What are the odds on a book that opens with a polysyllabic assault including mitochondria, prokaryocytes, eukaryotic, rhizobial on its first two pages selling more than 300,000 copies hard and paperback and being translated into nine languages?

How about a hospital president and research biologist who writes essays on weekends aimed at medical students and young doctors becoming one of the few physicians to make the best-seller list who doesn't discuss dieting, jogging or sex?

The book is "The Lives of the Cell," by Dr. Lewis Thomas, and this year he has repeated his success with his second collection of essays, "The Medusa and the Snail."

The appeal of Thomas' short and elegant essays is difficult to pin down, but a part must be his optimism about the earth and mankind.

"We are a spectacular, splendid manifestation of life," Thomas writes in a plea for a better press for humankind. He echoes Will Rogers' "I've never met a man I didn't like" and proclaims:

"There is nothing at all absurd about the human condition. We matter."

Thomas has been writing from time to time since he discovered Ezra Pound and T. S. Eliot while a Princeton undergraduate and tried his hand at poetry.

When he was an intern at Boston City Hospital, Thomas recalled, he "earned a fraction of an income" by selling poems to the Atlantic. The magazine paid$35 a poem. "That was better than selling blood," he said. Then, blood brought $25 a pint.

He only turned to the 1,200-word essays that have brought him his large audience by accident in 1970 when he was 56 and well-known within the biomedical profession, but almost unknown outside it.

A paper Thomas wrote on the phenomenon of inflammation was read by Franz Joseph Inelfinger, then the editor of the New England Journal of Medicine.

Ingelfinger, an old friend, offered him a column on the conditions that Thomas would meet a monthly deadline and not write more than one page of the Journal could hold. "The reward for this was that my copy wouldn't be edited," Thomas recalled with a smile.

He was at Yale then as dean of the medical school and wrote his essays during the weekends he spent at his house in Woods Hole, Mass. He would try to think out what he wanted to say in the car Friday night before starting to work on his yellow legal pads Saturday.

His prose is another reason for his success. Thomas writes as well as his liver takes care of its responsibilities. (Thomas once wrote that he is considerably less intelligent that his own liver.)

It's easy to come away from a Thomas essay with an enhanced pride of species. A slightly better feeling about what's going on.

Yet Thomas said in an interview in his office at Memorial Sloan-Kettering Cancer Center where he is president and chief executive officer, he is not as optimistic as some people think.

"I suppose I'm an optimist about the long term if we can get through the next 20 to 25 years," Thomas siad "I'm worried because of nuclear weapons. It seems to me that we only have 20 or 25 years to decide whether we're going to blow ourselves up or get rid of them. I think this is responsibile for a lot of despair in young people.

"There never been a time before when anxiety had to include the possibility that the world would end."

Thomas, 65, speaks clearly but softly, twisting his pipe in his hands. He is shy, but inspires confidence. It is easy to imagine him as a doctor with patients or a teacher with students, although most of his successful career has been devoted to medical administration and biological research.

Thomas is baffled that newspapers and journals discuss nuclear exchanges as though they were possibilities.

"Has nuclear war been talked about so much that it has lost its meaning?" he asks. "People talk as though you might have a war in which tens of millions of people were killed outright and still have hope of a society and a culture.

"I think society would come unhinged."

That would be a limited nuclear exchange. "If we let fly with everything that's around right now, there's a real risk of killing off plant life in the sea." Thomas said. Oxygen levels would plummet. Only microbial life would be sustained. The earth would be as it was 3 1/2 billion years ago.

Except for nuclear weapons, Thomas is optimistic about the problems that make headlines. Energy, food shortage and overpopulation will be solved somehow, he said.

Thomas is not sure how to explain the popularity of his books. Obviously, people are more interested in biologic science than might have been anticipated and Thomas is particularly pleased that there is "an interest in biological phenomena seen rather like puzzles."

He likes puzzles and enjoys the exhilaration of being dumbfounded by nature. In fact, how little man knows is one of his major themes. "We don't know very much. One of the things that's hopeful about us is that we're just getting underway," he said. In "The Medusa and the Snail" he wrote of human beings: "We are the newest, the youngest and the brightest thing around."

His second major theme is the importance of symbiosis -- the often elaborate partnerships that exist all around us.

The medusa (a jellyfish) and the snail (a sea slug) of his title are one stunning example. One type of slug lives with a small remnant of jellyfish attached close to its mouth as a parasite. However, the jellyfish can reproduce and when it does, the young of the slug float up into its tentacles, as though they were being eaten by the jellyfish.

It is not so easy to tell who is doing the eating, however. Gradually the jellyfish disappears, eaten by the slug, until only a small part remains affixed near the slug's mouth. The partnership is unique, Thomas writes, but it also reminds him of the whole earth.

Many of his essays repeat the theme that partnership is more the way of the world than attack. He follows his Will Rogerseque statement that "I've never encountered any genuinely, consistenly detestable human beings in all my life" by adding " . . . we haven't yet learned how to stay human when assembled in masses."

His books have brought him a lot of mail Thomas said, including letters from people who feel nature is a good deal more hostile than he portrays it. He smiles, Thomas cannot prove it, but he believes that symbiotic relationships between bacteria and their hosts are more common than infectious diseases.

His essay "On Disease" describes the high cost of a body's defense against an intruding virus in the case of a form of meningitis. It is the biological equivalent of the military man's statement: "We had to destroy the town to save it."

Some of Thomas' likes and dislikes crop up in his essays. He uses Bach's music frequently as an achievement of human development all human beings can take pride in. "I'm a skilled gramophone player," Thomas said. "I have lots of Bach. I may have everything he wrote on records."

Electric guitars go on his list of dislikes, but Thomas declined to elaborate on what makes him angry about 1979 American society except to say:

"I'm worried by the tendency to emphasize the importance of oneself." He thinks it is clear that much of the satisfaction humans get is from being useful. A self-imposed unhappiness can result from concentrating one the self rather than finding things to do or things to learn. The me generation doesn't win his applause.

The last 20th centruy is infected by a cultural sadness, Thomas wrote. One thing wrong, eating away at us is that "We do not know enough about oursleves."

Thomas wrote: "We have discovered now to ask important questions, and now we really do need, as an urgent matter, for the sake of our society and its culture, to obtain some answers.

Thomas plans to continue writing, but he has more or less stopped meeting his regular deadline. There is no question, however, that he will keep writing essays of the same type. "I'm stuck. I can't go above 1,200 words," Thomas said dryly.

In addition to his books and his job as president of Memorial Sloan-Kettering which combines a research institution with a large cancer financial aid to meet its $150 million annual cost. Thomas has kept a small research lab going, as he has wherever he has been.

He manages to spend a few hours in his lab almost every day early in the morning, at lunchtime or in the evening. He is also on the governing board of the National Academy of Sciences and is an overseer of Harvard and a trustee of the Rockefeller University. He reads ("a lot of things that I'm pointed to by my wife") and continues to write some poetry. It is not a schedule that leaves much time for idleness nor for giving interviews.

Although he is polite and gracious he makes it clear that he hopes the que;tions can be asked and photographs be taken as quickly as possible.

What Thomas sees around him in the cancer hospital strikes him as having something to do with the biological drive toward usefulness. The morale, he said, is higher than at any other hospital where he has been involved.

"Perhaps that's because there are no medically trivial problems. The stakes are perceived as life and death in each case," he said.

Death is another subject on which Thomas is reassuring, Nature, he has written in several essays, has worked it out so that the moment of death is not a moment of pain, but of an extraordinary kind of tranquility.

No scientific evidence supports his belief, but there is abundant anecdotal evidence from humans and it is also possible to observe an apparent time tranquility to overcome dying animals.

A good deal of anecdotal literature is available from people who have been saved from near death by modern medicine. Thomas quotes Montaigne, who almost died from a riding accident. "It seemed to me that my life was hanging only by the tip of my lips. I closed my eyes in order, it seemed to me, to,help push it out, and took pleasure in growing languid and letting myself go.

"In order to get used to the idea of death, I find there is nothing like coming close to it."

In a system in which dying is a part of living, a mechanism to turn off pain quickly when it is no longer useful, such as when it has been a signal to take your hand off the stove or to pull the thorn from your toe, is a marvel. "I could not think of a better way to manage," Thomas wrote.