It used to be said that the public attitude toward scientific discovery had three stages: 1) It's impossible. 2) Maybe it can be done, but it will be useless. 3) It's obvious.

To these we should today add a fourth: 4) Even if it can be done, it's a terrible idea and it will lead to all kinds of problems.

Five years ago, the average person thought that making an exact copy of a mammal - a clone - was impossible. Then in 1997 a Scottish lamb, Dolly, achieved worldwide fame because she was just such a clone, produced from a single cell taken from the udder of a ewe. She was, genetically speaking, an exact copy of her mother.

The outcry was strong and immediate. The worry, of course, was not about cloned sheep; it was about the possibility of cloned humans.

A church group, asked why they had not been concerned earlier about the possibility of making exact copies of humans, offered a frank and honest reply: We didn't worry about it, because we thought it was impossible. That was a most reasonable answer. Back in 1978, a book appeared which claimed to describe the cloning of a human, but every reputable biologist was convinced that the author was writing fiction. "The Cloning of a Man" and "The Boys From Brazil" notwithstanding, no one knew how to clone any mammal, still less a human. At the time, the biologists' reassurances were perfectly true. Today, when mammals from mice to sheep and cows have been successfully cloned, the chances are close to certainty that a human clone will be produced within the next ten to twenty years (legally or illegally).

How worried should we be by this? Should a human clone be regarded as "contrary to nature" and some kind of abomination?

Fortunately, I can give a very specific answer to those questions. Before doing so, however, let me point out that the idea of plant and animal cloning is neither new nor unnatural. Every time that a plant is grown from a cutting or from root division, the new one is an exact genetic copy - a clone - of the original. Many animals employ a form of reproduction known as parthenogenesis, in which females produce eggs but no male is involved. All the resulting offspring are female, and they are clones of the mother. Aphids (greenflies) often reproduce this way, and so do water fleas. It is as though some females, although quite able to reproduce sexually, have decided that the search for a mate is too much trouble.

Other animals can be induced by human intervention to reproduce without the presence of a male partner. Unfertilized eggs, suitably treated, will hatch out and develop into adult forms. The treatment ranges from heat or shaking, which will do the trick with starfish, to pricking an egg with a needle dipped in blood, which works for frogs (a full moon and suitable incantations are not needed).

These tricks for inducing parthenogenetic reproduction were known three-quarters of a century ago. However, greenflies, starfish, and frogs are not our close relatives. What about humans?

At this point I am going to confide a secret from my own past, something which is not known even to my own wife: long ago, I dated one or two girls.

When I say, "one or two girls" I mean that statement quite literally. Either I dated one girl, or I dated two, and I have never been sure which. A pair of identical twin sisters used to go to the same dance hall as I did on Saturday nights, and they had the devilish habit of alternating dresses. I could not tell them apart. Conversation on the dance floor, or walking one of them home afterwards, provided a real challenge because I was never sure which one I had been with last week.

Of course, their parents had no difficulty at all in telling which was which, regardless of who was wearing what dress. Identical twins are never truly identical (for one thing, they have different fingerprints). Yet from a genetic point of view, identical twins are exactly the same. They are clones - more similar, in fact, than the human clones who will surely one day be created, because unlike those future clones they share the same human womb at the same time and are usually brought up in a common home environment. Clones born a generation apart and raised in a much-changed society are likely to be far more different in behavior than identical twins.

Such twins are natural clones, and there tens of millions of them. About one human birth in every 250 is of identical twins. (There are more total twins than this, because a rather larger number of fraternal twins are born, siblings from a separate sperm and a separate ovum who happen to share the womb.) Since the world population is more than six billion, there are over twenty million identical twins - clones in everything but name - alive today.

No one, so far as I know, considers that this presents a moral dilemma. No one is suggesting that identical twins should be aborted or banned from the world, or that their existence is unnatural and offers ethical problems. Once the outcry dies away and the technology becomes standard, I expect that human cloning will become just another accepted way of having babies.

This does not mean that this is the right time to clone humans. I regard any such effort as highly premature, since most of the cloned mammals produced to date have deformities and many abnormalities. We need to understand exactly why this happens and be able to guarantee the development of a normal individual, before we risk a human life.

However, provided that we proceed with due care and caution, human cloning does not seem to me to involve questions of morality. In fact, I see only one real objection to it, and that is that the people most likely to want to form a clone of themselves will be the ones we would least like to see in duplicate versions. Are you ready for more than one Donald Trump, or for multiple Jerry Springers?

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