THE SHORT-LIVED Baby Fae made an even bigger splash with her transplanted baboon's heart than the USDA's experiments inserting human genes into sheep and pigs to make them grow larger. We have used heart valves from pigs to repair human hearts for years without anybody getting excited about species mixing. But baboon hearts! This time, have we gone too far?

How should we think about such questions? Consider two purposefully extreme hypothetical reports of possible biological futures. Let's examine them logically. For that will offer a way to think about whether, in other issues, man has "gone too far."

* Avant-Garde Hospital today reported a major breakthrough in the treatment of accident victims. Four-year-old Girl Grace, riding her tricycle along the sidewalk, lost her balane and fell into the path of a truck which drove over her right arm, destroying it. Grace was rushed to the shock-trauma unit, where a surgical team, noted for its pioneering successes in reattaching the severed limbs of accident victims, grafted the arm of a young baboon onto her body. Now, improved immunosuppressant drugs and promising animal experiments have lead to optimism that Grace, and others like her, will have functional digits and limbs despite the severity of their accidents. A hospital spokesman said, "We anticipate that, if the grafting is successful, Girl Grace may require some special counseling as she is growing up. But she will have two functioning arms! This new therapy does not save lives like the transplantation of vital organs. But it brings the patient closer to normal functioning, which is what most medical treatment is about. So we're very excited about it."


* The Defense Department revealed today that genetic engineers, working on a top-secret enterprise on the scale of the Manhattan Project, have bred a new class of foot soldier. The Pentagon's statement said, in part, "These new warriors greatly enhance our ability to secure peace through strength. They have immense stamina, physical power, and resistance to pain. Within the range of their abilities, they follow instructions superbly. They mature quickly, and have marvelous, uncomplaining dispositions. The nation is indebted to the dedicated scientists who, starting 30 years ago with genetic material from great athletes, mentally handicapped people, primates, and certain other mammals, learned to combine that material into this crucial part of the greatest force for peace the world has ever known."

What's our choke point on this sort of development, the first involving transplants and the second, gene-mixing? How should we assess the ethics of these possibilites?

We should begin, at least, by reasoning carefully, respecting important distinctions, and withholding judgments that exceed the warrant of the facts.

There's little point in simply shouting slogans back and forth at one another; we've got to make policy decisions that will serve us all well. For how the growing debate comes out will affect what medical cures will be available to us, how well we will eat, and even what kinds of beings there are and what humans look like.

For that matter, the way we structure the debate can help us decide whether we've "gone too far" on a range of other issues -- from Star Wars defense to abortion.

Let's start with using animal parts to treat humans. One complaint about Baby Fae's transplant is that it abused the baboon. The moral character of our relationship to animals is important, but I endorse the most prevalent sentiment: Provided no other treatment is comparably promising, a defense of the baboon's interests is not alone sufficient for opposing such transplant surgery.

Other complaints said that the transplant was flawed technology -- that it was not the best available treatment, was not sufficiently promising even in the absence of any other option, and was done without adequately informed parental consent.

These are all reasonable objections. But they distract us from considering the propriety of interspecies transfer. So, for the sake of argument, assume that the treatment cannot be opposed on such grounds.

We are left with the fundamental objection that seemed to cause the emotional response -- that it was a baboon heart. The charge is that such transplantation is wrong because it violates the integrity of natural species. Advocates of such a view speak of "the rights of each species to exist as a separate, identifiable creature," and cite the respect for species integrity that is reflected in our endangered-species laws.

In the aftermath of Baby Fae, we have learned how common it has become to use animal parts to repair humans -- even their hearts. Our use of pig heart valves to save 100,000 people seems unproblematic; why then the fuss about Baby Fae?

Some of it surely arises from the symbolic significance of the heart. On the one hand, the heart is just a vital organ -- one of the simpler ones, at that. It is a pump, nothing more. If it fails, the challenge is to fix or replace it. That task is prominent on the agenda of medical science and in the public consciousness because lots of hearts fail.

Ah, but then there is the other hand. For lots of hearts break, as well. And what would romantic poetry be without the heart? Could Lovelace have conveyed the impact of Gratiana's dancing by writing, "And when she ceas'd, we sighing saw the floor lay pav'd with broken spleens?" Would you want a valentine that said "I love you with all my liver?" Would you rush to see a film named "Kind Kidneys and Coronets?"

Our language and mythology imbue the heart with metaphorical power that shapes our emotional reactions to medical news. But cool reflection should lead us to separate the one hand from the other, and to make judgments about medical care strictly on the basis of relevant considerations.

So we reject the notion that species-mixing therapies are inherently wrong, recognizing that we have relied on them comfortably already. And we thus reject the charge that Baby Fae's transplant was wrong because it was a heart, rather than some other organ, or just because it was a whole heart, rather than only a valve.

Are we then unable to object to any future interspecies transplant that seems to be the best available medical technology? Are we committed to accepting the arm transplant for Girl Grace?

I think not. But if we are to resist it, we ought to be able to distinguish it from the cases we accept. We ought to be able to say something credible about why it is different.

Forget that it is not technically feasible today to graft a primate's limb onto a person. That could change quickly with a major research investment.

Assume no robotic limb could achieve comparable functional capacity.

Assume it is your arm that is lost. Would you accept the primate graft? If the patient were your daughter, would you grant consent for it?

In each case, I would not. And I was fairly sure of that before I had any clear idea why.

British philosopher Johnathan Glover, in his new book "What Sort of People Should There Be?," helps shed some light on the matter. "Our present practice is to act in quite different ways towards humans and towards members of other species," he writes. If we were to blur "the clear gap between monkeys and ourselves," we would face the need to develop appropriate patterns of behavior toward the resulting beings. And they would have to find ways of relating to us. It could be very difficult.

Baby Fae, had she lived, would have been unmistakably human, despite her heart. (Her elementary school chums, knowing her history, would have denied it, but that is a separate matter.)

Girl Grace, however, would be unmistakably anomalous. She would live surrounded by people who struggled to know what to make of her, how to react to her. She would live behind a wall of strangeness, making normal relationships impossible: Would you want a date to have a baboon arm?

By contrast, we do make our way -- often less well than we should -- with those who visibly fall far outside the range of normal human body images because of injury, illness, or congential anomaly. We know that their character and worth transcend the limitations of their bodies, and we know it is our own limitations that introduce awkwardness into our dealings with them.

Yet we also know that participation in the human social order is deeply rooted in having a human body. A body visibly constructed from the parts of other animals is, I would argue, one that we would respond to in a fundamentally different way than to a body that nature has misshapen, or that man has supplemented with machine-like prosthetics. And the thought of doctors creating such a body on purpose causes us to be revulsed or made very anxious. We know far better how to deal with amputees than with Girl Grace; they know better how to deal with us.

I have no proof that Girl Grace's operation is a bad idea. It is a matter of judgment, supported by reasons that can be persuasive without being conclusive. Even if you could gracefully accept your child having a baboon arm, surely you can imagine some combination of animal parts attached to a human brain that you would be unable to view as totally human. Then somewhere along the continuum from the unproblematic pig valve to the unacceptable limb grafting, the balance has shifted from "yes" to "no."

One objection may be that this position seems to assess human worth based on externals -- that it defines a person's humanity in terms of visible appearance rather than character, rationality and psychological capacity. After all, many severely deformed people not only function in society, but humble, instruct, and inspire us with their courage, tenacity, and accomplishment.

But the objection is a misunderstanding. I do not propose to diminish our respect for the victims of severe deformation due to accidents of birth or circumstances. I simply propose to acknowledge that they are victims, and to resist victimizing others knowingly, when we have a choice, by increasing their separation from their fellow human beings, thus diminishing their humanity.

Critics challenge this sort of position by asking "Where do you draw the line between 'yes' and 'no'?" They point to another new issue in medicine -- gene therapy. They fear that if we start altering genes to remove congenital defects, we will become committed to the brutalizing quest for genetically engineered human perfection.

The new breed of hybrid warrior would be clearly forbidden by the criterion I have suggested; we could not relate to such a consciously designed product as totally human. Yet I welcome the gene therapy presently under development.

No one now at work in genetic engineering intends to develop medical interventions that will modify the the "germline" of humans -- those characteristics that are passed on to future generations. Nor can anyone think of any practical way to develop such a technology that would reliably produce beneficial results.

The gene therapy now planned is a way to help patients who themselves suffer from specific, genetically-based, deadly diseases such as Lesch-Nyhan disease -- the victims of which uncontrollably shred their own flesh as they deteriorate. This kind of therapy is called "somatic" -- it treats an individual victim suffering from genetic disease. But it does not in any way affect the offspring getting the disease. In this way, the technology is not ethically different from a plastic surgery to "cure" a person of having a disagreeable nose, without affecting his offspring's likelihood of having such a feature. There is hope that other genetic disorders -- such as Tay-Sachs disease, which afflicts Eastern European-heritage Jews, and sickle- cell anemia, which plagues blacks -- will eventually be treatable in this fashion.

No proposals for human germline intervention, as opposed to somatic therapy, are under development. It is foolish -- indeed, cruel -- to oppose the responsible development of beneficial somatic therapies on the ground that germline projects of a sort not now underway could lead to bad outcomes if we were unable, once they were underway, to set sensible policies of limitation.

It would also be foolish to assume that no surveillance of developments in genetic engineering or interspecies transfer is needed. What is learned by advances in somatic therapy could lead to progress toward germline intervention, and that could lead to socially and ethically unpalatable consequences. But we need not fear that outcome, so long as we separate fact from fantasy, and openly debate the limits that should constrain our emerging skills.

Those who argue differently are devotees of the "slippery slope" approach. The structure of their argument is familiar: What is now contemplated may seem justifiable by itself, but it is the first step toward an intolerable outcome. If we allow that first step, we will have no way to halt the process, for drawing any line of limitation will be arbitrary and controversial. So we must not allow the first step. Such "slippery slope" arguments are often sound, but not always. Each instance must be assessed on its merits.

Consider this argument: if we allow research on human subjects, people will be exploited and harmed in the interest of medical progress. Yet we have drawn the line; we have an elaborate, imperfect, but largely successful set of guidelines and mechanisms to protect the subjects of medical research.

Consider: if we allow abortion without limit in the first trimester, we will lose all respect for life and will accept infanticide of any unwanted infant. That argument has been heard, but despite a liberal abortion policy, we have not moved even slightly in the direction of indifference toward infanticide. Indeed, our efforts to protect children from abuse and deprivation of all kinds seems to be more vigorous than ever, if news accounts can be credited.

So we must ask how good the slippery- slope arguments are against interspecies transplants and gene transfer. The answer is that they are not very good. It is easy to envision a distasteful outcome from unrestrained applications of any technical advance. But there is no need for the gloomy view that restraint is impossible. Our record is better than that.

Assuming, then, that curing an individual's genetic disease is, in principle, justifiable, and that the creation of hybrid soldiers is not, how can we defend a policy that says "yes" to the former and "no" to the latter? Where do we draw the line, and why?

No man turns from hirsute to bald when one last, crucial hair departs. There's a region between the clear cases in which it is impossible, and also unnecessary, to decide whether baldness has been achieved. Similarly, a man with a pig's valve in his heart is entirely human (you still have the same car even if you replace the alternator). But a human head transplanted onto the body of an ape would clearly be an interspecies hybrid (you would not have the same car if you kept only the engine and replaced everything else). Between, there are regions of grey, where it is impossible to make the call.

The "Where do you draw the line?" crowd tries to capitalize on that impossibility. But it doesn't matter where exactly such lines are drawn, so long as they include the clearly tolerable cases and rule out the clearly intolerable ones. Draw them anywhere, after suitable public debate, so long as they fall within the band of grey.

We've done that with abortion policy -- a good example precisely because there is such bitter dispute about it. The overwhelming majority position is that new-born infants count as people and the day-old embryo does not. Somewhere between, personhood emerges gradually, as the fetus develops the neurological capacity for sentience.

Personhood does not just snap into place, so we cannot cite a special point at which it is suddenly present. By allowing abortions early in this process of development, but resisting them later, the Supreme Court (perhaps on different reasoning) has drawn a line appropriately through a murky band of grey that separates two situations so different in degree as to be different in kind, despite the continuity of process that leads from one to the other.

And so it can be with interspecies transplants and genetic transfers. I would oppose procedures that produce what, on reflection, we cannot comfortably conclude is clearly human in all interpersonally significant respects. Others might set the limits somewhat differently. I trust that after open debate about the issues, public sentiment would favor a policy close enough to that to satisfy me.

To say all this is to place great reliance on our capacity to exercise judgment. It is to respect our ability to face an uncertain future, confident that we can bring reason to bear on new possibilities, to separate progress from madness. It is to say that we dare proceed, because we can exercise discretion, change direction, and say "This far is far enough. Here, we shall stop."

We need to do all that with regard to nuclear weapons, high-technology health care, and genetic engineering. I have no proof that we will succeed. But if we lack the capacity and resolve to do so, then what is so special about our humanity that we should worry so much about preserving its integrity?