In the fields I know best, among the life sciences, it is required that the most expert and sophisticated minds be capable of changing course - often with a great lurch - every few years.

We pass the word around; we ponder how the case is put by different people, we read the poetry; we meditate over the literature; we play the music; we change our minds; we reach an understanding. Society evolves this way, not by shouting each other down, but by the unique capacity of unique, individual human beings to comprehend each other.

I do not agree with this century's fashion of running down the human species as a failed try, a doomed sport. At our worst, we may be going through the early stages of adolescence, and everyone remembers what that is like.

We can take some gratification at having come a certain distance in just a few thousand years of our existence as language users, but it should be a deeper satisfaction, even an exhilaration, to recognize that we have such a distance still to go.

We are not made up, as we had always supposed, of successively enriched packets of our own parts. We are shared, rented, occupied. At the interior of our cells, driving them, providing the oxidative energy that sends us out for the improvement of each shining day, are the mitochondria, and in a strict sense they are not ours.

As a species, taking all in all, we are still too young, too juvenile, to be trusted. We have spread across the face of the earth in just a few thousand years, no time at all as evolution clocks time, covering all livable parts of the planet, endangering other forms of life, and now threatening ourselves.

I agree that you might clone some people who would look amazingly like their parental cell donors, but the odds are that they'd be almost as different as you or me, and certainly more different than any of today's identical twins.