism, mental illness - these will be the diseases left to drift randomly among the families of the underclass." The "GenRich" (i.e., genetically enhanced) minority, Silver continues, will control "all aspects of the economy, the media, the entertainment industry, and the knowledge industry." Eventually, "the GenRich class and the Natural class [i.e., the rest of us] will become ... entirely separate species with no ability to crossbreed, and with as much romantic interest in each other as a current human would have for a chimpanzee."

I call this un-American. As if to explicate the Declaration of Independence, Thomas Jefferson, its principal author, later wrote: "The general spread of the light of science has already laid open to every view the palpable truth, that the mass of mankind has not been born with saddles on their backs, nor a favored few booted and spurred, ready to ride them legitimately, by the grace of God." But one must ask, what science once gaveth, is science now about to take away? "The ultimate question raised by biotechnology," concludes Francis Fukuyama in his very astute Our Posthuman Future, is: "What will happen to political rights once we are able to, in effect, breed some people with saddles on their backs, and others with boots and spurs?" What will happen, pretty clearly, is the end of democracy and the American ideal. A modest proposal, therefore: shouldn't we get a little closer to attaining those things before we risk losing them forever so that a privileged minority can ascend to superhumanity (and, not incidentally, so that a powerful industry can reap enormous profits)?

Assuming, of course, that we have any choice in the matter. Perhaps, after all, we won't. "Whether we like it or not," Lee Silver admonishes, "the global marketplace will reign supreme." There you have it - the antagonism between democratic deliberation and free-market fundamentalism could not be more forthrightly, even contemptuously, expressed. There's a lot of money to be made in biotech, as there was in generating greenhouse gases, even if those activities turn our inner and outer worlds upside down. Case closed.

The political argument for restraint is straightforward and, it seems to me, unanswerable. But most of Enough is devoted to making a deeper, more ambitious, more difficult argument against human genetic engineering: an argument from meaning. The promise of the new technologies (McKibben also discusses robotics and nanotechnology, the use of molecules for computing, manufacturing, etc.) is fabulous. Newly formulated gene packages will improve our every physical, emotional, and intellectual capability, eventually without limit. Beginning with the more directly gene-dependent ones, every disease will be eradicated, including cancer - even including aging; immortality too is part of the program. Nanobots (contingents of molecule-size robots) with unimaginable information-processing capacity will synthesize food, assemble consumer goods, and clean up waste, thereby eliminating hunger, poverty, and pollution. And in the fullness of time, they will construct a vastly superior species, thereby eliminating us.

Some of these, as McKibben shows, are false promises. To take a longstanding one: the Green Revolution in agriculture has, it is true, invariably increased plant yields, but it has also invariably increased micronutrient deficiencies and small-farmer indebtedness as well. The latest genetically engineered miracle, "golden rice," is a will-of-the-wisp. Biotechnology is not the solution to world hunger; biodiversity is. Low-tech solutions are also readily available for Third World poverty and disease; though since they are less profitable than high-tech solutions, they require a little disinterested First World support. The brand-new promise of nanotechnology is also dubious, even if feasible: nanoweapons, or even nano-accidents, would be far more devastating than present-day biological or chemical weapons.

But whether feasible or not, post-humanity is, McKibben argues, a poisoned gift while we have not yet achieved humanity. All the proposed new technologies aim at a fundamentally similar result: more. They aim to make our genetic hardware, our resources, our actions in some way larger or faster or easier. More computational power or muscle mass or dexterity or serotonin or orgasmic potency; faster communication or transportation or food preparation; easier work or no work. More lifespan, sensory inputs, entertainments, et cetera ad infinitum. More and faster and easier are not, in general, bad. On the contrary, they are, other things being equal, good. But sometimes - as in the near future, perhaps, when a flood of new stimuli and experiences may overwhelm our sensory and imaginative capacity to assimilate them, not to mention our political capacity to keep them from rending humankind's already precarious community - the good can be the enemy of the best.

What then, if not more, is best for human beings now? McKibben bravely faces this all-important question and throws out some fruitful hints. He cites some fascinating psychological research on joy, in particular the "almost trance-like state" the subjects entered when their work was going especially well. In this state, "fatigue, hunger, or discomfort ceased to matter." This ecstatic experience seems to derive from "the intense concentration that risk and adversity entail"; it "arises more from sensory focus than sensory overload" - from going inward, that is, rather than simply adding on, whether data, circuits, or genes. The world, the research subjects testify in a striking formulation, "falls away." This sounds a good deal like the peak experiences described by mystics and meditators. It also bears some resemblance to the "deep time" of fully engaged reading, evoked memorably by literary critic Sven Birkerts in The Gutenberg Elegies. It even (or so it seems to me) remotely echoes Nietzsche's enigmatic allusions to "self-overcoming."

McKibben's point is that there's an inner world as well as an outer world, and penetrating the one may be as essential to human fulfillment as conquering the other. The new hypertechnologies will facilitate the latter but not the former. Since our inclination is usually to evade what's difficult, we may find an increasing disproportion between our power and our depth. If there really is, as he speculates, a "strange connection between effort and joy and pride and reward," then perhaps limitless abundance would be less satisfying than the techno-utopians suppose.

It is perfectly true, of course, that inwardness - or self-cultivation or self-overcoming or whatever you like to call it - requires a sufficiency of material goods. Underfed and overworked people should not be asked to forego further calories and conveniences. But as McKibben tirelessly reiterates, we in the developed world are already pretty close to the "enough" point. "We have reached a point of great comfort and ease relative to the past. The real question is whether, having reached that point, we want to trade it in for something essentially unknown." Turning back technologically, or even standing still, is no more desirable than it is possible. But it's at least thinkable that "the rush of technological innovation that's marked the last five hundred years can finally slow, and spread out to water the whole delta of human possibility." And to begin, for decency's sake, with those who are now much farther from the "enough" point than we are would provide a pretty challenging technological agenda for the rest of this century, at least.

Heartily as I agree with McKibben, I am tempted to amend his battle-cry to "Enough ... for now." We can't really know, after all, how a grown-up human race would regard this question. Perhaps someday in the distant future we will want, as a species, to assume a new form of life. Perhaps we will have together come to fruit, reached an unsurpassable ripeness, a stable equipoise of individuality and community, solidarity and self-assertion, such that nowhere on earth remained large pools of unnecessary pain or ignorance, such that all or most possible harmonies within us and among us had been sounded and danced to. Then, not from fear that one's future child may be left behind in a hectic competition for top pre-schools, or from any such desperately sad motive, but instead with a grateful reverence for the rounded humanity and exquisite comity we will by then have perfected together, some may volunteer to renounce that perfection and be tinkered with.

If that's how it happens, then so be it. But we today should leave to those who will be, by our own lights and through our own efforts, wiser than us, a decision that is beyond our present wisdom. We should, for the future's sake, acknowledge the limits of that wisdom. We are not wise enough - not now, not for a long time - to justify abdicating our specieshood, renouncing our humanity forever in favor of post-humanity. We are wise enough, though, to make a good life for our species now and to see a few steps in the direction of a better one.

At least, I hope we are. We will be, at any rate, if we can learn from Bill McKibben's humane and far-seeing book.