We are entering a new age, says Jeremy Rifkin in his book "Algeny." It will be an age when scientists will literally be able to make living beings to their own specifications, will use bioengineering to insert genes, delete genes, rearrange genes, and to borrow genes from one species and lend them to another. This use of bioengineering to upgrade life is what Rifkin means by the word "algeny."

But there is a dark side to algeny, Rifkin argues. Although scientists will be able to unlock the secrets of life and will be able to "change the essence of living things," we would do well to contemplate whether we really want to play God. Rifkin believes that if we do exercise all of our soon-to-be-gotten powers, life will be cheapened and trivialized. He pleads for scientists to restrain themselves from altering life.

Basically, Rifkin reasons that every age has a cosmology, a way it views the world, and every age uses its cosmology to justify itself. For example, Rifkin writes, Thomas Aquinas in the Middle Ages characterized nature in such a way as to reflect and legitimize medieval society. Darwin's theory of evolution reflected and legitimized the industrial revolution. And now Darwin is about to be replaced by a new cosmology that will reflect the computer age and that will legitimize the sort of tinkering with life that Rifkin so fears.

The question of whether molecular biologists should eventually agree to forget certain kinds of genetic engineering is provocative. The temptation, of course, is always to continue down the primrose path. But "Algeny," despite its veneer of a carefully constructed argument, ultimately does not make its case.

Since the central premise of this book is that scientists will soon be true genetic engineers, it is only fair to ask: How soon is soon? Many molecular biologists will doubtless be surprised to find that Rifkin expects them to engineer life within the next few decades. "Researchers," he writes, "believe that when today's babies are old enough to have children of their own, they may be able to select from a wide range of beneficial gene traits that they would like to have programmed directly into their offspring at the fetal stage, from manual dexterity skills to improved memory retentive capacity." Moreover, according to Rifkin, genetic engineering will also be used to prevent or cure diseases. "Other genes will be inserted that can help facilitate or retard growth, regenerate limbs, and perform a host of other medically useful activities."

Although molecular biologists have made enormous gains in their abilities to manipulate genes, they still do not know how to insert a gene in a predetermined place in an organism's DNA, they do not know how genes are turned on or off, they are far from understanding how genes influence such traits as manual dexterity or a good memory, and they believe that most genes interact with each other and with the environment in ways that the scientists can only vaguely grasp. In other words, we still need a series of stupendous advances before Rifkin's vision of the future becomes a reality.

But suppose Rifkin is right and that, 20 years from now, we can manipulate living organisms at will. Why should we not exercise this power if it could, for example, lead to better crops, better livestock, prevent cancer and heart disease and mental retardation?

According to Rifkin, if we alter life, we alter some sort of great cosmic ledger. He writes, "We live by the grace of sacrifice. Every amplification of our being owes its existence to some diminution somewhere else. In an ultimate sense, nothing that we claim as ours belongs to us, not even our fiber and sinew. Everything about us has been borrowed. We have been lent by nature. It has given over to us parts of itself, thus precluding their use for an infinite number of other things."

He then says that by not altering life, we will gain more than we will lose. "We gain immortality through our sacrifices. It is by giving something back, by leaving something unspent that we live on."

These quasi-religious arguments are nonarguments. Their appeal, I suspect, is to those who are frightened by the idea of genetic engineering and who do not realize how little scientists know about manipulating life. Rifkin plays too much to people's fears and, in the end, he fails to convince that his scenarios for the future make sense.