On trial in Arkansas is not just the future in the classroom of one small corner of the biological sciences. It is rather a test of this society's understanding of the scientific endeavor, which involves as essential elements: study of the natural universe, deduction from that study of the underlying mechanisms at work, formulation of generalizations based solely on known, natural phenomena from which predictions can be made and which can therefore be tested, and, finally, the steady refinement or abandonment of old generalizations that do not fit new data. In every one of these essentials, the so-called "scientific creationism," defined as science by Arkansas's new law, fails. The phrase itself is an oxymoron, a contradiction in terms.

Every field of science rests on one or a few great simplifying, organizing principles. In chemistry, it is atomic and molecular theory. In classical physics, it is the spare but far-reaching laws of Newtonian mechanics. In modern physics, it is relativity and quantum theory. And in biology, the unifying principle that informs and directs all understanding of the biological universe is the theory of evolution.

In broadest outline, Darwin's theory states that all biological species have a common origin, and that species evolve one from the other through the random production of diversity followed by selection of traits that confer some reproductive advantage, however slight, on the organism.

Like the earlier explanation offered by the French scientist, Lamarck, the theory rests on the observation that the biological universe is constantly changing. But Darwin's brilliant insight was able to account for the exquisite fit of biological species to so many, vastly different environments without recourse to supernatural explanations. Focusing on the individual, Lamarck was forced to postulate a built-in "drive toward perfection" and the ability to pass on acquired characteristics. Instead, Darwin saw populations and hence natural selection.

Among the predictions the theory makes is an ancient origin of life to allow time for the observed diversity of species. This is corroborated by the fossil record, which indicates that life began on Earth more than 3 billion years ago. The common origin of species is confirmed by the startling biochemical uniformity--in contrast to their external diversity--of all living things. Discrete units of inheritance are explained by Mendel's laws (unknown in Darwin's time), and the random production of diversity by the now understood chemistry of DNA. Natural selection can be observed in the laboratory in any species small enough and quick enough to reproduce so that many generations and millions of individuals can be observed in a convenient period.

Evolution illuminates modern biology at every level, from ecology and population genetics, through the study of behavior, and down to microscopic cellular stucture and the chemistry of genes and proteins. Darwin's principles are, at least implicitly, at the core of every biologist's approach to a new problem: what purpose does this structure or behavior serve; what advantage did it confer? Yet evolution is not a fixed and finished body of knowledge. It is still at the frontier of study, helping to explain the latest findings from the discovery of "nonsense" genes in higher organisms to the intricacies of social behavior in insects. It is itself still evolving. There are many gaps, and some of the mechanisms of evolution are matters of hot dispute. But because the theory has so far proved so powerful, self-consistent and compatible with the findings of other fields of research, the gaps are seen only as areas about which not enough is yet known.

Creationism, on the other hand, makes two claims: that the universe and the Earth are between 6,000 and 10,000 years old, and that all current and extinct forms of life are not the result of continuous change, but were created in six days, no more than 10,000 years ago. Acceptance of these assertions as fact does not just make hash out of all of biology. It also means that much of what is known about geology, paleontology, nuclear physics, astronomy, anthropology, archeology and history no longer makes sense.

The age and distance of the oldest stars and other types of evidence tell astronomers that the universe is about 15 billion years old. Radioactive decay rates give physicists the same answer and also indicate that the age of the Earth is not 10,000 years but 4,500,000,000 years. Geologists could no longer explain the process that produced coal and oil, how and why the continents are observed to move, or how mountains came to be formed out of seabeds in such a compressed time span.

Paleontologists, anthropologists, archeologists and historians would have to reject much of what they thought they knew. The tremendous excitement in these fields set off by recent hominid fossil discoveries would have to be set aside. In an article entitled "The Genesis of Equal Time," John Skow reports that a history text was disapproved earlier this year by the Alabama state board of education because it stated that humans developed spoken language during the Old Stone Age. Creationists opposed the book because: "How does (the author) know that there was no language in the beginning? . . . That is a very subtle way of telling us that we evolved through evolution. . . . There must have been language in the beginning . . . because . . . Adam named all the animals."

The Arkansas law, then, its companion in Louisiana, and the similar bills introduced in 11 other states, will affect the teaching of most, if not all, of the natural and physical sciences and of history and the social sciences as well. Its provisions require ''balanced treatment" in the classroom, in textbooks and in library holdings. Pressure from creationists has already led schoolbook publishers, many of whom follow a policy of avoiding controversy at whatever cost to their integrity, to alter texts far more than most Americans imagine. If the Arkansas law is struck down, creationists have ready a more carefully drafted successor that requires "the removal of bias."

To the scientist, that is like calling someone biased who no longer gives equal time to the notion that the Earth is at the center of the solar system. Yet surveys show that great numbers of Americans support equal time for creationism, perhaps out of ignorance, perhaps out of an instinctive but misguided sense of fair play. The final irony is that genetic engineering has opened a new age in biology, a revolution only just begun, which in all likelihood will affect our lives more profoundly and far more positively than the discovery of nuclear fission. If the creationists have their way, next generation's leaders in that revolution will no longer be Americans.